[PATCH v12 00/15] barrier: Add smp_cond_load_{relaxed,acquire}_timeout()

[PATCH v12 00/15] barrier: Add smp_cond_load_{relaxed,acquire}_timeout()

[Ankur Arora]

Hi,

Main change in this version:

  - addressed some review comments from sashiko (see commit notes)
    - The one notable change is to the implementation of
      smp_cond_load_acquire_timeout() where there was a missed
      control dependency in the timeout case.
      All the others are minor.
  - fixed a low probability race in the kunit test added in v11.
  - added a bunch of kunit tests validating the implementation's
    use of the clock.

Andrew, if the changes look okay, could we take this in your mm-nomm
tree as before?

The core kernel often uses smp_cond_load_{relaxed,acquire}() to spin
on condition variables with architectural primitives used to avoid
hammering the relevant cachelines.

(This primitive can vary greatly across architectures: on x86 it's a
cpu_relax() to slow down the pipeline. On arm64, this is a __cmpwait()
which waits for a cacheline to change state in a time limited fashion.)

Regardless of architectural details, typical smp_cond_load*() usage
does not allow for termination until the condition change occurs.

Beyond the core kernel, there are cases where it is useful to additionally
terminate on a timeout. Two cases:

  - cpuidle poll_idle(): wait for need-resched until the cpuidle polling
    duration expires.

  - rqspinlock: nested qspinlock acquisition that terminates on timeout
    or deadlock.

Accordingly add two interfaces (with their generic and arm64 specific
implementations):

   smp_cond_load_relaxed_timeout(ptr, cond_expr, time_expr, timeout)
   smp_cond_load_acquire_timeout(ptr, cond_expr, time_expr, timeout)

Also add tif_need_resched_relaxed_wait() which wraps the polling
pattern and its scheduler specific details in poll_idle().
In addition add atomic_cond_read_*_timeout(),
atomic64_cond_read_*_timeout(), and atomic_long wrappers.

Structurally, both the smp_cond_load_*_timeout() interfaces are similar
to smp_cond_load*(), with the addition of a rate-limited time-check.

Usage
==

These interfaces drop straight-forwardly into the rqspinlock logic
since qspinlock already uses smp_cond_load*(), and the time-check
extension can now be used for timeout and deadlock handling.

Using tif_need_resched_relaxed_wait() in poll_idle() removes any
architectural details allowing arm64 to straight-forwardly support
that path.
(However, for efficiency reasons cpuidle/poll_state.c continues to
depend on ARCH_HAS_CPU_RELAX since that is defined on architectures
with an optimized architectural primitive.)


Performance
==

Apart from simplifications due to this change, supporting polling in
cpuidle on arm64 helps improve wakeup latency (needs a few cpuidle/acpi
patches):


  # perf stat -r 5 --cpu 4,5 -e task-clock,cycles,instructions,sched:sched_wake_idle_without_ipi \
  perf bench sched pipe -l 1000000 -c 4

  # No haltpoll (and, no TIF_POLLING_NRFLAG):

  Performance counter stats for 'CPU(s) 4,5' (5 runs):

         25,229.57 msec task-clock                       #    2.000 CPUs utilized               ( +-  7.75% )
    45,821,250,284      cycles                           #    1.816 GHz                         ( +- 10.07% )
    26,557,496,665      instructions                     #    0.58  insn per cycle              ( +-  0.21% )
                 0      sched:sched_wake_idle_without_ipi #    0.000 /sec

       12.615 +- 0.977 seconds time elapsed  ( +-  7.75% )


  # Haltpoll:

  Performance counter stats for 'CPU(s) 4,5' (5 runs):

         15,131.58 msec task-clock                       #    2.000 CPUs utilized               ( +- 10.00% )
    34,158,188,839      cycles                           #    2.257 GHz                         ( +-  6.91% )
    20,824,950,916      instructions                     #    0.61  insn per cycle              ( +-  0.09% )
         1,983,822      sched:sched_wake_idle_without_ipi #  131.105 K/sec                       ( +-  0.78% )

        7.566 +- 0.756 seconds time elapsed  ( +- 10.00% )

  We get improved latency because we don't switch in and out of a
  deeper sleep state or from the hypervisor. This also causes us to
  execute ~20% fewer instructions.


Haris Okanovic also saw improvement in real workloads due to the
cpuidle changes: "observed 4-6% improvements in memcahed, cassandra,
mysql, and postgresql under certain loads. Other applications likely
benefit too." [12]


Changelog:
  v11 [13] (as listed above):
    - addressed some review comments from sashiko (see commit notes)
      - The one notable change is to the implementation of
        smp_cond_load_acquire_timeout() where there was a missed
        control dependency in the timeout case.
      All the others are minor.
    - fixed a low probability race in the kunit test added in v11.
    - added a bunch of kunit tests validating the implementation's
      use of the clock.

  v10 [10]:
   - add a comment mentioning that smp_cond_load_relaxed_timeout() might
     be using architectural primitives that don't support MMIO.
     (David Laight, Catalin Marinas)
   - added a kunit test for smp_cond_load_relaxed_timeout() (Andrew
     Morton.)

  v9 [9]:
   - s/@cond/@cond_expr/ (Randy Dunlap)
   - Clarify that SMP_TIMEOUT_POLL_COUNT is only around memory
     addresses. (David Laight)
   - Add the missing config ARCH_HAS_CPU_RELAX in arch/arm64/Kconfig.
     (Catalin Marinas).
   - Switch to arch_counter_get_cntvct_stable() (via __delay_cycles())
     in the cmpwait path instead of using arch_timer_read_counter().
     (Catalin Marinas)

  v8 [0]:
   - Defer evaluation of @time_expr_ns to when we hit the slowpath.
      (comment from Alexei Starovoitov).

   - Mention that cpu_poll_relax() is better than raw CPU polling
     only where ARCH_HAS_CPU_RELAX is defined.
     - also define ARCH_HAS_CPU_RELAX for arm64.
      (Came out of a discussion with Will Deacon.)

   - Split out WFET and WFE handling. I was doing both of these
     in a common handler.
     (From Will Deacon and in an earlier revision by Catalin Marinas.)

   - Add mentions of atomic_cond_read_{relaxed,acquire}(),
     atomic_cond_read_{relaxed,acquire}_timeout() in
     Documentation/atomic_t.txt.

   - Use the BIT() macro to do the checking in tif_bitset_relaxed_wait().

   - Cleanup unnecessary assignments, casts etc in poll_idle().
     (From Rafael Wysocki.)

   - Fixup warnings from kernel build robot


  v7 [1]:
   - change the interface to separately provide the timeout. This is
     useful for supporting WFET and similar primitives which can do
     timed waiting (suggested by Arnd Bergmann).

   - Adapting rqspinlock code to this changed interface also
     necessitated allowing time_expr to fail.
   - rqspinlock changes to adapt to the new smp_cond_load_acquire_timeout().

   - add WFET support (suggested by Arnd Bergmann).
   - add support for atomic-long wrappers.
   - add a new scheduler interface tif_need_resched_relaxed_wait() which
     encapsulates the polling logic used by poll_idle().
     - interface suggested by (Rafael J. Wysocki).


  v6 [2]:
   - fixup missing timeout parameters in atomic64_cond_read_*_timeout()
   - remove a race between setting of TIF_NEED_RESCHED and the call to
     smp_cond_load_relaxed_timeout(). This would mean that dev->poll_time_limit
     would be set even if we hadn't spent any time waiting.
     (The original check compared against local_clock(), which would have been
     fine, but I was instead using a cheaper check against _TIF_NEED_RESCHED.)
   (Both from meta-CI bot)


  v5 [3]:
   - use cpu_poll_relax() instead of cpu_relax().
   - instead of defining an arm64 specific
     smp_cond_load_relaxed_timeout(), just define the appropriate
     cpu_poll_relax().
   - re-read the target pointer when we exit due to the time-check.
   - s/SMP_TIMEOUT_SPIN_COUNT/SMP_TIMEOUT_POLL_COUNT/
   (Suggested by Will Deacon)

   - add atomic_cond_read_*_timeout() and atomic64_cond_read_*_timeout()
     interfaces.
   - rqspinlock: use atomic_cond_read_acquire_timeout().
   - cpuidle: use smp_cond_load_relaxed_tiemout() for polling.
   (Suggested by Catalin Marinas)

   - rqspinlock: define SMP_TIMEOUT_POLL_COUNT to be 16k for non arm64


  v4 [4]:
    - naming change 's/timewait/timeout/'
    - resilient spinlocks: get rid of res_smp_cond_load_acquire_waiting()
      and fixup use of RES_CHECK_TIMEOUT().
    (Both suggested by Catalin Marinas)

  v3 [5]:
    - further interface simplifications (suggested by Catalin Marinas)

  v2 [6]:
    - simplified the interface (suggested by Catalin Marinas)
       - get rid of wait_policy, and a multitude of constants
       - adds a slack parameter
      This helped remove a fair amount of duplicated code duplication and in
      hindsight unnecessary constants.

  v1 [7]:
     - add wait_policy (coarse and fine)
     - derive spin-count etc at runtime instead of using arbitrary
       constants.

Haris Okanovic tested v4 of this series with poll_idle()/haltpoll patches. [8]

Comments appreciated!

Thanks
Ankur

 [0] https://lore.kernel.org/lkml/20251215044919.460086-1-ankur.a.arora@oracle.com/
 [1] https://lore.kernel.org/lkml/20251028053136.692462-1-ankur.a.arora@oracle.com/
 [2] https://lore.kernel.org/lkml/20250911034655.3916002-1-ankur.a.arora@oracle.com/
 [3] https://lore.kernel.org/lkml/20250911034655.3916002-1-ankur.a.arora@oracle.com/
 [4] https://lore.kernel.org/lkml/20250829080735.3598416-1-ankur.a.arora@oracle.com/
 [5] https://lore.kernel.org/lkml/20250627044805.945491-1-ankur.a.arora@oracle.com/
 [6] https://lore.kernel.org/lkml/20250502085223.1316925-1-ankur.a.arora@oracle.com/
 [7] https://lore.kernel.org/lkml/20250203214911.898276-1-ankur.a.arora@oracle.com/
 [8] https://lore.kernel.org/lkml/2cecbf7fb23ee83a4ce027e1be3f46f97efd585c.camel@amazon.com/
 [9] https://lore.kernel.org/lkml/20260209023153.2661784-1-ankur.a.arora@oracle.com/
 [10] https://lore.kernel.org/lkml/20260316013651.3225328-1-ankur.a.arora@oracle.com/
 [11] https://lore.kernel.org/lkml/20230809134837.GM212435@hirez.programming.kicks-ass.net/
 [12] https://lore.kernel.org/lkml/c6f3c8d3f1f2e89a9dc7ae22482973b5a51b08cb.camel@amazon.com/

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: bpf@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-pm@vger.kernel.org

Ankur Arora (15):
  asm-generic: barrier: Add smp_cond_load_relaxed_timeout()
  arm64: barrier: Support smp_cond_load_relaxed_timeout()
  arm64/delay: move some constants out to a separate header
  arm64: support WFET in smp_cond_load_relaxed_timeout()
  arm64: rqspinlock: Remove private copy of
    smp_cond_load_acquire_timewait()
  asm-generic: barrier: Add smp_cond_load_acquire_timeout()
  atomic: Add atomic_cond_read_*_timeout()
  locking/atomic: scripts: build atomic_long_cond_read_*_timeout()
  bpf/rqspinlock: switch check_timeout() to a clock interface
  bpf/rqspinlock: Use smp_cond_load_acquire_timeout()
  sched: add need-resched timed wait interface
  cpuidle/poll_state: Wait for need-resched via
    tif_need_resched_relaxed_wait()
  arm64/delay: enable testing smp_cond_load_relaxed_timeout()
  barrier: add tests for smp_cond_load_*_timeout()
  barrier: add clock tests for smp_cond_load_relaxed_timeout()

 Documentation/atomic_t.txt           |  14 +-
 arch/arm64/Kconfig                   |   3 +
 arch/arm64/include/asm/barrier.h     |  23 ++++
 arch/arm64/include/asm/cmpxchg.h     |  62 +++++++--
 arch/arm64/include/asm/delay-const.h |  28 ++++
 arch/arm64/include/asm/rqspinlock.h  |  85 ------------
 arch/arm64/lib/delay.c               |  17 +--
 drivers/clocksource/arm_arch_timer.c |   2 +
 drivers/cpuidle/poll_state.c         |  21 +--
 drivers/soc/qcom/rpmh-rsc.c          |   8 +-
 include/asm-generic/barrier.h        |  97 ++++++++++++++
 include/linux/atomic.h               |  10 ++
 include/linux/atomic/atomic-long.h   |  18 ++-
 include/linux/sched/idle.h           |  29 +++++
 kernel/bpf/rqspinlock.c              |  77 +++++++----
 lib/Kconfig.debug                    |  10 ++
 lib/tests/Makefile                   |   1 +
 lib/tests/barrier-timeout-test.c     | 185 +++++++++++++++++++++++++++
 scripts/atomic/gen-atomic-long.sh    |  16 ++-
 19 files changed, 528 insertions(+), 178 deletions(-)
 create mode 100644 arch/arm64/include/asm/delay-const.h
 create mode 100644 lib/tests/barrier-timeout-test.c

-- 
2.31.1

[PATCH v12 01/15] asm-generic: barrier: Add smp_cond_load_relaxed_timeout()

[Ankur Arora]

Add smp_cond_load_relaxed_timeout(), which extends
smp_cond_load_relaxed() to allow waiting for a duration.

We loop around waiting for the condition variable to change while
peridically doing a time-check. The loop uses cpu_poll_relax() to slow
down the busy-wait, which, unless overridden by the architecture
code, amounts to a cpu_relax().

