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Guilherme G. Piccoli 39e32fe65e x86/tsc: Always save/restore TSC sched_clock() on suspend/resume 
commit d90c9de9de upstream.

TSC could be reset in deep ACPI sleep states, even with invariant TSC.

That's the reason we have sched_clock() save/restore functions, to deal
with this situation. But what happens is that such functions are guarded
with a check for the stability of sched_clock - if not considered stable,
the save/restore routines aren't executed.

On top of that, we have a clear comment in native_sched_clock() saying
that *even* with TSC unstable, we continue using TSC for sched_clock due
to its speed.

In other words, if we have a situation of TSC getting detected as unstable,
it marks the sched_clock as unstable as well, so subsequent S3 sleep cycles
could bring bogus sched_clock values due to the lack of the save/restore
mechanism, causing warnings like this:

  [22.954918] ------------[ cut here ]------------
  [22.954923] Delta way too big! 18446743750843854390 ts=18446744072977390405 before=322133536015 after=322133536015 write stamp=18446744072977390405
  [22.954923] If you just came from a suspend/resume,
  [22.954923] please switch to the trace global clock:
  [22.954923]   echo global > /sys/kernel/tracing/trace_clock
  [22.954923] or add trace_clock=global to the kernel command line
  [22.954937] WARNING: CPU: 2 PID: 5728 at kernel/trace/ring_buffer.c:2890 rb_add_timestamp+0x193/0x1c0

Notice that the above was reproduced even with "trace_clock=global".

The fix for that is to _always_ save/restore the sched_clock on suspend
cycle _if TSC is used_ as sched_clock - only if we fallback to jiffies
the sched_clock_stable() check becomes relevant to save/restore the
sched_clock.

Debugged-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: stable@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250215210314.351480-1-gpiccoli@igalia.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2025-04-10 14:33:42 +02:00
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README 

Linux kernel
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