[PATCH v4 24/30] context-tracking: Introduce work deferral infrastructure

Valentin Schneider
2025-01-14T18:03:03Z

smp_call_function() & friends have the unfortunate habit of sending IPIs to
isolated, NOHZ_FULL, in-userspace CPUs, as they blindly target all online
CPUs.

Some callsites can be bent into doing the right, such as done by commit:

cc9e303c91f5 ("x86/cpu: Disable frequency requests via aperfmperf IPI for nohz_full CPUs")

Unfortunately, not all SMP callbacks can be omitted in this
fashion. However, some of them only affect execution in kernelspace, which
means they don't have to be executed *immediately* if the target CPU is in
userspace: stashing the callback and executing it upon the next kernel entry
would suffice. x86 kernel instruction patching or kernel TLB invalidation
are prime examples of it.

Reduce the RCU dynticks counter width to free up some bits to be used as a
deferred callback bitmask. Add some build-time checks to validate that
setup.

Presence of CT_STATE_KERNEL in the ct_state prevents queuing deferred work.

Later commits introduce the bit:callback mappings.

Link: https://lore.kernel.org/all/20210929151723.162004989@infradead.org/
Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
arch/Kconfig | 9 +++
arch/x86/Kconfig | 1 +
arch/x86/include/asm/context_tracking_work.h | 16 +++++
include/linux/context_tracking.h | 21 +++++++
include/linux/context_tracking_state.h | 30 ++++++---
include/linux/context_tracking_work.h | 26 ++++++++
kernel/context_tracking.c | 66 +++++++++++++++++++-
kernel/time/Kconfig | 5 ++
8 files changed, 162 insertions(+), 12 deletions(-)
create mode 100644 arch/x86/include/asm/context_tracking_work.h
create mode 100644 include/linux/context_tracking_work.h

diff --git a/arch/Kconfig b/arch/Kconfig
index 6682b2a53e342..b637f20f0fc68 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -952,6 +952,15 @@ config HAVE_CONTEXT_TRACKING_USER_OFFSTACK
- No use of instrumentation, unless instrumentation_begin() got
called.

+config HAVE_CONTEXT_TRACKING_WORK
+ bool
+ help
+ Architecture supports deferring work while not in kernel context.
+ This is especially useful on setups with isolated CPUs that might
+ want to avoid being interrupted to perform housekeeping tasks (for
+ ex. TLB invalidation or icache invalidation). The housekeeping
+ operations are performed upon re-entering the kernel.
+
config HAVE_TIF_NOHZ
bool
help
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 9d7bd0ae48c42..ca5dd4cfc354b 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -216,6 +216,7 @@ config X86
select HAVE_CMPXCHG_LOCAL
select HAVE_CONTEXT_TRACKING_USER if X86_64
select HAVE_CONTEXT_TRACKING_USER_OFFSTACK if HAVE_CONTEXT_TRACKING_USER
+ select HAVE_CONTEXT_TRACKING_WORK if X86_64
select HAVE_C_RECORDMCOUNT
select HAVE_OBJTOOL_MCOUNT if HAVE_OBJTOOL
select HAVE_OBJTOOL_NOP_MCOUNT if HAVE_OBJTOOL_MCOUNT
diff --git a/arch/x86/include/asm/context_tracking_work.h b/arch/x86/include/asm/context_tracking_work.h
new file mode 100644
index 0000000000000..5f3b2d0977235
--- /dev/null
+++ b/arch/x86/include/asm/context_tracking_work.h
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_CONTEXT_TRACKING_WORK_H
+#define _ASM_X86_CONTEXT_TRACKING_WORK_H
+
+static __always_inline void arch_context_tracking_work(enum ct_work work)
+{
+ switch (work) {
+ case CT_WORK_n:
+ // Do work...
+ break;
+ case CT_WORK_MAX:
+ WARN_ON_ONCE(true);
+ }
+}
+
+#endif
diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index af9fe87a09225..0b0faa040e9b5 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -5,6 +5,7 @@
#include <linux/sched.h>
#include <linux/vtime.h>
#include <linux/context_tracking_state.h>
+#include <linux/context_tracking_work.h>
#include <linux/instrumentation.h>

#include <asm/ptrace.h>
@@ -137,6 +138,26 @@ static __always_inline unsigned long ct_state_inc(int incby)
return raw_atomic_add_return(incby, this_cpu_ptr(&context_tracking.state));
}