Note that there are two ways for the time-check to fail: the timeout
case or, @time_expr_ns returning an invalid value (negative or zero).
The second failure mode allows for clocks attached to the clock-domain
of @cond_expr --  which might cease to operate meaningfully once some
state internal to @cond_expr has changed -- to fail.

Evaluation of @time_expr_ns: in the fastpath we want to keep the
performance close to smp_cond_load_relaxed(). So defer evaluation
of the potentially costly @time_expr_ns to the slowpath.

This also means that there will always be some hardware dependent
duration that has passed in cpu_poll_relax() iterations at the time
of first evaluation. Additionally cpu_poll_relax() is not guaranteed
to return at timeout boundary. In sum, expect timeout overshoot when
we exit due to expiration of the timeout.

The number of spin iterations before time-check, SMP_TIMEOUT_POLL_COUNT
is chosen to be 200 by default. With a cpu_poll_relax() iteration
taking ~20-30 cycles (measured on a variety of x86 platforms), we
expect a time-check every ~4000-6000 cycles.

The outer limit of the overshoot is double that when working with the
parameters above. This might be higher or lower depending on the
implementation of cpu_poll_relax() across architectures.

Lastly, config option ARCH_HAS_CPU_RELAX indicates availability of a
cpu_poll_relax() that is cheaper than polling. This might be relevant
for cases with a long timeout.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-arch@vger.kernel.org
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---

Note: addresses a few Sashiko comments from [1].

We leave unaddressed a couple of potential timeout range issues (around
S64_MAX, or during early boot). I had proposed a version earlier that
would address those in [2]. Since then, however, I've come to the view
that these issues are best handled in code review instead of
overcomplicating the implementation.

[1] https://sashiko.dev/#/patchset/20260408122538.3610871-1-ankur.a.arora%40oracle.com
[2] https://lore.kernel.org/lkml/874iklm1uy.fsf@oracle.com/
---
 include/asm-generic/barrier.h | 69 +++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)

diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index d4f581c1e21d..c56df9513a08 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -273,6 +273,75 @@ do {									\
 })
 #endif
 
+/*
+ * Number of times we iterate in the loop before doing the time check.
+ * Note that the iteration count assumes that the loop condition is
+ * relatively cheap.
+ */
+#ifndef SMP_TIMEOUT_POLL_COUNT
+#define SMP_TIMEOUT_POLL_COUNT		200
+#endif
+
+/*
+ * Platforms with ARCH_HAS_CPU_RELAX have a cpu_poll_relax() implementation
+ * that is expected to be cheaper (lower power) than pure polling.
+ */
+#ifndef cpu_poll_relax
+#define cpu_poll_relax(ptr, val, timeout_ns)	cpu_relax()
+#endif
+
+/**
+ * smp_cond_load_relaxed_timeout() - (Spin) wait for cond with no ordering
+ * guarantees until a timeout expires.
+ * @ptr: pointer to the variable to wait on.
+ * @cond_expr: boolean expression to wait for.
+ * @time_expr_ns: expression that evaluates to monotonic time (in ns) or,
+ *  on failure, returns a negative value.
+ * @timeout_ns: timeout value in ns
+ * Both of the above are assumed to be compatible with s64; the signed
+ * value is used to handle the failure case in @time_expr_ns.
+ *
+ * Equivalent to using READ_ONCE() on the condition variable.
+ *
+ * Callers that expect to wait for prolonged durations might want
+ * to take into account the availability of ARCH_HAS_CPU_RELAX.
+ *
+ * Note that @ptr is expected to point to a memory address. Using this
+ * interface with MMIO will be slower (since SMP_TIMEOUT_POLL_COUNT is
+ * tuned for memory) and might also break in interesting architecture
+ * dependent ways.
+ */
+#ifndef smp_cond_load_relaxed_timeout
+#define smp_cond_load_relaxed_timeout(ptr, cond_expr,			\
+				      time_expr_ns, timeout_ns)		\
+({									\
+	typeof(ptr) __PTR = (ptr);					\
+	__unqual_scalar_typeof(*(ptr)) VAL;				\
+	u32 __count = 0, __spin = SMP_TIMEOUT_POLL_COUNT;		\
+	s64 __timeout = (s64)timeout_ns;				\
+	s64 __time_now, __time_end = 0;					\
+									\
+	for (;;) {							\
+		VAL = READ_ONCE(*__PTR);				\
+		if (cond_expr)						\
+			break;						\
+		cpu_poll_relax(__PTR, VAL, (u64)__timeout);		\
+		if (++__count < __spin)					\
+			continue;					\
+		__time_now = (s64)(time_expr_ns);			\
+		if (unlikely(__time_end == 0))				\
+			__time_end = __time_now + __timeout;		\
+		__timeout = __time_end - __time_now;			\
+		if (__time_now <= 0 || __timeout <= 0) {		\
+			VAL = READ_ONCE(*__PTR);			\
+			break;						\
+		}							\
+		__count = 0;						\
+	}								\
+	(typeof(*(ptr)))VAL;						\
+})
+#endif
+
 /*
  * pmem_wmb() ensures that all stores for which the modification
  * are written to persistent storage by preceding instructions have
-- 
2.31.1

[PATCH v12 02/15] arm64: barrier: Support smp_cond_load_relaxed_timeout()

[Ankur Arora]

Support waiting in smp_cond_load_relaxed_timeout() via
__cmpwait_relaxed(). To ensure that we wake from waiting in
WFE periodically and don't block forever if there are no stores
to ptr, this path is only used when the event-stream is enabled.

Note that when using __cmpwait_relaxed() we ignore the timeout
value, allowing an overshoot by up to the event-stream period.
And, in the unlikely event that the event-stream is unavailable,
fallback to spin-waiting.

Also set SMP_TIMEOUT_POLL_COUNT to 1 so we do the time-check in
each iteration of smp_cond_load_relaxed_timeout().

And finally define ARCH_HAS_CPU_RELAX to indicate that we have
an optimized implementation of cpu_poll_relax().

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Suggested-by: Will Deacon <will@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---

Notes: Doesn't address sashiko comments [1]:

  - event stream being disabled or broken: the event stream being
    disabled is not a production use case; doesn't make sense to
    add a fastpath check for this.

  - arch_timer_evtstrm_available() isn't exported. On arm64, this
    means that smp_cond_load_relaxed_timeout() won't be available
    to modules: tight now the use of this interface (and others of
    this ilk) is fairly limited. Not worth solving until there's a
    use case.

[1] https://sashiko.dev/#/patchset/20260408122538.3610871-1-ankur.a.arora%40oracle.com
---
 arch/arm64/Kconfig               |  3 +++
 arch/arm64/include/asm/barrier.h | 21 +++++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index fe60738e5943..fa676428ec3f 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1606,6 +1606,9 @@ config ARCH_SUPPORTS_CRASH_DUMP
 config ARCH_DEFAULT_CRASH_DUMP
 	def_bool y
 
+config ARCH_HAS_CPU_RELAX
+	def_bool y
+
 config ARCH_HAS_GENERIC_CRASHKERNEL_RESERVATION
 	def_bool CRASH_RESERVE
 
diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index 9495c4441a46..6190e178db51 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -12,6 +12,7 @@
 #include <linux/kasan-checks.h>
 
 #include <asm/alternative-macros.h>
+#include <asm/vdso/processor.h>
 
 #define __nops(n)	".rept	" #n "\nnop\n.endr\n"
 #define nops(n)		asm volatile(__nops(n))
@@ -219,6 +220,26 @@ do {									\
 	(typeof(*ptr))VAL;						\
 })
 
+/* Re-declared here to avoid include dependency. */
+extern bool arch_timer_evtstrm_available(void);
+
+/*
+ * In the common case, cpu_poll_relax() sits waiting in __cmpwait_relaxed()
+ * for the ptr value to change.
+ *
+ * Since this period is reasonably long, choose SMP_TIMEOUT_POLL_COUNT
+ * to be 1, so smp_cond_load_{relaxed,acquire}_timeout() does a
+ * time-check in each iteration.
+ */
+#define SMP_TIMEOUT_POLL_COUNT	1
+
+#define cpu_poll_relax(ptr, val, timeout_ns) do {			\
+	if (arch_timer_evtstrm_available())				\
+		__cmpwait_relaxed(ptr, val);				\
+	else								\
+		cpu_relax();						\
+} while (0)
+
 #include <asm-generic/barrier.h>
 
 #endif	/* __ASSEMBLER__ */
-- 
2.31.1

[PATCH v12 03/15] arm64/delay: move some constants out to a separate header

[Ankur Arora]

Moves some constants and functions related to xloops, cycles computation
out to a new header. Also make __delay_cycles() available outside of
arch/arm64/lib/delay.c.

Rename some macros in qcom/rpmh-rsc.c which were occupying the same
namespace.

No functional change.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Bjorn Andersson <andersson@kernel.org>
Cc: Konrad Dybcio <konradybcio@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Reviewed-by: Christoph Lameter <cl@linux.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---

Notes:
  - include <linux/types.h> (flagged by sashiko)
---
 arch/arm64/include/asm/delay-const.h | 28 ++++++++++++++++++++++++++++
 arch/arm64/lib/delay.c               | 15 ++++-----------
 drivers/soc/qcom/rpmh-rsc.c          |  8 ++++----
 3 files changed, 36 insertions(+), 15 deletions(-)
 create mode 100644 arch/arm64/include/asm/delay-const.h

diff --git a/arch/arm64/include/asm/delay-const.h b/arch/arm64/include/asm/delay-const.h
new file mode 100644
index 000000000000..2a5acfb7bff1
--- /dev/null
+++ b/arch/arm64/include/asm/delay-const.h
@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef _ASM_DELAY_CONST_H
+#define _ASM_DELAY_CONST_H
+
+#include <linux/types.h>
+#include <asm/param.h>	/* For HZ */
+
+/* 2**32 / 1000000 (rounded up) */
+#define __usecs_to_xloops_mult	0x10C7UL
+
+/* 2**32 / 1000000000 (rounded up) */
+#define __nsecs_to_xloops_mult	0x5UL
+
+extern unsigned long loops_per_jiffy;
+static inline unsigned long xloops_to_cycles(unsigned long xloops)
+{
+	return (xloops * loops_per_jiffy * HZ) >> 32;
+}
+
+#define USECS_TO_CYCLES(time_usecs) \
+	xloops_to_cycles((time_usecs) * __usecs_to_xloops_mult)
+
+#define NSECS_TO_CYCLES(time_nsecs) \
+	xloops_to_cycles((time_nsecs) * __nsecs_to_xloops_mult)
+
+u64 notrace __delay_cycles(void);
+
+#endif	/* _ASM_DELAY_CONST_H */
diff --git a/arch/arm64/lib/delay.c b/arch/arm64/lib/delay.c
index e278e060e78a..c660a7ea26dd 100644
--- a/arch/arm64/lib/delay.c
+++ b/arch/arm64/lib/delay.c
@@ -12,17 +12,10 @@
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/timex.h>
+#include <asm/delay-const.h>
 
 #include <clocksource/arm_arch_timer.h>
 
-#define USECS_TO_CYCLES(time_usecs)			\
-	xloops_to_cycles((time_usecs) * 0x10C7UL)
-
-static inline unsigned long xloops_to_cycles(unsigned long xloops)
-{
-	return (xloops * loops_per_jiffy * HZ) >> 32;
-}
-
 /*
  * Force the use of CNTVCT_EL0 in order to have the same base as WFxT.
  * This avoids some annoying issues when CNTVOFF_EL2 is not reset 0 on a
@@ -32,7 +25,7 @@ static inline unsigned long xloops_to_cycles(unsigned long xloops)
  * Note that userspace cannot change the offset behind our back either,
  * as the vcpu mutex is held as long as KVM_RUN is in progress.
  */
-static cycles_t notrace __delay_cycles(void)
+u64 notrace __delay_cycles(void)
 {
 	guard(preempt_notrace)();
 	return __arch_counter_get_cntvct_stable();
@@ -73,12 +66,12 @@ EXPORT_SYMBOL(__const_udelay);
 
 void __udelay(unsigned long usecs)
 {
-	__const_udelay(usecs * 0x10C7UL); /* 2**32 / 1000000 (rounded up) */
+	__const_udelay(usecs * __usecs_to_xloops_mult);
 }
 EXPORT_SYMBOL(__udelay);
 
 void __ndelay(unsigned long nsecs)
 {
-	__const_udelay(nsecs * 0x5UL); /* 2**32 / 1000000000 (rounded up) */
+	__const_udelay(nsecs * __nsecs_to_xloops_mult);
 }
 EXPORT_SYMBOL(__ndelay);
diff --git a/drivers/soc/qcom/rpmh-rsc.c b/drivers/soc/qcom/rpmh-rsc.c
index c6f7d5c9c493..ad5ec5c0de0a 100644
--- a/drivers/soc/qcom/rpmh-rsc.c
+++ b/drivers/soc/qcom/rpmh-rsc.c
@@ -146,10 +146,10 @@ enum {
  *  +---------------------------------------------------+
  */
 
-#define USECS_TO_CYCLES(time_usecs)			\
-	xloops_to_cycles((time_usecs) * 0x10C7UL)
+#define RPMH_USECS_TO_CYCLES(time_usecs)		\
+	rpmh_xloops_to_cycles((time_usecs) * 0x10C7UL)
 
-static inline unsigned long xloops_to_cycles(u64 xloops)
+static inline unsigned long rpmh_xloops_to_cycles(u64 xloops)
 {
 	return (xloops * loops_per_jiffy * HZ) >> 32;
 }
@@ -819,7 +819,7 @@ void rpmh_rsc_write_next_wakeup(struct rsc_drv *drv)
 	wakeup_us = ktime_to_us(wakeup);
 
 	/* Convert the wakeup to arch timer scale */
-	wakeup_cycles = USECS_TO_CYCLES(wakeup_us);
+	wakeup_cycles = RPMH_USECS_TO_CYCLES(wakeup_us);
 	wakeup_cycles += arch_timer_read_counter();
 
 exit:
-- 
2.31.1

[PATCH v12 04/15] arm64: support WFET in smp_cond_load_relaxed_timeout()

[Ankur Arora]

To handle WFET use __cmpwait_timeout() similarly to __cmpwait(). These
call out to the respective __cmpwait_case_timeout_##sz(),
__cmpwait_case_##sz() functions.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---

Notes:

Does not address sashiko [1] comments on:

  - timeout overshoot: intentional, not meant to be a precise
    interface.

