[RFC][PATCH 8/9] sched/clock: Provide local_clock_noinstr()

Peter Zijlstra
2023-05-08T21:35:55Z

Now that all ARCH_WANTS_NO_INSTR architectures (arm64, loongarch,
s390, x86) provide sched_clock_noinstr(), use this to provide
local_clock_noinstr().

This local_clock_noinstr() will be safe to use from noinstr code with
the assumption that any such noinstr code is non-preemptible (it had
better be, entry code will have IRQs disabled while __cpuidle must
have preemption disabled).

Specifically, preempt_enable_notrace(), a common part of many a
sched_clock() implementation calls out to schedule() -- even though,
per the above, it will never trigger -- which frustrates noinstr
validation.

vmlinux.o: warning: objtool: local_clock+0xb5: call to preempt_schedule_notrace_thunk() leaves .noinstr.text section

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/sched/clock.h | 17 ++++++++++++++++-
kernel/sched/clock.c | 19 +++++++++++++------
2 files changed, 29 insertions(+), 7 deletions(-)

--- a/include/linux/sched/clock.h
+++ b/include/linux/sched/clock.h
@@ -12,7 +12,16 @@
*
* Please use one of the three interfaces below.
*/
-extern unsigned long long notrace sched_clock(void);
+extern u64 sched_clock(void);
+
+#if defined(CONFIG_ARCH_WANTS_NO_INSTR) || defined(CONFIG_GENERIC_SCHED_CLOCK)
+extern u64 sched_clock_noinstr(void);
+#else
+__always_inline u64 sched_clock_noinstr(void)
+{
+ return sched_clock();
+}
+#endif

/*
* See the comment in kernel/sched/clock.c
@@ -45,6 +54,11 @@ static inline u64 cpu_clock(int cpu)
return sched_clock();
}

+static __always_inline u64 local_clock_noinstr(void)
+{
+ return sched_clock_noinstr();
+}
+
static __always_inline u64 local_clock(void)
{
return sched_clock();
@@ -79,6 +93,7 @@ static inline u64 cpu_clock(int cpu)
return sched_clock_cpu(cpu);
}

+extern u64 local_clock_noinstr(void);
extern u64 local_clock(void);

#endif
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -266,7 +266,7 @@ static __always_inline u64 sched_clock_l
s64 delta;

again:
- now = sched_clock();
+ now = sched_clock_noinstr();
delta = now - scd->tick_raw;
if (unlikely(delta < 0))
delta = 0;
@@ -293,22 +293,29 @@ static __always_inline u64 sched_clock_l
return clock;
}

-noinstr u64 local_clock(void)
+noinstr u64 local_clock_noinstr(void)
{
u64 clock;

if (static_branch_likely(&__sched_clock_stable))
- return sched_clock() + __sched_clock_offset;
+ return sched_clock_noinstr() + __sched_clock_offset;

if (!static_branch_likely(&sched_clock_running))
- return sched_clock();
+ return sched_clock_noinstr();

- preempt_disable_notrace();
clock = sched_clock_local(this_scd());
- preempt_enable_notrace();

return clock;
}
+
+u64 local_clock(void)
+{
+ u64 now;
+ preempt_disable_notrace();
+ now = local_clock_noinstr();
+ preempt_enable_notrace();
+ return now;
+}
EXPORT_SYMBOL_GPL(local_clock);

static notrace u64 sched_clock_remote(struct sched_clock_data *scd)