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

[K Prateek Nayak <kprateek.nayak@amd.com>]

Hello Vincent,

On 1/23/2026 9:28 PM, Vincent Guittot wrote:
>> If TSC / any potentially unstable clock is marked unstable before
>> sched_clock_init_late(), we never flip __sched_clock_stable.
>>
>> On x86 side, since we already start using TSC for native_sched_clock()
>> super early, we have defensive measures (and a __use_tsc static key on
>> the x86 side too) to switch the sched clock back to jiffies if we
>> realise TSC was unstable before we hit sched_clock_init_late().
>>
>> This is necessary since post the sched_clock_init(),
>> "sched_clock_running" is true and the local clocks will always uses
>> sched_clock_noinstr() and it must be switch to jiffies on the x86 side
>> since arch defines it.
> 
> sched_clock_init_late() is always called so can't we check if
> irqtime_enabled and clock is unstable then we disable irqtime
> accounting

That can be done, yes.

Wangyang, can you also consolidate the irqtime disable bits (TSC and
generic) to trigger at sched_clock_init_late() if the clock was found
unstable and in __sched_clock_work()? 

Both context can directly do a static_branch_disable() without
needing an extra delayed work.

-- 
Thanks and Regards,
Prateek