  - range edge cases: as mentioned before, better addressed in
    code review.

  - loadable kernel modules using this will fail to build: niche
    interface, not worth fixing unless necessary.

[1] https://sashiko.dev/#/patchset/20260408122538.3610871-1-ankur.a.arora%40oracle.com
---
 arch/arm64/include/asm/barrier.h |  8 +++--
 arch/arm64/include/asm/cmpxchg.h | 62 +++++++++++++++++++++++++-------
 2 files changed, 55 insertions(+), 15 deletions(-)

diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index 6190e178db51..fbd71cd4ef4e 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -224,8 +224,8 @@ do {									\
 extern bool arch_timer_evtstrm_available(void);
 
 /*
- * In the common case, cpu_poll_relax() sits waiting in __cmpwait_relaxed()
- * for the ptr value to change.
+ * In the common case, cpu_poll_relax() sits waiting in __cmpwait_relaxed()/
+ * __cmpwait_relaxed_timeout() for the ptr value to change.
  *
  * Since this period is reasonably long, choose SMP_TIMEOUT_POLL_COUNT
  * to be 1, so smp_cond_load_{relaxed,acquire}_timeout() does a
@@ -234,7 +234,9 @@ extern bool arch_timer_evtstrm_available(void);
 #define SMP_TIMEOUT_POLL_COUNT	1
 
 #define cpu_poll_relax(ptr, val, timeout_ns) do {			\
-	if (arch_timer_evtstrm_available())				\
+	if (alternative_has_cap_unlikely(ARM64_HAS_WFXT))		\
+		__cmpwait_relaxed_timeout(ptr, val, timeout_ns);	\
+	else if (arch_timer_evtstrm_available())			\
 		__cmpwait_relaxed(ptr, val);				\
 	else								\
 		cpu_relax();						\
diff --git a/arch/arm64/include/asm/cmpxchg.h b/arch/arm64/include/asm/cmpxchg.h
index 6cf3cd6873f5..9e4cdc9e41d1 100644
--- a/arch/arm64/include/asm/cmpxchg.h
+++ b/arch/arm64/include/asm/cmpxchg.h
@@ -12,6 +12,7 @@
 
 #include <asm/barrier.h>
 #include <asm/lse.h>
+#include <asm/delay-const.h>
 
 /*
  * We need separate acquire parameters for ll/sc and lse, since the full
@@ -212,7 +213,8 @@ __CMPXCHG_GEN(_mb)
 
 #define __CMPWAIT_CASE(w, sfx, sz)					\
 static inline void __cmpwait_case_##sz(volatile void *ptr,		\
-				       unsigned long val)		\
+				       unsigned long val,		\
+				       u64 __maybe_unused timeout_ns)	\
 {									\
 	unsigned long tmp;						\
 									\
@@ -235,20 +237,52 @@ __CMPWAIT_CASE( ,  , 64);
 
 #undef __CMPWAIT_CASE
 
-#define __CMPWAIT_GEN(sfx)						\
-static __always_inline void __cmpwait##sfx(volatile void *ptr,		\
-				  unsigned long val,			\
-				  int size)				\
+#define __CMPWAIT_TIMEOUT_CASE(w, sfx, sz)				\
+static inline void __cmpwait_case_timeout_##sz(volatile void *ptr,	\
+					       unsigned long val,	\
+					       u64 timeout_ns)		\
+{									\
+	unsigned long tmp;						\
+	u64 ecycles = __delay_cycles() +				\
+			NSECS_TO_CYCLES(timeout_ns);			\
+	asm volatile(							\
+	"	sevl\n"							\
+	"	wfe\n"							\
+	"	ldxr" #sfx "\t%" #w "[tmp], %[v]\n"			\
+	"	eor	%" #w "[tmp], %" #w "[tmp], %" #w "[val]\n"	\
+	"	cbnz	%" #w "[tmp], 2f\n"				\
+	"	msr s0_3_c1_c0_0, %[ecycles]\n"				\
+	"2:"								\
+	: [tmp] "=&r" (tmp), [v] "+Q" (*(u##sz *)ptr)			\
+	: [val] "r" (val), [ecycles] "r" (ecycles));			\
+}
+
+__CMPWAIT_TIMEOUT_CASE(w, b, 8);
+__CMPWAIT_TIMEOUT_CASE(w, h, 16);
+__CMPWAIT_TIMEOUT_CASE(w,  , 32);
+__CMPWAIT_TIMEOUT_CASE( ,  , 64);
+
+#undef __CMPWAIT_TIMEOUT_CASE
+
+#define __CMPWAIT_GEN(timeout, sfx)					\
+static __always_inline void __cmpwait##timeout##sfx(volatile void *ptr,	\
+						    unsigned long val,	\
+						    u64 timeout_ns,	\
+						    int size)		\
 {									\
 	switch (size) {							\
 	case 1:								\
-		return __cmpwait_case##sfx##_8(ptr, (u8)val);		\
+		return __cmpwait_case##timeout##sfx##_8(ptr, (u8)val,	\
+							timeout_ns);	\
 	case 2:								\
-		return __cmpwait_case##sfx##_16(ptr, (u16)val);		\
+		return __cmpwait_case##timeout##sfx##_16(ptr, (u16)val,	\
+							 timeout_ns);	\
 	case 4:								\
-		return __cmpwait_case##sfx##_32(ptr, val);		\
+		return __cmpwait_case##timeout##sfx##_32(ptr, val,	\
+							 timeout_ns);	\
 	case 8:								\
-		return __cmpwait_case##sfx##_64(ptr, val);		\
+		return __cmpwait_case##timeout##sfx##_64(ptr, val,	\
+							 timeout_ns);	\
 	default:							\
 		BUILD_BUG();						\
 	}								\
@@ -256,11 +290,15 @@ static __always_inline void __cmpwait##sfx(volatile void *ptr,		\
 	unreachable();							\
 }
 
-__CMPWAIT_GEN()
+__CMPWAIT_GEN(        , )
+__CMPWAIT_GEN(_timeout, )
 
 #undef __CMPWAIT_GEN
 
-#define __cmpwait_relaxed(ptr, val) \
-	__cmpwait((ptr), (unsigned long)(val), sizeof(*(ptr)))
+#define __cmpwait_relaxed_timeout(ptr, val, timeout_ns)			\
+	__cmpwait_timeout((ptr), (unsigned long)(val), timeout_ns, sizeof(*(ptr)))
+
+#define __cmpwait_relaxed(ptr, val)					\
+	__cmpwait((ptr), (unsigned long)(val), 0, sizeof(*(ptr)))
 
 #endif	/* __ASM_CMPXCHG_H */
-- 
2.31.1

[PATCH v12 05/15] arm64: rqspinlock: Remove private copy of smp_cond_load_acquire_timewait()

[Ankur Arora]

In preparation for defining smp_cond_load_acquire_timeout(), remove
the private copy. Lacking this, the rqspinlock code falls back to using
smp_cond_load_acquire().

Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: bpf@vger.kernel.org
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Haris Okanovic <harisokn@amazon.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---

Notes:

Sashiko mentions that this introduces a bisection hole: this has been
discussed in prior versions and doing it this way seems the cleanest.

---
 arch/arm64/include/asm/rqspinlock.h | 85 -----------------------------
 1 file changed, 85 deletions(-)

diff --git a/arch/arm64/include/asm/rqspinlock.h b/arch/arm64/include/asm/rqspinlock.h
index 9ea0a74e5892..a385603436e9 100644
--- a/arch/arm64/include/asm/rqspinlock.h
+++ b/arch/arm64/include/asm/rqspinlock.h
@@ -3,91 +3,6 @@
 #define _ASM_RQSPINLOCK_H
 
 #include <asm/barrier.h>
-
-/*
- * Hardcode res_smp_cond_load_acquire implementations for arm64 to a custom
- * version based on [0]. In rqspinlock code, our conditional expression involves
- * checking the value _and_ additionally a timeout. However, on arm64, the
- * WFE-based implementation may never spin again if no stores occur to the
- * locked byte in the lock word. As such, we may be stuck forever if
- * event-stream based unblocking is not available on the platform for WFE spin
- * loops (arch_timer_evtstrm_available).
- *
- * Once support for smp_cond_load_acquire_timewait [0] lands, we can drop this
- * copy-paste.
- *
- * While we rely on the implementation to amortize the cost of sampling
- * cond_expr for us, it will not happen when event stream support is
- * unavailable, time_expr check is amortized. This is not the common case, and
- * it would be difficult to fit our logic in the time_expr_ns >= time_limit_ns
- * comparison, hence just let it be. In case of event-stream, the loop is woken
- * up at microsecond granularity.
- *
- * [0]: https://lore.kernel.org/lkml/20250203214911.898276-1-ankur.a.arora@oracle.com
- */
-
-#ifndef smp_cond_load_acquire_timewait
-
-#define smp_cond_time_check_count	200
-
-#define __smp_cond_load_relaxed_spinwait(ptr, cond_expr, time_expr_ns,	\
-					 time_limit_ns) ({		\
-	typeof(ptr) __PTR = (ptr);					\
-	__unqual_scalar_typeof(*ptr) VAL;				\
-	unsigned int __count = 0;					\
-	for (;;) {							\
-		VAL = READ_ONCE(*__PTR);				\
-		if (cond_expr)						\
-			break;						\
-		cpu_relax();						\
-		if (__count++ < smp_cond_time_check_count)		\
-			continue;					\
-		if ((time_expr_ns) >= (time_limit_ns))			\
-			break;						\
-		__count = 0;						\
-	}								\
-	(typeof(*ptr))VAL;						\
-})
-
-#define __smp_cond_load_acquire_timewait(ptr, cond_expr,		\
-					 time_expr_ns, time_limit_ns)	\
-({									\
-	typeof(ptr) __PTR = (ptr);					\
-	__unqual_scalar_typeof(*ptr) VAL;				\
-	for (;;) {							\
-		VAL = smp_load_acquire(__PTR);				\
-		if (cond_expr)						\
-			break;						\
-		__cmpwait_relaxed(__PTR, VAL);				\
-		if ((time_expr_ns) >= (time_limit_ns))			\
-			break;						\
-	}								\
-	(typeof(*ptr))VAL;						\
-})
-
-#define smp_cond_load_acquire_timewait(ptr, cond_expr,			\
-				      time_expr_ns, time_limit_ns)	\
-({									\
-	__unqual_scalar_typeof(*ptr) _val;				\
-	int __wfe = arch_timer_evtstrm_available();			\
-									\
-	if (likely(__wfe)) {						\
-		_val = __smp_cond_load_acquire_timewait(ptr, cond_expr,	\
-							time_expr_ns,	\
-							time_limit_ns);	\
-	} else {							\
-		_val = __smp_cond_load_relaxed_spinwait(ptr, cond_expr,	\
-							time_expr_ns,	\
-							time_limit_ns);	\
-		smp_acquire__after_ctrl_dep();				\
-	}								\
-	(typeof(*ptr))_val;						\
-})
-
-#endif
-
-#define res_smp_cond_load_acquire(v, c) smp_cond_load_acquire_timewait(v, c, 0, 1)
-
 #include <asm-generic/rqspinlock.h>
 
 #endif /* _ASM_RQSPINLOCK_H */
-- 
2.31.1

[PATCH v12 06/15] asm-generic: barrier: Add smp_cond_load_acquire_timeout()

[Ankur Arora]

Add the acquire variant of smp_cond_load_relaxed_timeout(). This
reuses the relaxed variant, with additional LOAD->LOAD ordering
via smp_acquire__after_ctrl_dep().

To ensure that the necessary control dependency on the dereference
of @ptr exists (which does not in the timeout path), introduce
an empty evaluation of the cond_expr branch.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-arch@vger.kernel.org
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Haris Okanovic <harisokn@amazon.com>
Tested-by: Haris Okanovic <harisokn@amazon.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---

Notes:

Sashiko notes [1] that there's a missed control dependency in the
timeout path. Fix that by forcing evaluation of the cond_expr branch.

Catalin, Haris: I've kept your R-by on this. Please let me know if
you aren't okay with this change.

[1] https://sashiko.dev/#/patchset/20260408122538.3610871-1-ankur.a.arora%40oracle.com
---
 include/asm-generic/barrier.h | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index c56df9513a08..0ab26e98842c 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -342,6 +342,34 @@ do {									\
 })
 #endif
 
+/**
+ * smp_cond_load_acquire_timeout() - (Spin) wait for cond with ACQUIRE ordering
+ * until a timeout expires.
+ * @ptr: pointer to the variable to wait on.
+ * @cond_expr: boolean expression to wait for.
+ * @time_expr_ns: monotonic expression that evaluates to time in ns or,
+ *  on failure, returns a negative value.
+ * @timeout_ns: timeout value in ns
+ * (Both of the above are assumed to be compatible with s64.)
+ *
+ * Equivalent to using smp_cond_load_acquire() on the condition variable with
+ * a timeout.
+ */
+#ifndef smp_cond_load_acquire_timeout
+#define smp_cond_load_acquire_timeout(ptr, cond_expr,			\
+				      time_expr_ns, timeout_ns)		\
+({									\
+	__unqual_scalar_typeof(*(ptr)) VAL;				\
+	VAL = smp_cond_load_relaxed_timeout(ptr, cond_expr,		\
+					     time_expr_ns,		\
+					     timeout_ns);		\
+	if (cond_expr)							\
+		barrier();						\
+	smp_acquire__after_ctrl_dep();					\
+	(typeof(*(ptr)))VAL;						\
+})
+#endif
+
 /*
  * pmem_wmb() ensures that all stores for which the modification
  * are written to persistent storage by preceding instructions have
-- 
2.31.1

[PATCH v12 07/15] atomic: Add atomic_cond_read_*_timeout()

[Ankur Arora]

Add atomic load wrappers, atomic_cond_read_*_timeout() and
atomic64_cond_read_*_timeout() for the cond-load timeout interfaces.

