linux-kernel - Re: [PATCH] x86/tsc: Always save/restore TSC sched

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Message-ID: <3bcf3eac-555c-6f30-8e32-b8c39967eefe@igalia.com>
Date: Mon, 17 Feb 2025 15:49:13 -0300
From: "Guilherme G. Piccoli" <gpiccoli@...lia.com>
To: linux-kernel@...r.kernel.org, x86@...nel.org
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, hpa@...or.com, rostedt@...dmis.org,
 kernel-dev@...lia.com, kernel@...ccoli.net, stable@...r.kernel.org,
 Thadeu Lima de Souza Cascardo <cascardo@...lia.com>
Subject: Re: [PATCH] x86/tsc: Always save/restore TSC sched_clock on
 suspend/resume

On 15/02/2025 17:58, Guilherme G. Piccoli wrote:
> 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.
> 

Hi folks, I would like to ask if possible to add the following tag:

Debugged-by: Thadeu Lima de Souza Cascardo <cascardo@...lia.com>

Cascardo helped me a lot on debugging this issue but I forgot to add it
earlier, so nothing more fair than add it now!

Thanks,


Guilherme

> Cc: stable@...r.kernel.org
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@...lia.com>
> [...]