+#ifdef CONFIG_CONTEXT_TRACKING_WORK
+static __always_inline unsigned long ct_state_inc_clear_work(int incby)
+{
+ struct context_tracking *ct = this_cpu_ptr(&context_tracking);
+ unsigned long new, old, state;
+
+ state = arch_atomic_read(&ct->state);
+ do {
+ old = state;
+ new = old & ~CT_WORK_MASK;
+ new += incby;
+ state = arch_atomic_cmpxchg(&ct->state, old, new);
+ } while (old != state);
+
+ return new;
+}
+#else
+#define ct_state_inc_clear_work(x) ct_state_inc(x)
+#endif
+
static __always_inline bool warn_rcu_enter(void)
{
bool ret = false;
diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
index eb2149b20baef..b6ddfccba9714 100644
--- a/include/linux/context_tracking_state.h
+++ b/include/linux/context_tracking_state.h
@@ -5,6 +5,7 @@
#include <linux/percpu.h>
#include <linux/static_key.h>
#include <linux/context_tracking_irq.h>
+#include <linux/context_tracking_work.h>

/* Offset to allow distinguishing irq vs. task-based idle entry/exit. */
#define CT_NESTING_IRQ_NONIDLE ((LONG_MAX / 2) + 1)
@@ -39,16 +40,19 @@ struct context_tracking {
};

/*
- * We cram two different things within the same atomic variable:
+ * We cram up to three different things within the same atomic variable:
*
- * CT_RCU_WATCHING_START CT_STATE_START
- * | |
- * v v
- * MSB [ RCU watching counter ][ context_state ] LSB
- * ^ ^
- * | |
- * CT_RCU_WATCHING_END CT_STATE_END
+ * CT_RCU_WATCHING_START CT_STATE_START
+ * | CT_WORK_START |
+ * | | |
+ * v v v
+ * MSB [ RCU watching counter ][ context work ][ context_state ] LSB
+ * ^ ^ ^
+ * | | |
+ * | CT_WORK_END |
+ * CT_RCU_WATCHING_END CT_STATE_END
*
+ * The [ context work ] region spans 0 bits if CONFIG_CONTEXT_WORK=n
* Bits are used from the LSB upwards, so unused bits (if any) will always be in
* upper bits of the variable.
*/
@@ -59,18 +63,24 @@ struct context_tracking {
#define CT_STATE_START 0
#define CT_STATE_END (CT_STATE_START + CT_STATE_WIDTH - 1)

-#define CT_RCU_WATCHING_MAX_WIDTH (CT_SIZE - CT_STATE_WIDTH)
+#define CT_WORK_WIDTH (IS_ENABLED(CONFIG_CONTEXT_TRACKING_WORK) ? CT_WORK_MAX_OFFSET : 0)
+#define CT_WORK_START (CT_STATE_END + 1)
+#define CT_WORK_END (CT_WORK_START + CT_WORK_WIDTH - 1)
+
+#define CT_RCU_WATCHING_MAX_WIDTH (CT_SIZE - CT_WORK_WIDTH - CT_STATE_WIDTH)
#define CT_RCU_WATCHING_WIDTH (IS_ENABLED(CONFIG_RCU_DYNTICKS_TORTURE) ? 2 : CT_RCU_WATCHING_MAX_WIDTH)
-#define CT_RCU_WATCHING_START (CT_STATE_END + 1)
+#define CT_RCU_WATCHING_START (CT_WORK_END + 1)
#define CT_RCU_WATCHING_END (CT_RCU_WATCHING_START + CT_RCU_WATCHING_WIDTH - 1)
#define CT_RCU_WATCHING BIT(CT_RCU_WATCHING_START)

#define CT_STATE_MASK GENMASK(CT_STATE_END, CT_STATE_START)
+#define CT_WORK_MASK GENMASK(CT_WORK_END, CT_WORK_START)
#define CT_RCU_WATCHING_MASK GENMASK(CT_RCU_WATCHING_END, CT_RCU_WATCHING_START)

#define CT_UNUSED_WIDTH (CT_RCU_WATCHING_MAX_WIDTH - CT_RCU_WATCHING_WIDTH)