Also add a short description for the atomic_cond_read_{relaxed,acquire}(),
and the atomic_cond_read_{relaxed,acquire}_timeout() interfaces.

Cc: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 Documentation/atomic_t.txt | 14 +++++++++-----
 include/linux/atomic.h     | 10 ++++++++++
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
index bee3b1bca9a7..0e53f6ccb558 100644
--- a/Documentation/atomic_t.txt
+++ b/Documentation/atomic_t.txt
@@ -16,6 +16,10 @@ Non-RMW ops:
   atomic_read(), atomic_set()
   atomic_read_acquire(), atomic_set_release()
 
+Non-RMW, non-atomic_t ops:
+
+  atomic_cond_read_{relaxed,acquire}()
+  atomic_cond_read_{relaxed,acquire}_timeout()
 
 RMW atomic operations:
 
@@ -79,11 +83,11 @@ SEMANTICS
 
 Non-RMW ops:
 
-The non-RMW ops are (typically) regular LOADs and STOREs and are canonically
-implemented using READ_ONCE(), WRITE_ONCE(), smp_load_acquire() and
-smp_store_release() respectively. Therefore, if you find yourself only using
-the Non-RMW operations of atomic_t, you do not in fact need atomic_t at all
-and are doing it wrong.
+The non-RMW ops are (typically) regular, or conditional LOADs and STOREs and
+are canonically implemented using READ_ONCE(), WRITE_ONCE(),
+smp_load_acquire() and smp_store_release() respectively. Therefore, if you
+find yourself only using the Non-RMW operations of atomic_t, you do not in
+fact need atomic_t at all and are doing it wrong.
 
 A note for the implementation of atomic_set{}() is that it must not break the
 atomicity of the RMW ops. That is:
diff --git a/include/linux/atomic.h b/include/linux/atomic.h
index 8dd57c3a99e9..5bcb86e07784 100644
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -31,6 +31,16 @@
 #define atomic64_cond_read_acquire(v, c) smp_cond_load_acquire(&(v)->counter, (c))
 #define atomic64_cond_read_relaxed(v, c) smp_cond_load_relaxed(&(v)->counter, (c))
 
+#define atomic_cond_read_acquire_timeout(v, c, e, t) \
+	smp_cond_load_acquire_timeout(&(v)->counter, (c), (e), (t))
+#define atomic_cond_read_relaxed_timeout(v, c, e, t) \
+	smp_cond_load_relaxed_timeout(&(v)->counter, (c), (e), (t))
+
+#define atomic64_cond_read_acquire_timeout(v, c, e, t) \
+	smp_cond_load_acquire_timeout(&(v)->counter, (c), (e), (t))
+#define atomic64_cond_read_relaxed_timeout(v, c, e, t) \
+	smp_cond_load_relaxed_timeout(&(v)->counter, (c), (e), (t))
+
 /*
  * The idea here is to build acquire/release variants by adding explicit
  * barriers on top of the relaxed variant. In the case where the relaxed
-- 
2.31.1

[PATCH v12 08/15] locking/atomic: scripts: build atomic_long_cond_read_*_timeout()

[Ankur Arora]

Add the atomic long wrappers for the cond-load timeout interfaces.

Cc: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 include/linux/atomic/atomic-long.h | 18 +++++++++++-------
 scripts/atomic/gen-atomic-long.sh  | 16 ++++++++++------
 2 files changed, 21 insertions(+), 13 deletions(-)

diff --git a/include/linux/atomic/atomic-long.h b/include/linux/atomic/atomic-long.h
index 6a4e47d2db35..553b6b0e0258 100644
--- a/include/linux/atomic/atomic-long.h
+++ b/include/linux/atomic/atomic-long.h
@@ -11,14 +11,18 @@
 
 #ifdef CONFIG_64BIT
 typedef atomic64_t atomic_long_t;
-#define ATOMIC_LONG_INIT(i)		ATOMIC64_INIT(i)
-#define atomic_long_cond_read_acquire	atomic64_cond_read_acquire
-#define atomic_long_cond_read_relaxed	atomic64_cond_read_relaxed
+#define ATOMIC_LONG_INIT(i)			ATOMIC64_INIT(i)
+#define atomic_long_cond_read_acquire		atomic64_cond_read_acquire
+#define atomic_long_cond_read_relaxed		atomic64_cond_read_relaxed
+#define atomic_long_cond_read_acquire_timeout	atomic64_cond_read_acquire_timeout
+#define atomic_long_cond_read_relaxed_timeout	atomic64_cond_read_relaxed_timeout
 #else
 typedef atomic_t atomic_long_t;
-#define ATOMIC_LONG_INIT(i)		ATOMIC_INIT(i)
-#define atomic_long_cond_read_acquire	atomic_cond_read_acquire
-#define atomic_long_cond_read_relaxed	atomic_cond_read_relaxed
+#define ATOMIC_LONG_INIT(i)			ATOMIC_INIT(i)
+#define atomic_long_cond_read_acquire		atomic_cond_read_acquire
+#define atomic_long_cond_read_relaxed		atomic_cond_read_relaxed
+#define atomic_long_cond_read_acquire_timeout	atomic_cond_read_acquire_timeout
+#define atomic_long_cond_read_relaxed_timeout	atomic_cond_read_relaxed_timeout
 #endif
 
 /**
@@ -1809,4 +1813,4 @@ raw_atomic_long_dec_if_positive(atomic_long_t *v)
 }
 
 #endif /* _LINUX_ATOMIC_LONG_H */
-// 4b882bf19018602c10816c52f8b4ae280adc887b
+// 79c1f4acb5774376ceed559843d5d9ed1348df99
diff --git a/scripts/atomic/gen-atomic-long.sh b/scripts/atomic/gen-atomic-long.sh
index 9826be3ba986..874643dc74bd 100755
--- a/scripts/atomic/gen-atomic-long.sh
+++ b/scripts/atomic/gen-atomic-long.sh
@@ -79,14 +79,18 @@ cat << EOF
 
 #ifdef CONFIG_64BIT
 typedef atomic64_t atomic_long_t;
-#define ATOMIC_LONG_INIT(i)		ATOMIC64_INIT(i)
-#define atomic_long_cond_read_acquire	atomic64_cond_read_acquire
-#define atomic_long_cond_read_relaxed	atomic64_cond_read_relaxed
+#define ATOMIC_LONG_INIT(i)			ATOMIC64_INIT(i)
+#define atomic_long_cond_read_acquire		atomic64_cond_read_acquire
+#define atomic_long_cond_read_relaxed		atomic64_cond_read_relaxed
+#define atomic_long_cond_read_acquire_timeout	atomic64_cond_read_acquire_timeout
+#define atomic_long_cond_read_relaxed_timeout	atomic64_cond_read_relaxed_timeout
 #else
 typedef atomic_t atomic_long_t;
-#define ATOMIC_LONG_INIT(i)		ATOMIC_INIT(i)
-#define atomic_long_cond_read_acquire	atomic_cond_read_acquire
-#define atomic_long_cond_read_relaxed	atomic_cond_read_relaxed
+#define ATOMIC_LONG_INIT(i)			ATOMIC_INIT(i)
+#define atomic_long_cond_read_acquire		atomic_cond_read_acquire
+#define atomic_long_cond_read_relaxed		atomic_cond_read_relaxed
+#define atomic_long_cond_read_acquire_timeout	atomic_cond_read_acquire_timeout
+#define atomic_long_cond_read_relaxed_timeout	atomic_cond_read_relaxed_timeout
 #endif
 
 EOF
-- 
2.31.1

[PATCH v12 09/15] bpf/rqspinlock: switch check_timeout() to a clock interface

[Ankur Arora]

check_timeout() gets the current time value and depending on how
much time has passed, checks for deadlock or times out, returning 0
or -errno on deadlock or timeout.

Switch this out to a clock style interface, where it functions as a
clock in the "lock-domain", returning the current time until a
deadlock or timeout occurs. Once a deadlock or timeout has occurred,
it stops functioning as a clock and returns error.

Also adjust the RES_CHECK_TIMEOUT macro to discard the clock value
when updating the explicit return status.

Cc: bpf@vger.kernel.org
Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 kernel/bpf/rqspinlock.c | 45 +++++++++++++++++++++++++++--------------
 1 file changed, 30 insertions(+), 15 deletions(-)

diff --git a/kernel/bpf/rqspinlock.c b/kernel/bpf/rqspinlock.c
index e4e338cdb437..0ec17ebb67c1 100644
--- a/kernel/bpf/rqspinlock.c
+++ b/kernel/bpf/rqspinlock.c
@@ -196,8 +196,12 @@ static noinline int check_deadlock_ABBA(rqspinlock_t *lock, u32 mask)
 	return 0;
 }
 
-static noinline int check_timeout(rqspinlock_t *lock, u32 mask,
-				  struct rqspinlock_timeout *ts)
+/*
+ * Returns current monotonic time in ns on success or, negative errno
+ * value on failure due to timeout expiration or detection of deadlock.
+ */
+static noinline s64 clock_deadlock(rqspinlock_t *lock, u32 mask,
+				   struct rqspinlock_timeout *ts)
 {
 	u64 prev = ts->cur;
 	u64 time;
@@ -207,7 +211,7 @@ static noinline int check_timeout(rqspinlock_t *lock, u32 mask,
 			return -EDEADLK;
 		ts->cur = ktime_get_mono_fast_ns();
 		ts->timeout_end = ts->cur + ts->duration;
-		return 0;
+		return (s64)ts->cur;
 	}
 
 	time = ktime_get_mono_fast_ns();
@@ -219,11 +223,15 @@ static noinline int check_timeout(rqspinlock_t *lock, u32 mask,
 	 * checks.
 	 */
 	if (prev + NSEC_PER_MSEC < time) {
+		int ret;
 		ts->cur = time;
-		return check_deadlock_ABBA(lock, mask);
+		ret = check_deadlock_ABBA(lock, mask);
+		if (ret)
+			return ret;
+
 	}
 
-	return 0;
+	return (s64)time;
 }
 
 /*
@@ -231,15 +239,22 @@ static noinline int check_timeout(rqspinlock_t *lock, u32 mask,
  * as the macro does internal amortization for us.
  */
 #ifndef res_smp_cond_load_acquire
-#define RES_CHECK_TIMEOUT(ts, ret, mask)                              \
-	({                                                            \
-		if (!(ts).spin++)                                     \
-			(ret) = check_timeout((lock), (mask), &(ts)); \
-		(ret);                                                \
+#define RES_CHECK_TIMEOUT(ts, ret, mask)					\
+	({									\
+		s64 __timeval_err = 0;						\
+		if (!(ts).spin++)						\
+			__timeval_err = clock_deadlock((lock), (mask), &(ts));	\
+		(ret) = __timeval_err < 0 ? __timeval_err : 0;			\
+		__timeval_err;							\
 	})
 #else
-#define RES_CHECK_TIMEOUT(ts, ret, mask)			      \
-	({ (ret) = check_timeout((lock), (mask), &(ts)); })
+#define RES_CHECK_TIMEOUT(ts, ret, mask)					\
+	({									\
+		s64 __timeval_err;						\
+		__timeval_err = clock_deadlock((lock), (mask), &(ts));		\
+		(ret) = __timeval_err < 0 ? __timeval_err : 0;			\
+		__timeval_err;							\
+	})
 #endif
 
 /*
@@ -281,7 +296,7 @@ int __lockfunc resilient_tas_spin_lock(rqspinlock_t *lock)
 	val = atomic_read(&lock->val);
 
 	if (val || !atomic_try_cmpxchg(&lock->val, &val, 1)) {
-		if (RES_CHECK_TIMEOUT(ts, ret, ~0u))
+		if (RES_CHECK_TIMEOUT(ts, ret, ~0u) < 0)
 			goto out;
 		cpu_relax();
 		goto retry;
@@ -406,7 +421,7 @@ int __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val)
 	 */
 	if (val & _Q_LOCKED_MASK) {
 		RES_RESET_TIMEOUT(ts, RES_DEF_TIMEOUT);
-		res_smp_cond_load_acquire(&lock->locked, !VAL || RES_CHECK_TIMEOUT(ts, ret, _Q_LOCKED_MASK));
+		res_smp_cond_load_acquire(&lock->locked, !VAL || RES_CHECK_TIMEOUT(ts, ret, _Q_LOCKED_MASK) < 0);
 	}
 
 	if (ret) {
@@ -568,7 +583,7 @@ int __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val)
 	 */
 	RES_RESET_TIMEOUT(ts, RES_DEF_TIMEOUT * 2);
 	val = res_atomic_cond_read_acquire(&lock->val, !(VAL & _Q_LOCKED_PENDING_MASK) ||
-					   RES_CHECK_TIMEOUT(ts, ret, _Q_LOCKED_PENDING_MASK));
+					   RES_CHECK_TIMEOUT(ts, ret, _Q_LOCKED_PENDING_MASK) < 0);
 
 	/* Disable queue destruction when we detect deadlocks. */
 	if (ret == -EDEADLK) {
-- 
2.31.1

[PATCH v12 10/15] bpf/rqspinlock: Use smp_cond_load_acquire_timeout()

[Ankur Arora]

Switch out the conditional load interfaces used by rqspinlock
to smp_cond_read_acquire_timeout() and its wrapper,
atomic_cond_read_acquire_timeout().

