Re: [PATCH v3] sched/clock: Avoid false sharing for sched_clock_irqtime

[K Prateek Nayak]

Hello Vincent,

On 1/16/2026 10:11 PM, Vincent Guittot wrote:
>>> I think that disabling irq time accounting if it was enabled in
>>> __sched_clock_work() should be good
>>
>> I though about this and you can have this particular case as a result
>> of when TSC vs sched_clock is marked 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; irqtime_enabled() is true
>>
>>
>> The current approach from Wangyang covers this case so the v3 should be
>> good as is.
> 
> The need of a workqueue is only for CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
> and I'd like to keep it there instead of making the use of a workqueue
> the default behavior for disabling irq time accounting whereas it's
> not needed
> 
> So either we can use a single workqueue in unstable clock to handle
> the disabling of one or both __sched_clock_stable and
> sched_clock_irqtime
> 
> or  unstable clock need another workqueue for disabling sched_clock_irqtime

Ack! We'll likely need a second workqueue and call it early in
__clear_sched_clock_stable().

AFAICT, PA-RISC already suffers from this since it marks
sched_clock_unstable() on SMP but since it uses the generic sched_clock,
on a >= 1MHz processor, it enabled irqtime anyways and nothing disables
it later (unlike TSC, the clocksource doesn't have a "mark_unstable"
callback).

If we can delay enabling the irqtime accounting until we do
__set_sched_clock_stable() for CONFIG_HAVE_UNSTABLE_SCHED_CLOCK (I have
no clue if this is acceptable of not), then we can move the enabling /
disabling of irqtime to the same spots as when we flip
__sched_clock_stable.

-- 
Thanks and Regards,
Prateek