linux-kernel - Re: [PATCH v2] clocksource: Defer marking clocksources unstable to kthread

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Message-ID: <CALOAHbDhKzaj93PcQReEGk-omw7s5xXVj_X=+by1r6c0G4aV3A@mail.gmail.com>
Date: Sun, 9 Mar 2025 19:36:05 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: paulmck@...nel.org, linux-kernel@...r.kernel.org, jstultz@...gle.com, 
	sboyd@...nel.org, christian@...sel.eu, kernel-team@...a.com, 
	Peter Zijlstra <peterz@...radead.org>, Michal Koutný <mkoutny@...e.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH v2] clocksource: Defer marking clocksources unstable to kthread

On Sun, Mar 9, 2025 at 12:38 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Thu, Mar 06 2025 at 08:06, Paul E. McKenney wrote:
> > The clocksource watchdog marks clocksources unstable from within a timer
> > handler.  On x86, this marking involves an on_each_cpu_cond_mask(),
> > which in turn invokes smp_call_function_many_cond(), which may not be
> > invoked from a timer handler.  Doing so results in:
> >
> > WARNING: CPU: 3 PID: 0 at kernel/smp.c:815 smp_call_function_many_cond+0x46b/0x4c0
> >
> > Fix this by deferring the marking to the clocksource watchdog kthread.
> > Note that marking unstable is already deferred, so deferring it a bit
> > more should be just fine.
>
> While this can be done, that's papering over the underlying problem,
> which was introduced with:
>
>   8722903cbb8f ("sched: Define sched_clock_irqtime as static key")
>
> That added the static key switch, which is causing the problem. And
> "fixing" this in the clocksource watchdog is incomplete because the same
> problem exists during CPU hotplug when the TSC synchronization declares
> the TSC unstable. It's the exactly same problem as was fixed via:
>
>  6577e42a3e16 ("sched/clock: Fix up clear_sched_clock_stable()")
>
> So as this got introduced in the 6.14 merge window, the proper fix is to
> revert commit 8722903cbb8f and send it back to the drawing board. It was
> clearly never tested with the various possibilities which invoke
> mark_tsc*_unstable().

Hello Thomas,

It has been reverted by the following commit
b9f2b29b9494 ("sched: Don't define sched_clock_irqtime as static key")

https://web.git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?id=b9f2b29b94943b08157e3dfc970baabc7944dbc3

-- 
Regards
Yafang