Both these handle the timeout and amortize as needed, so use the
non-amortized RES_CHECK_TIMEOUT.

RES_CHECK_TIMEOUT does double duty here -- presenting the current
clock value, the timeout/deadlock error from clock_deadlock() to
the cond-load and, returning the error value via ret.

For correctness, we need to ensure that the error case of the
cond-load interface always agrees with that in clock_deadlock().

For the most part, this is fine because there's no independent clock,
or double reads from the clock in cond-load -- either of which could
lead to its internal state going out of sync from that of
clock_deadlock().

There is, however, an edge case where clock_deadlock() checks for:

        if (time > ts->timeout_end)
                return -ETIMEDOUT;

while smp_cond_load_acquire_timeout() checks for:

        __time_now = (time_expr_ns);
        if (__time_now <= 0 || __time_now >= __time_end) {
                VAL = READ_ONCE(*__PTR);
                break;
        }

This runs into a problem when (__time_now == __time_end) since
clock_deadlock() does not treat it as a timeout condition but
the second clause in the conditional above does.
So, add an equality check in clock_deadlock().

Finally, redefine SMP_TIMEOUT_POLL_COUNT to be 16k to be similar to
the spin-count used in the amortized version. We only do this for
non-arm64 as that uses a waiting implementation.

Cc: bpf@vger.kernel.org
Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 kernel/bpf/rqspinlock.c | 40 +++++++++++++++++++++++-----------------
 1 file changed, 23 insertions(+), 17 deletions(-)

diff --git a/kernel/bpf/rqspinlock.c b/kernel/bpf/rqspinlock.c
index 0ec17ebb67c1..e5e27266b813 100644
--- a/kernel/bpf/rqspinlock.c
+++ b/kernel/bpf/rqspinlock.c
@@ -215,7 +215,7 @@ static noinline s64 clock_deadlock(rqspinlock_t *lock, u32 mask,
 	}
 
 	time = ktime_get_mono_fast_ns();
-	if (time > ts->timeout_end)
+	if (time >= ts->timeout_end)
 		return -ETIMEDOUT;
 
 	/*
@@ -235,11 +235,10 @@ static noinline s64 clock_deadlock(rqspinlock_t *lock, u32 mask,
 }
 
 /*
- * Do not amortize with spins when res_smp_cond_load_acquire is defined,
- * as the macro does internal amortization for us.
+ * Spin amortized version of RES_CHECK_TIMEOUT. Used when busy-waiting in
+ * atomic_try_cmpxchg().
  */
-#ifndef res_smp_cond_load_acquire
-#define RES_CHECK_TIMEOUT(ts, ret, mask)					\
+#define RES_CHECK_TIMEOUT_AMORTIZED(ts, ret, mask)				\
 	({									\
 		s64 __timeval_err = 0;						\
 		if (!(ts).spin++)						\
@@ -247,7 +246,7 @@ static noinline s64 clock_deadlock(rqspinlock_t *lock, u32 mask,
 		(ret) = __timeval_err < 0 ? __timeval_err : 0;			\
 		__timeval_err;							\
 	})
-#else
+
 #define RES_CHECK_TIMEOUT(ts, ret, mask)					\
 	({									\
 		s64 __timeval_err;						\
@@ -255,7 +254,6 @@ static noinline s64 clock_deadlock(rqspinlock_t *lock, u32 mask,
 		(ret) = __timeval_err < 0 ? __timeval_err : 0;			\
 		__timeval_err;							\
 	})
-#endif
 
 /*
  * Initialize the 'spin' member.
@@ -269,6 +267,17 @@ static noinline s64 clock_deadlock(rqspinlock_t *lock, u32 mask,
  */
 #define RES_RESET_TIMEOUT(ts, _duration) ({ (ts).timeout_end = 0; (ts).duration = _duration; })
 
+/*
+ * Limit how often we invoke clock_deadlock() while spin-waiting in
+ * smp_cond_load_acquire_timeout() or atomic_cond_read_acquire_timeout().
+ *
+ * We only override the default value not superceding ARM64's override.
+ */
+#ifndef CONFIG_ARM64
+#undef SMP_TIMEOUT_POLL_COUNT
+#define SMP_TIMEOUT_POLL_COUNT	(16*1024)
+#endif
+
 /*
  * Provide a test-and-set fallback for cases when queued spin lock support is
  * absent from the architecture.
@@ -296,7 +305,7 @@ int __lockfunc resilient_tas_spin_lock(rqspinlock_t *lock)
 	val = atomic_read(&lock->val);
 
 	if (val || !atomic_try_cmpxchg(&lock->val, &val, 1)) {
-		if (RES_CHECK_TIMEOUT(ts, ret, ~0u) < 0)
+		if (RES_CHECK_TIMEOUT_AMORTIZED(ts, ret, ~0u) < 0)
 			goto out;
 		cpu_relax();
 		goto retry;
@@ -319,12 +328,6 @@ EXPORT_SYMBOL_GPL(resilient_tas_spin_lock);
  */
 static DEFINE_PER_CPU_ALIGNED(struct qnode, rqnodes[_Q_MAX_NODES]);
 
-#ifndef res_smp_cond_load_acquire
-#define res_smp_cond_load_acquire(v, c) smp_cond_load_acquire(v, c)
-#endif
-
-#define res_atomic_cond_read_acquire(v, c) res_smp_cond_load_acquire(&(v)->counter, (c))
-
 /**
  * resilient_queued_spin_lock_slowpath - acquire the queued spinlock
  * @lock: Pointer to queued spinlock structure
@@ -421,7 +424,9 @@ int __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val)
 	 */
 	if (val & _Q_LOCKED_MASK) {
 		RES_RESET_TIMEOUT(ts, RES_DEF_TIMEOUT);
-		res_smp_cond_load_acquire(&lock->locked, !VAL || RES_CHECK_TIMEOUT(ts, ret, _Q_LOCKED_MASK) < 0);
+		smp_cond_load_acquire_timeout(&lock->locked, !VAL,
+					      RES_CHECK_TIMEOUT(ts, ret, _Q_LOCKED_MASK),
+					      ts.duration);
 	}
 
 	if (ret) {
@@ -582,8 +587,9 @@ int __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val)
 	 * us.
 	 */
 	RES_RESET_TIMEOUT(ts, RES_DEF_TIMEOUT * 2);
-	val = res_atomic_cond_read_acquire(&lock->val, !(VAL & _Q_LOCKED_PENDING_MASK) ||
-					   RES_CHECK_TIMEOUT(ts, ret, _Q_LOCKED_PENDING_MASK) < 0);
+	val = atomic_cond_read_acquire_timeout(&lock->val, !(VAL & _Q_LOCKED_PENDING_MASK),
+					       RES_CHECK_TIMEOUT(ts, ret, _Q_LOCKED_PENDING_MASK),
+					       ts.duration);
 
 	/* Disable queue destruction when we detect deadlocks. */
 	if (ret == -EDEADLK) {
-- 
2.31.1

[PATCH v12 11/15] sched: add need-resched timed wait interface

[Ankur Arora]

Add tif_bitset_relaxed_wait() (and tif_need_resched_relaxed_wait()
which wraps it) which takes the thread_info bit and timeout duration
as parameters and waits until the bit is set or for the expiration
of the timeout.

The wait is implemented via smp_cond_load_relaxed_timeout().

smp_cond_load_relaxed_timeout() essentially provides the pattern used
in poll_idle() where we spin in a loop waiting for the flag to change
until a timeout occurs.

tif_need_resched_relaxed_wait() allows us to abstract out the internals
of waiting, scheduler specific details etc.

Placed in linux/sched/idle.h instead of linux/thread_info.h to work
around recursive include hell.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rafael J. Wysocki <rafael@kernel.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: linux-pm@vger.kernel.org
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 include/linux/sched/idle.h | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/include/linux/sched/idle.h b/include/linux/sched/idle.h
index 8465ff1f20d1..ddee9b019895 100644
--- a/include/linux/sched/idle.h
+++ b/include/linux/sched/idle.h
@@ -3,6 +3,7 @@
 #define _LINUX_SCHED_IDLE_H
 
 #include <linux/sched.h>
+#include <linux/sched/clock.h>
 
 enum cpu_idle_type {
 	__CPU_NOT_IDLE = 0,
@@ -113,4 +114,32 @@ static __always_inline void current_clr_polling(void)
 }
 #endif
 
+/*
+ * Caller needs to make sure that the thread context cannot be preempted
+ * or migrated, so current_thread_info() cannot change from under us.
+ *
+ * This also allows us to safely stay in the local_clock domain.
+ */
+static __always_inline bool tif_bitset_relaxed_wait(int tif, u64 timeout_ns)
+{
+	unsigned long flags;
+
+	flags = smp_cond_load_relaxed_timeout(&current_thread_info()->flags,
+					      (VAL & BIT(tif)),
+					      local_clock_noinstr(),
+					      timeout_ns);
+	return flags & BIT(tif);
+}
+
+/**
+ * tif_need_resched_relaxed_wait() - Wait for need-resched being set
+ * with no ordering guarantees until a timeout expires.
+ *
+ * @timeout_ns: timeout value.
+ */
+static __always_inline bool tif_need_resched_relaxed_wait(u64 timeout_ns)
+{
+	return tif_bitset_relaxed_wait(TIF_NEED_RESCHED, timeout_ns);
+}
+
 #endif /* _LINUX_SCHED_IDLE_H */
-- 
2.31.1

[PATCH v12 12/15] cpuidle/poll_state: Wait for need-resched via tif_need_resched_relaxed_wait()

[Ankur Arora]

The inner loop in poll_idle() polls over the thread_info flags,
waiting to see if the thread has TIF_NEED_RESCHED set. The loop
exits once the condition is met, or if the poll time limit has
been exceeded.

To minimize the number of instructions executed in each iteration,
the time check is rate-limited. In addition, each loop iteration
executes cpu_relax() which on certain platforms provides a hint to
the pipeline that the loop busy-waits, allowing the processor to
reduce power consumption.

Switch over to tif_need_resched_relaxed_wait() instead, since that
provides exactly that.

However, since we want to minimize power consumption in idle, building
of cpuidle/poll_state.c continues to depend on CONFIG_ARCH_HAS_CPU_RELAX
as that serves as an indicator that the platform supports an optimized
version of tif_need_resched_relaxed_wait() (via
smp_cond_load_acquire_timeout()).

Cc: Rafael J. Wysocki <rafael@kernel.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: linux-pm@vger.kernel.org
Suggested-by: Rafael J. Wysocki <rafael@kernel.org>
Acked-by: Rafael J. Wysocki (Intel) <rafael@kernel.org>
Tested-by: Haris Okanovic <harisokn@amazon.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---

Notes:

Sashiko notes [1] that lazy initialization of the timeout deadline will
cause an overshoot of the wakeup deadline: this was discussed earlier 
and shouldn't be a big concern [2]. Cpuidle ranges aren't meant to be
precise and in any case we are only waiting to go into a deeper idle
state.

[1] https://sashiko.dev/#/patchset/20260408122538.3610871-1-ankur.a.arora%40oracle.com
[2] https://lore.kernel.org/lkml/CAJZ5v0izSBR0_DeH5HVnSLFGRfV9WoSzbu9Mh5yvvuyrvw7fLg@mail.gmail.com/
---
 drivers/cpuidle/poll_state.c | 21 +--------------------
 1 file changed, 1 insertion(+), 20 deletions(-)

diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
index c7524e4c522a..7443b3e971ba 100644
--- a/drivers/cpuidle/poll_state.c
+++ b/drivers/cpuidle/poll_state.c
@@ -6,41 +6,22 @@
 #include <linux/cpuidle.h>
 #include <linux/export.h>
 #include <linux/irqflags.h>
-#include <linux/sched.h>
-#include <linux/sched/clock.h>
 #include <linux/sched/idle.h>
 #include <linux/sprintf.h>
 #include <linux/types.h>
 
-#define POLL_IDLE_RELAX_COUNT	200
-
 static int __cpuidle poll_idle(struct cpuidle_device *dev,
 			       struct cpuidle_driver *drv, int index)
 {
-	u64 time_start;
-
-	time_start = local_clock_noinstr();
-
 	dev->poll_time_limit = false;
 
 	raw_local_irq_enable();
 	if (!current_set_polling_and_test()) {
-		unsigned int loop_count = 0;
 		u64 limit;
 
 		limit = cpuidle_poll_time(drv, dev);
 
-		while (!need_resched()) {
-			cpu_relax();
-			if (loop_count++ < POLL_IDLE_RELAX_COUNT)
-				continue;
-
-			loop_count = 0;
-			if (local_clock_noinstr() - time_start > limit) {
-				dev->poll_time_limit = true;
-				break;
-			}
-		}
+		dev->poll_time_limit = !tif_need_resched_relaxed_wait(limit);
 	}
 	raw_local_irq_disable();
 
-- 
2.31.1

[PATCH v12 13/15] arm64/delay: enable testing smp_cond_load_relaxed_timeout()

[Ankur Arora]

This enables the barrier tests to be built as a module.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 arch/arm64/lib/delay.c               | 2 ++
 drivers/clocksource/arm_arch_timer.c | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/arch/arm64/lib/delay.c b/arch/arm64/lib/delay.c
index c660a7ea26dd..dfb102ce3009 100644
--- a/arch/arm64/lib/delay.c
+++ b/arch/arm64/lib/delay.c
@@ -12,6 +12,7 @@
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/timex.h>
+#include <kunit/visibility.h>
 #include <asm/delay-const.h>
 
 #include <clocksource/arm_arch_timer.h>
@@ -30,6 +31,7 @@ u64 notrace __delay_cycles(void)
 	guard(preempt_notrace)();
 	return __arch_counter_get_cntvct_stable();
 }
+EXPORT_SYMBOL_IF_KUNIT(__delay_cycles);
 
 void __delay(unsigned long cycles)
 {
diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
index 90aeff44a276..1de63e1a2cd2 100644
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c
@@ -28,6 +28,7 @@
 #include <linux/acpi.h>
 #include <linux/arm-smccc.h>
 #include <linux/ptp_kvm.h>
+#include <kunit/visibility.h>
 
 #include <asm/arch_timer.h>
 #include <asm/virt.h>
@@ -896,6 +897,7 @@ bool arch_timer_evtstrm_available(void)
 	 */
 	return cpumask_test_cpu(raw_smp_processor_id(), &evtstrm_available);
 }
+EXPORT_SYMBOL_IF_KUNIT(arch_timer_evtstrm_available);
 
 static struct arch_timer_kvm_info arch_timer_kvm_info;
 
-- 
2.31.1

[PATCH v12 14/15] barrier: add tests for smp_cond_load_*_timeout()

[Ankur Arora]

Add success and failure case tests for smp_cond_load_*_timeout().

