[PATCH] x86/tsc: Always save/restore TSC sched_clock on suspend/resume

["Guilherme G. Piccoli" <gpiccoli@igalia.com>]

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 happens 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 on 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.

Cc: stable@vger.kernel.org
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
---
 arch/x86/kernel/tsc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 34dec0b72ea8..88e5a4ed9db3 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -959,7 +959,7 @@ static unsigned long long cyc2ns_suspend;
 
 void tsc_save_sched_clock_state(void)
 {
-	if (!sched_clock_stable())
+	if (!static_branch_likely(&__use_tsc) && !sched_clock_stable())
 		return;
 
 	cyc2ns_suspend = sched_clock();
@@ -979,7 +979,7 @@ void tsc_restore_sched_clock_state(void)
 	unsigned long flags;
 	int cpu;
 
-	if (!sched_clock_stable())
+	if (!static_branch_likely(&__use_tsc) && !sched_clock_stable())
 		return;
 
 	local_irq_save(flags);
-- 
2.47.1