static_assert(CT_STATE_WIDTH +
+ CT_WORK_WIDTH +
CT_RCU_WATCHING_WIDTH +
CT_UNUSED_WIDTH ==
CT_SIZE);
diff --git a/include/linux/context_tracking_work.h b/include/linux/context_tracking_work.h
new file mode 100644
index 0000000000000..c68245f8d77c5
--- /dev/null
+++ b/include/linux/context_tracking_work.h
@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_CONTEXT_TRACKING_WORK_H
+#define _LINUX_CONTEXT_TRACKING_WORK_H
+
+#include <linux/bitops.h>
+
+enum {
+ CT_WORK_n_OFFSET,
+ CT_WORK_MAX_OFFSET
+};
+
+enum ct_work {
+ CT_WORK_n = BIT(CT_WORK_n_OFFSET),
+ CT_WORK_MAX = BIT(CT_WORK_MAX_OFFSET)
+};
+
+#include <asm/context_tracking_work.h>
+
+#ifdef CONFIG_CONTEXT_TRACKING_WORK
+extern bool ct_set_cpu_work(unsigned int cpu, enum ct_work work);
+#else
+static inline bool
+ct_set_cpu_work(unsigned int cpu, unsigned int work) { return false; }
+#endif
+
+#endif
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index 15f10ddec8cbe..f7019c12269f9 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -72,6 +72,67 @@ static __always_inline void rcu_task_trace_heavyweight_exit(void)
#endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
}

+#ifdef CONFIG_CONTEXT_TRACKING_WORK
+static noinstr void ct_work_flush(unsigned long seq)
+{
+ int bit;
+
+ seq = (seq & CT_WORK_MASK) >> CT_WORK_START;
+
+ /*
+ * arch_context_tracking_work() must be noinstr, non-blocking,
+ * and NMI safe.
+ */
+ for_each_set_bit(bit, &seq, CT_WORK_MAX)
+ arch_context_tracking_work(BIT(bit));
+}
+
+/**
+ * ct_set_cpu_work - set work to be run at next kernel context entry
+ *
+ * If @cpu is not currently executing in kernelspace, it will execute the
+ * callback mapped to @work (see arch_context_tracking_work()) at its next
+ * transition to CT_KERNEL_STATE.
+ *
+ * If it is already in CT_KERNEL_STATE, this will be a no-op.
+ */
+bool ct_set_cpu_work(unsigned int cpu, enum ct_work work)
+{
+ struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
+ unsigned int old;
+ bool ret = false;
+
+ if (!ct->active)
+ return false;
+
+ preempt_disable();
+
+ old = atomic_read(&ct->state);
+
+ /*
+ * We only want to set the work bit if the target CPU is not in
+ * kernelspace, so we clear the KERNEL bit here and let the cmpxchg do
+ * the check for us - the state could change between the atomic_read() and
+ * the cmpxchg().
+ */
+ old &= ~CT_STATE_KERNEL;
+ /*
+ * Try setting the work until either
+ * - the target CPU has entered kernelspace
+ * - the work has been set
+ */
+ do {
+ ret = atomic_try_cmpxchg(&ct->state, &old, old | (work << CT_WORK_START));
+ } while (!ret && ((old & CT_STATE_MASK) != CT_STATE_KERNEL));
+
+ preempt_enable();
+ return ret;
+}
+#else
+static __always_inline void ct_work_flush(unsigned long work) { }
+static __always_inline void ct_work_clear(struct context_tracking *ct) { }
+#endif
+
/*
* Record entry into an extended quiescent state. This is only to be
* called when not already in an extended quiescent state, that is,
@@ -88,7 +149,7 @@ static noinstr void ct_kernel_exit_state(int offset)
* next idle sojourn.
*/
rcu_task_trace_heavyweight_enter(); // Before CT state update!
- seq = ct_state_inc(offset);
+ seq = ct_state_inc_clear_work(offset);
// RCU is no longer watching. Better be in extended quiescent state!
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && (seq & CT_RCU_WATCHING));
}
@@ -100,7 +161,7 @@ static noinstr void ct_kernel_exit_state(int offset)
*/
static noinstr void ct_kernel_enter_state(int offset)
{
- int seq;
+ unsigned long seq;

/*
* CPUs seeing atomic_add_return() must see prior idle sojourns,
@@ -108,6 +169,7 @@ static noinstr void ct_kernel_enter_state(int offset)
* critical section.
*/
seq = ct_state_inc(offset);
+ ct_work_flush(seq);
// RCU is now watching. Better not be in an extended quiescent state!
rcu_task_trace_heavyweight_exit(); // After CT state update!
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !(seq & CT_RCU_WATCHING));
diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index b0b97a60aaa6f..7e8106a0d981f 100644
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -181,6 +181,11 @@ config CONTEXT_TRACKING_USER_FORCE
Say N otherwise, this option brings an overhead that you
don't want in production.

+config CONTEXT_TRACKING_WORK
+ bool
+ depends on HAVE_CONTEXT_TRACKING_WORK && CONTEXT_TRACKING_USER
+ default y
+
config NO_HZ
bool "Old Idle dynticks config"
help
--
2.43.0