Success or failure cases depend on the expected bit being set (or not).
Additionally in failure cases smp_cond_load_*_timeout() cannot return
before timeout.

Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---

Note: This fixes an error in the test case reported by Mark Brown
in https://lore.kernel.org/lkml/agr_RxvNtfASfevg@sirena.org.uk/.

There are three changes:

  - One of the test conditions used in the test was much too strict.
    The test was treating:
      success => (runtime <= timeout_ns).

    Instead, it makes greater sense to treat:
      !success => (runtime >= timeout_ns).

  - The test can run in a wide variety of environments including
    emulated qemu. To get rid of potential failures due to timing issues,
    remove the kthreaded case.

  - Parametrize the test cases.
---
 lib/Kconfig.debug                |  10 +++
 lib/tests/Makefile               |   1 +
 lib/tests/barrier-timeout-test.c | 128 +++++++++++++++++++++++++++++++
 3 files changed, 139 insertions(+)
 create mode 100644 lib/tests/barrier-timeout-test.c

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 8ff5adcfe1e0..ad5131776f68 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2406,6 +2406,16 @@ config FPROBE_SANITY_TEST
 
 	  Say N if you are unsure.
 
+config BARRIER_TIMEOUT_TEST
+	tristate "KUnit tests for smp_cond_load_relaxed_timeout()"
+	depends on KUNIT
+	default KUNIT_ALL_TESTS
+	help
+	  Builds KUnit tests that validate wake-up and timeout handling paths
+	  in smp_cond_load_relaxed_timeout().
+
+	  Say N if you are unsure.
+
 config BACKTRACE_SELF_TEST
 	tristate "Self test for the backtrace code"
 	depends on DEBUG_KERNEL
diff --git a/lib/tests/Makefile b/lib/tests/Makefile
index 7e9c2fa52e35..19c1d6b17856 100644
--- a/lib/tests/Makefile
+++ b/lib/tests/Makefile
@@ -20,6 +20,7 @@ CFLAGS_fortify_kunit.o += $(DISABLE_STRUCTLEAK_PLUGIN)
 obj-$(CONFIG_FORTIFY_KUNIT_TEST) += fortify_kunit.o
 CFLAGS_test_fprobe.o += $(CC_FLAGS_FTRACE)
 obj-$(CONFIG_FPROBE_SANITY_TEST) += test_fprobe.o
+obj-$(CONFIG_BARRIER_TIMEOUT_TEST) += barrier-timeout-test.o
 obj-$(CONFIG_GLOB_KUNIT_TEST) += glob_kunit.o
 obj-$(CONFIG_HASHTABLE_KUNIT_TEST) += hashtable_test.o
 obj-$(CONFIG_HASH_KUNIT_TEST) += test_hash.o
diff --git a/lib/tests/barrier-timeout-test.c b/lib/tests/barrier-timeout-test.c
new file mode 100644
index 000000000000..2160844b27b8
--- /dev/null
+++ b/lib/tests/barrier-timeout-test.c
@@ -0,0 +1,128 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KUnit tests exercising smp_cond_load_relaxed_timeout().
+ *
+ * Copyright (c) 2026, Oracle Corp.
+ * Author: Ankur Arora <ankur.a.arora@oracle.com>
+ */
+
+#include <linux/bitops.h>
+#include <linux/types.h>
+#include <linux/sched/clock.h>
+#include <linux/delay.h>
+#include <asm/barrier.h>
+#include <kunit/test.h>
+#include <kunit/visibility.h>
+
+MODULE_IMPORT_NS("EXPORTED_FOR_KUNIT_TESTING");
+
+struct clock_state {
+	s64	start_time;
+	s64	end_time;
+};
+
+#define TIMEOUT_MSEC	2
+#define TEST_FLAG_VAL	BIT(2)
+static unsigned int flag;
+
+static s64 basic_clock(struct clock_state *clk)
+{
+	clk->end_time = local_clock();
+	return clk->end_time;
+}
+
+static void update_flags(void)
+{
+	WRITE_ONCE(flag, TEST_FLAG_VAL);
+}
+
+static s64 mocked_clock(struct clock_state *clk)
+{
+	s64 clk_mid = clk->start_time + (TIMEOUT_MSEC * NSEC_PER_MSEC)/2;
+
+	clk->end_time = local_clock();
+	if (clk->end_time >= clk_mid)
+		update_flags();
+	return clk->end_time;
+}
+
+typedef s64 (*clkfn_t)(struct clock_state *);
+struct smp_cond_update_params {
+	clkfn_t	clock;
+	bool	acquire;
+	bool	succeeds;
+};
+
+static const struct smp_cond_update_params update_params_list[] = {
+	/* mocked-clock updates flag inline. */
+	{ .clock = &mocked_clock, .succeeds = true, .acquire = false, },
+	{ .clock = &mocked_clock, .succeeds = true, .acquire = true,  },
+
+	/* basic-clock doesn't update flag. */
+	{ .clock = &basic_clock, .succeeds = false,  .acquire = true, },
+	{ .clock = &basic_clock, .succeeds = false,  .acquire = false, },
+};
+
+static void param_to_desc(const struct smp_cond_update_params *p, char *desc)
+{
+	char *clk, *update;
+
+	if (p->clock == &mocked_clock) {
+		clk = "mocked";
+		update = "inline";
+	} else if (p->clock == &basic_clock) {
+		clk = "basic";
+		update = "none";
+	}
+
+
+	snprintf(desc, KUNIT_PARAM_DESC_SIZE, "smp_cond_%s_timeout: clock-%s, update=%s",
+		p->acquire ? "acquire" : "relaxed", clk, update);
+}
+
+KUNIT_ARRAY_PARAM(smp_cond_update_params, update_params_list, param_to_desc);
+
+
+static void test_smp_cond_timeout(struct kunit *test)
+{
+	const struct smp_cond_update_params *p = test->param_value;
+	struct clock_state clk = {
+		.start_time = local_clock(),
+		.end_time = local_clock(),
+	};
+	s64 runtime, timeout_ns = TIMEOUT_MSEC * NSEC_PER_MSEC;
+	unsigned int result;
+
+	flag = 0;
+	if (p->acquire) {
+		result = smp_cond_load_acquire_timeout(&flag,
+						       (VAL & TEST_FLAG_VAL),
+						       p->clock(&clk),
+						       timeout_ns);
+	} else {
+		result = smp_cond_load_relaxed_timeout(&flag,
+						       (VAL & TEST_FLAG_VAL),
+						       p->clock(&clk),
+						       timeout_ns);
+	}
+
+	runtime = clk.end_time - clk.start_time;
+	KUNIT_EXPECT_EQ(test, (bool)(result & TEST_FLAG_VAL), p->succeeds);
+	if (!p->succeeds)
+		KUNIT_EXPECT_GE(test, runtime, timeout_ns);
+}
+
+static struct kunit_case barrier_timeout_test_cases[] = {
+	KUNIT_CASE_PARAM(test_smp_cond_timeout, smp_cond_update_params_gen_params),
+	{}
+};
+
+static struct kunit_suite barrier_timeout_test_suite = {
+	.name = "smp-cond-load-*-timeout",
+	.test_cases = barrier_timeout_test_cases,
+};
+
+kunit_test_suite(barrier_timeout_test_suite);
+
+MODULE_DESCRIPTION("KUnit tests for smp_cond_load_relaxed_timeout()");
+MODULE_LICENSE("GPL");
-- 
2.31.1

[PATCH v12 15/15] barrier: add clock tests for smp_cond_load_relaxed_timeout()

[Ankur Arora]

Add a few clock tests for smp_cond_load_relaxed_timeout(). These
ensure that the implementation doesn't do anything funny stuff with the
clock (like multiple accesses per iteration.)

Also ensure that we handle edge cases sanely. Note that two edge cases
fail: S64_MAX and U64_MAX. However, both of those are quite far out
and if needed, can be addressed in the implementation of the interface.

Also, this tests only smp_cond_load_relaxed_timeout(). The acquire
variant uses an identical clock path and testing wouldn't add anything.

Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 lib/tests/barrier-timeout-test.c | 57 ++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/lib/tests/barrier-timeout-test.c b/lib/tests/barrier-timeout-test.c
index 2160844b27b8..ec9dc0aa65d1 100644
--- a/lib/tests/barrier-timeout-test.c
+++ b/lib/tests/barrier-timeout-test.c
@@ -19,6 +19,8 @@ MODULE_IMPORT_NS("EXPORTED_FOR_KUNIT_TESTING");
 struct clock_state {
 	s64	start_time;
 	s64	end_time;
+	s64	extra;
+	u32	niters;
 };
 
 #define TIMEOUT_MSEC	2
@@ -112,8 +114,63 @@ static void test_smp_cond_timeout(struct kunit *test)
 		KUNIT_EXPECT_GE(test, runtime, timeout_ns);
 }
 
+static s64 synthetic_clock(struct clock_state *clk)
+{
+	clk->end_time += clk->extra;
+	clk->niters++;
+
+	return clk->end_time;
+}
+
+
+struct smp_cond_expiry_params {
+	char	*desc;
+	s64	timeout;
+	s64	clk_inc;
+	u32	niters;
+};
+
+static const struct smp_cond_expiry_params expiry_params_list[] = {
+	{ .clk_inc = (0x1ULL << 28), .timeout = -1LL,		.niters = 1,			.desc = "-1LL",    },
+	{ .clk_inc = (0x1ULL << 28), .timeout = (0x1ULL << 30), .niters = 1 + (1 << (30-28)),	.desc = "1<<30",   },
+	{ .clk_inc = (0x1ULL << 28), .timeout = S32_MAX,	.niters = 1 + (1 << (31-28)),	.desc = "S32_MAX", },
+	{ .clk_inc = (0x1ULL << 28), .timeout = U32_MAX,	.niters = 1 + (1 << (32-28)),	.desc = "U32_MAX", },
+	{ .clk_inc = (0x1ULL << 28), .timeout = (0x1ULL << 33), .niters = 1 + (1 << (33-28)),	.desc = "1<<33",   },
+};
+
+static void expiry_param_to_desc(const struct smp_cond_expiry_params *p, char *desc)
+{
+	snprintf(desc, KUNIT_PARAM_DESC_SIZE, "smp_cond_%s_timeout: clock-%s, timeout=%s, iterations=%u",
+		"relaxed", "synthetic", p->desc, p->niters);
+}
+
+static void test_smp_cond_expiry(struct kunit *test)
+{
+	const struct smp_cond_expiry_params *p = test->param_value;
+	struct clock_state clk = {
+		.start_time = 0,
+		.end_time = 0,
+		.extra = p->clk_inc,
+		.niters = 0,
+	};
+	s64 runtime;
+
+	flag = 0;
+	smp_cond_load_relaxed_timeout(&flag,
+				      0,
+				      synthetic_clock(&clk),
+				      p->timeout);
+
+	runtime = (u64)clk.end_time - (u64)clk.start_time;
+	KUNIT_EXPECT_EQ(test, clk.niters, p->niters);
+	KUNIT_EXPECT_GE(test, runtime, p->timeout);
+}
+
+
+KUNIT_ARRAY_PARAM(smp_cond_expiry_params, expiry_params_list, expiry_param_to_desc);
 static struct kunit_case barrier_timeout_test_cases[] = {
 	KUNIT_CASE_PARAM(test_smp_cond_timeout, smp_cond_update_params_gen_params),
+	KUNIT_CASE_PARAM(test_smp_cond_expiry, smp_cond_expiry_params_gen_params),
 	{}
 };
 
-- 
2.31.1

Re: [PATCH v12 01/15] asm-generic: barrier: Add smp_cond_load_relaxed_timeout()

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 2681 bytes --]

> diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
> index d4f581c1e21d..c56df9513a08 100644
> --- a/include/asm-generic/barrier.h
> +++ b/include/asm-generic/barrier.h
> @@ -273,6 +273,75 @@ do {									\

[ ... ]

> +#ifndef smp_cond_load_relaxed_timeout
> +#define smp_cond_load_relaxed_timeout(ptr, cond_expr,			\
> +				      time_expr_ns, timeout_ns)		\
> +({									\
> +	typeof(ptr) __PTR = (ptr);					\
> +	__unqual_scalar_typeof(*(ptr)) VAL;				\
> +	u32 __count = 0, __spin = SMP_TIMEOUT_POLL_COUNT;		\
> +	s64 __timeout = (s64)timeout_ns;				\
> +	s64 __time_now, __time_end = 0;					\
> +									\
> +	for (;;) {							\
> +		VAL = READ_ONCE(*__PTR);				\
> +		if (cond_expr)						\
> +			break;						\
> +		cpu_poll_relax(__PTR, VAL, (u64)__timeout);		\

__timeout is held as a signed s64 but is cast to u64 here.  On the first
iterations __timeout still holds the raw timeout_ns argument before any
time-check has run.  If a caller passes a negative or already-expired
timeout_ns, the (u64) cast turns it into a value close to U64_MAX.

