linux-kernel - Re: [PATCH v7] sched/clock: Avoid false sharing for sched_clock

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Message-ID: <7c6236a5-82b7-4d1b-9bb2-3f0d2ac186f8@linux.ibm.com>
Date: Tue, 27 Jan 2026 16:34:38 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Wangyang Guo <wangyang.guo@...el.com>,
        K Prateek Nayak <kprateek.nayak@....com>
Cc: linux-kernel@...r.kernel.org, Benjamin Lei <benjamin.lei@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Tianyou Li <tianyou.li@...el.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH v7] sched/clock: Avoid false sharing for
 sched_clock_irqtime

Hi Wangyang, Prateek.

On 1/27/26 12:55 PM, Wangyang Guo wrote:
> Read-mostly sched_clock_irqtime may share the same cacheline with
> frequently updated nohz struct. Make it as static_key to avoid
> false sharing issue.
> 
> The only user of disable_sched_clock_irqtime()
> is tsc_.*mark_unstable() which may be invoked under atomic context
> and require a workqueue to disable static_key. But both of them
> calls clear_sched_clock_stable() just before doing
> disable_sched_clock_irqtime(). We can reuse
> "sched_clock_work" to also disable sched_clock_irqtime().
> 
> One additional case need to handle is if the tsc is marked unstable
> before late_initcall() phase, sched_clock_work will not be invoked
> and sched_clock_irqtime will stay enabled although clock is unstable:
>    tsc_init()
>      enable_sched_clock_irqtime() # irqtime accounting is enabled here
>      ...
>      if (unsynchronized_tsc()) # true
>        mark_tsc_unstable()
>          clear_sched_clock_stable()
>            __sched_clock_stable_early = 0;
>            ...
>            if (static_key_count(&sched_clock_running.key) == 2)
>              # Only happens at sched_clock_init_late()
>              __clear_sched_clock_stable(); # Never executed
>    ...
> 
>    # late_initcall() phase
>    sched_clock_init_late()
>      if (__sched_clock_stable_early) # Already false
>        __set_sched_clock_stable(); # sched_clock is never marked stable
>    # TSC unstable, but sched_clock_work won't run to disable irqtime
> 
> So we need to disable_sched_clock_irqtime() in sched_clock_init_late()
> if clock is unstable.
> 

Do you this as a valid case? have you tested with CONFIG_PARAVIRT?

Lets say you have a non native sched clock such as kvm_sched_clock_read.
tsc_init -> sets enable_sched_clock_irqtime()
          ->mark_tsc_unstable -> if using_native_sched_clock -> clear_sched_clock_stable

In this case, since clear_sched_clock_stable won't be called you may not disable the
sched clock irqtime since __sched_clock_stable_early is reset only in clear_sched_clock_stable


Bigger concern(maybe) is clock source marked as stable still, though called mark_tsc_unstable in
non native sched clock?

Disclaimer: (just curious, seeing this x86 code for first time, so may not know all paths)