This is harmless while cpu_poll_relax() ignores its timeout argument and
falls back to cpu_relax().  Would it misbehave on an architecture whose
cpu_poll_relax() consumes the value, such as an arm64 WFET variant that
uses it as an event-stream deadline?

David Laight raised this on v11
(https://lore.kernel.org/lkml/20260508093206.389d9af2@pumpkin/):

  Yes, the code is ok if cpu_poll_relax() ignores the timeout. But for
  WFE it is all broken. ... if you assume/require that the converted
  time is either 0 or greater than 200 then it can be used as the
  initialiser for __n.

The Sashiko bot repeated the same concern as a medium finding on v12.
The v12 cover note opts to document the valid range rather than clamp at
runtime, but the code still passes (u64)__timeout unguarded, so would it
be worth converting timeout_ns into an absolute end-time (or 0 when WFE
is unused) before it reaches cpu_poll_relax()?

> +		if (++__count < __spin)					\
> +			continue;					\
> +		__time_now = (s64)(time_expr_ns);			\
> +		if (unlikely(__time_end == 0))				\
> +			__time_end = __time_now + __timeout;		\
> +		__timeout = __time_end - __time_now;			\
> +		if (__time_now <= 0 || __timeout <= 0) {		\
> +			VAL = READ_ONCE(*__PTR);			\
> +			break;						\
> +		}							\
> +		__count = 0;						\
> +	}								\
> +	(typeof(*(ptr)))VAL;						\
> +})
> +#endif


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Re: [PATCH v12 05/15] arm64: rqspinlock: Remove private copy of smp_cond_load_acquire_timewait()

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 3202 bytes --]

> diff --git a/arch/arm64/include/asm/rqspinlock.h b/arch/arm64/include/asm/rqspinlock.h
> index 9ea0a74e5892..a385603436e9 100644
> --- a/arch/arm64/include/asm/rqspinlock.h
> +++ b/arch/arm64/include/asm/rqspinlock.h
> @@ -3,91 +3,6 @@
>  #define _ASM_RQSPINLOCK_H
>
>  #include <asm/barrier.h>
> -
> -/*
> - * Hardcode res_smp_cond_load_acquire implementations for arm64 to a custom
> - * version based on [0]. In rqspinlock code, our conditional expression involves
> - * checking the value _and_ additionally a timeout. However, on arm64, the
> - * WFE-based implementation may never spin again if no stores occur to the
> - * locked byte in the lock word. As such, we may be stuck forever if
> - * event-stream based unblocking is not available on the platform for WFE spin
> - * loops (arch_timer_evtstrm_available).
> - *

[ ... ]

> -#endif
> -
> -#define res_smp_cond_load_acquire(v, c) smp_cond_load_acquire_timewait(v, c, 0, 1)
> -
>  #include <asm-generic/rqspinlock.h>

With res_smp_cond_load_acquire removed here, does arm64 now fall back to
the generic definition in kernel/bpf/rqspinlock.c?

  #ifndef res_smp_cond_load_acquire
  #define res_smp_cond_load_acquire(v, c) smp_cond_load_acquire(v, c)
  #endif

On arm64, smp_cond_load_acquire() uses __cmpwait_relaxed(), which issues a
WFE that wakes only on a store to the monitored cache line or on an
event-stream tick.

The consumer is resilient_queued_spin_lock_slowpath():

  kernel/bpf/rqspinlock.c:
      res_smp_cond_load_acquire(&lock->locked,
          !VAL || RES_CHECK_TIMEOUT(ts, ret, _Q_LOCKED_MASK));

The RES_CHECK_TIMEOUT() escape is part of cond_expr, so it is only
re-evaluated when smp_cond_load_acquire() returns from its WFE.

On platforms where arch_timer_evtstrm_available() is false, if the current
lock owner is stuck or dead and issues no further store to lock->locked,
can the WFE ever wake, and can the RES_CHECK_TIMEOUT timeout ever fire?

This looks like the exact case the removed comment described:

> - * checking the value _and_ additionally a timeout. However, on arm64, the
> - * WFE-based implementation may never spin again if no stores occur to the
> - * locked byte in the lock word. As such, we may be stuck forever if
> - * event-stream based unblocking is not available on the platform for WFE spin
> - * loops (arch_timer_evtstrm_available).

The removed private copy avoided this by falling back to a cpu_relax()-based
spinwait that always re-checks the timeout when the event stream is
unavailable.

Forward search of the series shows this is resolved by the later commit
7abb03c21db2 ("bpf/rqspinlock: Use smp_cond_load_acquire_timeout()"), which
switches rqspinlock to smp_cond_load_acquire_timeout(); on arm64 that uses
a waiting implementation handling the timeout and event-stream-absent case.
Should the change-over happen in the same commit, or is the window where a
kernel built or bisected at this commit can block indefinitely acceptable?


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Re: [PATCH v12 02/15] arm64: barrier: Support smp_cond_load_relaxed_timeout()

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 1930 bytes --]

> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index fe60738e5943..fa676428ec3f 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1606,6 +1606,9 @@ config ARCH_SUPPORTS_CRASH_DUMP
>  config ARCH_DEFAULT_CRASH_DUMP
>  	def_bool y
>
> +config ARCH_HAS_CPU_RELAX
> +	def_bool y
> +

ARCH_HAS_CPU_RELAX isn't private to the barrier code.  Before this it was
defined only by arch/x86, and the generic kernel already keys cpuidle
behavior off it in two places that arm64 builds.

In drivers/cpuidle/Makefile:

  obj-$(CONFIG_ARCH_HAS_CPU_RELAX)	  += poll_state.o

and in drivers/acpi/processor_idle.c:

  #define ACPI_IDLE_STATE_START	(IS_ENABLED(CONFIG_ARCH_HAS_CPU_RELAX) ? 1 : 0)
  ...
  if (IS_ENABLED(CONFIG_ARCH_HAS_CPU_RELAX)) {
          cpuidle_poll_state_init(drv);
          count = 1;
  }

On arm64, ACPI_PROCESSOR is "default y" and depends on ARM64, and it
selects ACPI_PROCESSOR_IDLE which selects CPU_IDLE.  So on an ACPI arm64
system this now builds poll_state.o and makes acpi_processor_setup_cstates()
register a POLL idle state as C-state index 0, shifting the firmware
C-states up by one.

The commit message describes this symbol only as indicating "an optimized
implementation of cpu_poll_relax()", and the generic patch describes it as
"a cpu_poll_relax() implementation that is expected to be cheaper".  Neither
mentions the cpuidle/ACPI poll_state side effect.

Is enabling the cpuidle polling idle state on arm64 intended here, or is
ARCH_HAS_CPU_RELAX only meant to feed smp_cond_load_relaxed_timeout()?  If
the latter, would a dedicated symbol (or decoupling poll_state/ACPI from
ARCH_HAS_CPU_RELAX) avoid the unintended idle-path change?


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Re: [PATCH v12 10/15] bpf/rqspinlock: Use smp_cond_load_acquire_timeout()

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 3229 bytes --]

> bpf/rqspinlock: Use smp_cond_load_acquire_timeout()
>
> Switch out the conditional load interfaces used by rqspinlock
> to smp_cond_read_acquire_timeout() and its wrapper,
> atomic_cond_read_acquire_timeout().
>
> Both these handle the timeout and amortize as needed, so use the
> non-amortized RES_CHECK_TIMEOUT.
>
> RES_CHECK_TIMEOUT does double duty here -- presenting the current
> clock value, the timeout/deadlock error from clock_deadlock() to
> the cond-load and, returning the error value via ret.
>
> For correctness, we need to ensure that the error case of the
> cond-load interface always agrees with that in clock_deadlock().
>
> For the most part, this is fine because there's no independent clock,
> or double reads from the clock in cond-load -- either of which could
> lead to its internal state going out of sync from that of
> clock_deadlock().
>
> There is, however, an edge case where clock_deadlock() checks for:
>
>         if (time > ts->timeout_end)
>                 return -ETIMEDOUT;
>
> while smp_cond_load_acquire_timeout() checks for:
>
>         __time_now = (time_expr_ns);
>         if (__time_now <= 0 || __time_now >= __time_end) {
>                 VAL = READ_ONCE(*__PTR);
>                 break;
>         }
>
> This runs into a problem when (__time_now == __time_end) since
> clock_deadlock() does not treat it as a timeout condition but
> the second clause in the conditional above does.
> So, add an equality check in clock_deadlock().
>
> Finally, redefine SMP_TIMEOUT_POLL_COUNT to be 16k to be similar to
> the spin-count used in the amortized version. We only do this for
> non-arm64 as that uses a waiting implementation.
>
> Cc: bpf@vger.kernel.org
> Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>

Besides switching to the new cond-load interface, this commit also
changes the boundary comparison in clock_deadlock() from:

        if (time > ts->timeout_end)
                return -ETIMEDOUT;

to:

        if (time >= ts->timeout_end)
                return -ETIMEDOUT;

so that time == timeout_end is now treated as a timeout, matching the
__time_now >= __time_end check in smp_cond_load_acquire_timeout().

The time > ts->timeout_end check, and the timeout_end field itself, were
first introduced in the then-named check_timeout() helper by:

  commit 14c48ee81452 ("rqspinlock: Add support for timeouts")

Should this commit carry a Fixes: tag for that boundary change?

  Fixes: 14c48ee81452 ("rqspinlock: Add support for timeouts")

This is offered tentatively: under the previous res_smp_cond_load_acquire
interface the '>' comparison was internally consistent, and the boundary
divergence only becomes observable once this commit switches to the
'__time_now >= __time_end' interface. So this may read more as a
self-contained adjustment than a fix for a pre-existing bug.


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Re: [PATCH v12 02/15] arm64: barrier: Support smp_cond_load_relaxed_timeout()

[Ankur Arora]

bot+bpf-ci@kernel.org writes:

>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index fe60738e5943..fa676428ec3f 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -1606,6 +1606,9 @@ config ARCH_SUPPORTS_CRASH_DUMP
>>  config ARCH_DEFAULT_CRASH_DUMP
>>  	def_bool y
>>
>> +config ARCH_HAS_CPU_RELAX
>> +	def_bool y
>> +
>
> ARCH_HAS_CPU_RELAX isn't private to the barrier code.  Before this it was
> defined only by arch/x86, and the generic kernel already keys cpuidle
> behavior off it in two places that arm64 builds.
>
> In drivers/cpuidle/Makefile:
>
>   obj-$(CONFIG_ARCH_HAS_CPU_RELAX)	  += poll_state.o
>
> and in drivers/acpi/processor_idle.c:
>
>   #define ACPI_IDLE_STATE_START	(IS_ENABLED(CONFIG_ARCH_HAS_CPU_RELAX) ? 1 : 0)
>   ...
>   if (IS_ENABLED(CONFIG_ARCH_HAS_CPU_RELAX)) {
>           cpuidle_poll_state_init(drv);
>           count = 1;
>   }
>
> On arm64, ACPI_PROCESSOR is "default y" and depends on ARM64, and it
> selects ACPI_PROCESSOR_IDLE which selects CPU_IDLE.  So on an ACPI arm64
> system this now builds poll_state.o and makes acpi_processor_setup_cstates()
> register a POLL idle state as C-state index 0, shifting the firmware
> C-states up by one.

Yes, this is a bug. The additional POLL idle state change wasn't intended
to be registered as part of this patch.

With this change the first c-state will end up executing a NOP poll_idle()
since current_set_polling_and_test() is tif_need_resched() when
TIF_POLLING_NRFLAG is not defined but this does unnecessarily register
the additional c-state.

I missed that in my testing because my test machine has LPI states which
need an explicit change to enable this.
(https://lore.kernel.org/lkml/20250218213337.377987-7-ankur.a.arora@oracle.com/)

Ankur


> The commit message describes this symbol only as indicating "an optimized
> implementation of cpu_poll_relax()", and the generic patch describes it as
> "a cpu_poll_relax() implementation that is expected to be cheaper".  Neither
> mentions the cpuidle/ACPI poll_state side effect.
>
>
> Is enabling the cpuidle polling idle state on arm64 intended here, or is
> ARCH_HAS_CPU_RELAX only meant to feed smp_cond_load_relaxed_timeout()?  If
> the latter, would a dedicated symbol (or decoupling poll_state/ACPI from
> ARCH_HAS_CPU_RELAX) avoid the unintended idle-path change?

>
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
>
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/27125050324


--
ankur

Re: [PATCH v12 00/15] barrier: Add smp_cond_load_{relaxed,acquire}_timeout()

[Ankur Arora]

Summarizing all of the bot reviews (sashiko/bpf-bot):

Most of the comments are same as v11. Let me outline the ones I think
are notable:

  - edge cases around (timeout is -1, S64_MAX, U64_MAX).

    I've noted in the first patch how these cases are probably best
    addressed at review time instead of complicating the implementation
    like in https://lore.kernel.org/lkml/874iklm1uy.fsf@oracle.com/

  - as a side-effect of enabling ARCH_HAS_CPU_RELAX, acpi_processor_setup_cstate()
    enables a NOP poll_idle() unintentionally (patch-2). I've described
    it in more detail in my reply to that patch.

    Will fix this.

  - potentially missed control dependency in the timeout case of
    smp_cond_load_acquire_timeout(). Probably need a better fix for
    this than I have.

    Need more thinktime as the bots would say.  Will address this one
    and the one below in reply to patches 6, 7.

  - possibly torn reads with atomic64_cond_read_*_timeout() on 32-bit
    architectures.

Ankur

Ankur Arora <ankur.a.arora@oracle.com> writes:

> Hi,
>
> Main change in this version:
>
>   - addressed some review comments from sashiko (see commit notes)
>     - The one notable change is to the implementation of
>       smp_cond_load_acquire_timeout() where there was a missed
>       control dependency in the timeout case.
>       All the others are minor.
>   - fixed a low probability race in the kunit test added in v11.
>   - added a bunch of kunit tests validating the implementation's
>     use of the clock.
>
> Andrew, if the changes look okay, could we take this in your mm-nomm
> tree as before?
>
> The core kernel often uses smp_cond_load_{relaxed,acquire}() to spin
> on condition variables with architectural primitives used to avoid
> hammering the relevant cachelines.
>
> (This primitive can vary greatly across architectures: on x86 it's a
> cpu_relax() to slow down the pipeline. On arm64, this is a __cmpwait()
> which waits for a cacheline to change state in a time limited fashion.)
>
> Regardless of architectural details, typical smp_cond_load*() usage
> does not allow for termination until the condition change occurs.
>
> Beyond the core kernel, there are cases where it is useful to additionally
> terminate on a timeout. Two cases:
>
>   - cpuidle poll_idle(): wait for need-resched until the cpuidle polling
>     duration expires.
>
>   - rqspinlock: nested qspinlock acquisition that terminates on timeout
>     or deadlock.
>
> Accordingly add two interfaces (with their generic and arm64 specific
> implementations):
>
>    smp_cond_load_relaxed_timeout(ptr, cond_expr, time_expr, timeout)
>    smp_cond_load_acquire_timeout(ptr, cond_expr, time_expr, timeout)
>
> Also add tif_need_resched_relaxed_wait() which wraps the polling
> pattern and its scheduler specific details in poll_idle().
> In addition add atomic_cond_read_*_timeout(),
> atomic64_cond_read_*_timeout(), and atomic_long wrappers.
>
> Structurally, both the smp_cond_load_*_timeout() interfaces are similar
> to smp_cond_load*(), with the addition of a rate-limited time-check.
>
> Usage
> ==
>
> These interfaces drop straight-forwardly into the rqspinlock logic
> since qspinlock already uses smp_cond_load*(), and the time-check
> extension can now be used for timeout and deadlock handling.
>
> Using tif_need_resched_relaxed_wait() in poll_idle() removes any
> architectural details allowing arm64 to straight-forwardly support
> that path.
> (However, for efficiency reasons cpuidle/poll_state.c continues to
> depend on ARCH_HAS_CPU_RELAX since that is defined on architectures
> with an optimized architectural primitive.)
>
>
> Performance
> ==
>
> Apart from simplifications due to this change, supporting polling in
> cpuidle on arm64 helps improve wakeup latency (needs a few cpuidle/acpi
> patches):
>
>
>   # perf stat -r 5 --cpu 4,5 -e task-clock,cycles,instructions,sched:sched_wake_idle_without_ipi \
>   perf bench sched pipe -l 1000000 -c 4
>
>   # No haltpoll (and, no TIF_POLLING_NRFLAG):
>
>   Performance counter stats for 'CPU(s) 4,5' (5 runs):
>
>          25,229.57 msec task-clock                       #    2.000 CPUs utilized               ( +-  7.75% )
>     45,821,250,284      cycles                           #    1.816 GHz                         ( +- 10.07% )
>     26,557,496,665      instructions                     #    0.58  insn per cycle              ( +-  0.21% )
>                  0      sched:sched_wake_idle_without_ipi #    0.000 /sec
>
>        12.615 +- 0.977 seconds time elapsed  ( +-  7.75% )
>
>
>   # Haltpoll:
>
>   Performance counter stats for 'CPU(s) 4,5' (5 runs):
>
>          15,131.58 msec task-clock                       #    2.000 CPUs utilized               ( +- 10.00% )
>     34,158,188,839      cycles                           #    2.257 GHz                         ( +-  6.91% )
>     20,824,950,916      instructions                     #    0.61  insn per cycle              ( +-  0.09% )
>          1,983,822      sched:sched_wake_idle_without_ipi #  131.105 K/sec                       ( +-  0.78% )
>
>         7.566 +- 0.756 seconds time elapsed  ( +- 10.00% )
>
>   We get improved latency because we don't switch in and out of a
>   deeper sleep state or from the hypervisor. This also causes us to
>   execute ~20% fewer instructions.
>
>
> Haris Okanovic also saw improvement in real workloads due to the
> cpuidle changes: "observed 4-6% improvements in memcahed, cassandra,
> mysql, and postgresql under certain loads. Other applications likely
> benefit too." [12]
>
>
> Changelog:
>   v11 [13] (as listed above):
>     - addressed some review comments from sashiko (see commit notes)
>       - The one notable change is to the implementation of
>         smp_cond_load_acquire_timeout() where there was a missed
>         control dependency in the timeout case.
>       All the others are minor.
>     - fixed a low probability race in the kunit test added in v11.
>     - added a bunch of kunit tests validating the implementation's
>       use of the clock.
>
>   v10 [10]:
>    - add a comment mentioning that smp_cond_load_relaxed_timeout() might
>      be using architectural primitives that don't support MMIO.
>      (David Laight, Catalin Marinas)
>    - added a kunit test for smp_cond_load_relaxed_timeout() (Andrew
>      Morton.)
>
>   v9 [9]:
>    - s/@cond/@cond_expr/ (Randy Dunlap)
>    - Clarify that SMP_TIMEOUT_POLL_COUNT is only around memory
>      addresses. (David Laight)
>    - Add the missing config ARCH_HAS_CPU_RELAX in arch/arm64/Kconfig.
>      (Catalin Marinas).
>    - Switch to arch_counter_get_cntvct_stable() (via __delay_cycles())
>      in the cmpwait path instead of using arch_timer_read_counter().
>      (Catalin Marinas)
>
>   v8 [0]:
>    - Defer evaluation of @time_expr_ns to when we hit the slowpath.
>       (comment from Alexei Starovoitov).
>
>    - Mention that cpu_poll_relax() is better than raw CPU polling
>      only where ARCH_HAS_CPU_RELAX is defined.
>      - also define ARCH_HAS_CPU_RELAX for arm64.
>       (Came out of a discussion with Will Deacon.)
>
>    - Split out WFET and WFE handling. I was doing both of these
>      in a common handler.
>      (From Will Deacon and in an earlier revision by Catalin Marinas.)
>
>    - Add mentions of atomic_cond_read_{relaxed,acquire}(),
>      atomic_cond_read_{relaxed,acquire}_timeout() in
>      Documentation/atomic_t.txt.
>
>    - Use the BIT() macro to do the checking in tif_bitset_relaxed_wait().
>
>    - Cleanup unnecessary assignments, casts etc in poll_idle().
>      (From Rafael Wysocki.)
>
>    - Fixup warnings from kernel build robot
>
>
>   v7 [1]:
>    - change the interface to separately provide the timeout. This is
>      useful for supporting WFET and similar primitives which can do
>      timed waiting (suggested by Arnd Bergmann).
>
>    - Adapting rqspinlock code to this changed interface also
>      necessitated allowing time_expr to fail.
>    - rqspinlock changes to adapt to the new smp_cond_load_acquire_timeout().
>
>    - add WFET support (suggested by Arnd Bergmann).
>    - add support for atomic-long wrappers.
>    - add a new scheduler interface tif_need_resched_relaxed_wait() which
>      encapsulates the polling logic used by poll_idle().
>      - interface suggested by (Rafael J. Wysocki).
>
>
>   v6 [2]:
>    - fixup missing timeout parameters in atomic64_cond_read_*_timeout()
>    - remove a race between setting of TIF_NEED_RESCHED and the call to
>      smp_cond_load_relaxed_timeout(). This would mean that dev->poll_time_limit
>      would be set even if we hadn't spent any time waiting.
>      (The original check compared against local_clock(), which would have been
>      fine, but I was instead using a cheaper check against _TIF_NEED_RESCHED.)
>    (Both from meta-CI bot)
>
>
>   v5 [3]:
>    - use cpu_poll_relax() instead of cpu_relax().
>    - instead of defining an arm64 specific
>      smp_cond_load_relaxed_timeout(), just define the appropriate
>      cpu_poll_relax().
>    - re-read the target pointer when we exit due to the time-check.
>    - s/SMP_TIMEOUT_SPIN_COUNT/SMP_TIMEOUT_POLL_COUNT/
>    (Suggested by Will Deacon)
>
>    - add atomic_cond_read_*_timeout() and atomic64_cond_read_*_timeout()
>      interfaces.
>    - rqspinlock: use atomic_cond_read_acquire_timeout().
>    - cpuidle: use smp_cond_load_relaxed_tiemout() for polling.
>    (Suggested by Catalin Marinas)
>
>    - rqspinlock: define SMP_TIMEOUT_POLL_COUNT to be 16k for non arm64
>
>
>   v4 [4]:
>     - naming change 's/timewait/timeout/'
>     - resilient spinlocks: get rid of res_smp_cond_load_acquire_waiting()
>       and fixup use of RES_CHECK_TIMEOUT().
>     (Both suggested by Catalin Marinas)
>
>   v3 [5]:
>     - further interface simplifications (suggested by Catalin Marinas)
>
>   v2 [6]:
>     - simplified the interface (suggested by Catalin Marinas)
>        - get rid of wait_policy, and a multitude of constants
>        - adds a slack parameter
>       This helped remove a fair amount of duplicated code duplication and in
>       hindsight unnecessary constants.
>
>   v1 [7]:
>      - add wait_policy (coarse and fine)
>      - derive spin-count etc at runtime instead of using arbitrary
>        constants.
>
> Haris Okanovic tested v4 of this series with poll_idle()/haltpoll patches. [8]
>
> Comments appreciated!
>
> Thanks
> Ankur
>
>  [0] https://lore.kernel.org/lkml/20251215044919.460086-1-ankur.a.arora@oracle.com/
>  [1] https://lore.kernel.org/lkml/20251028053136.692462-1-ankur.a.arora@oracle.com/
>  [2] https://lore.kernel.org/lkml/20250911034655.3916002-1-ankur.a.arora@oracle.com/
>  [3] https://lore.kernel.org/lkml/20250911034655.3916002-1-ankur.a.arora@oracle.com/
>  [4] https://lore.kernel.org/lkml/20250829080735.3598416-1-ankur.a.arora@oracle.com/
>  [5] https://lore.kernel.org/lkml/20250627044805.945491-1-ankur.a.arora@oracle.com/
>  [6] https://lore.kernel.org/lkml/20250502085223.1316925-1-ankur.a.arora@oracle.com/
>  [7] https://lore.kernel.org/lkml/20250203214911.898276-1-ankur.a.arora@oracle.com/
>  [8] https://lore.kernel.org/lkml/2cecbf7fb23ee83a4ce027e1be3f46f97efd585c.camel@amazon.com/
>  [9] https://lore.kernel.org/lkml/20260209023153.2661784-1-ankur.a.arora@oracle.com/
>  [10] https://lore.kernel.org/lkml/20260316013651.3225328-1-ankur.a.arora@oracle.com/
>  [11] https://lore.kernel.org/lkml/20230809134837.GM212435@hirez.programming.kicks-ass.net/
>  [12] https://lore.kernel.org/lkml/c6f3c8d3f1f2e89a9dc7ae22482973b5a51b08cb.camel@amazon.com/
>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Will Deacon <will@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
> Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: bpf@vger.kernel.org
> Cc: linux-arch@vger.kernel.org
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-pm@vger.kernel.org
>
> Ankur Arora (15):
>   asm-generic: barrier: Add smp_cond_load_relaxed_timeout()
>   arm64: barrier: Support smp_cond_load_relaxed_timeout()
>   arm64/delay: move some constants out to a separate header
>   arm64: support WFET in smp_cond_load_relaxed_timeout()
>   arm64: rqspinlock: Remove private copy of
>     smp_cond_load_acquire_timewait()
>   asm-generic: barrier: Add smp_cond_load_acquire_timeout()
>   atomic: Add atomic_cond_read_*_timeout()
>   locking/atomic: scripts: build atomic_long_cond_read_*_timeout()
>   bpf/rqspinlock: switch check_timeout() to a clock interface
>   bpf/rqspinlock: Use smp_cond_load_acquire_timeout()
>   sched: add need-resched timed wait interface
>   cpuidle/poll_state: Wait for need-resched via
>     tif_need_resched_relaxed_wait()
>   arm64/delay: enable testing smp_cond_load_relaxed_timeout()
>   barrier: add tests for smp_cond_load_*_timeout()
>   barrier: add clock tests for smp_cond_load_relaxed_timeout()
>
>  Documentation/atomic_t.txt           |  14 +-
>  arch/arm64/Kconfig                   |   3 +
>  arch/arm64/include/asm/barrier.h     |  23 ++++
>  arch/arm64/include/asm/cmpxchg.h     |  62 +++++++--
>  arch/arm64/include/asm/delay-const.h |  28 ++++
>  arch/arm64/include/asm/rqspinlock.h  |  85 ------------
>  arch/arm64/lib/delay.c               |  17 +--
>  drivers/clocksource/arm_arch_timer.c |   2 +
>  drivers/cpuidle/poll_state.c         |  21 +--
>  drivers/soc/qcom/rpmh-rsc.c          |   8 +-
>  include/asm-generic/barrier.h        |  97 ++++++++++++++
>  include/linux/atomic.h               |  10 ++
>  include/linux/atomic/atomic-long.h   |  18 ++-
>  include/linux/sched/idle.h           |  29 +++++
>  kernel/bpf/rqspinlock.c              |  77 +++++++----
>  lib/Kconfig.debug                    |  10 ++
>  lib/tests/Makefile                   |   1 +
>  lib/tests/barrier-timeout-test.c     | 185 +++++++++++++++++++++++++++
>  scripts/atomic/gen-atomic-long.sh    |  16 ++-
>  19 files changed, 528 insertions(+), 178 deletions(-)
>  create mode 100644 arch/arm64/include/asm/delay-const.h
>  create mode 100644 lib/tests/barrier-timeout-test.c


--
ankur