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Subject: Re: scheduler performance regression since v6.11
To: Peter Zijlstra <peterz@infradead.org>
Cc: Chris Mason <clm@meta.com>, linux-kernel@vger.kernel.org,
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From: Dietmar Eggemann <dietmar.eggemann@arm.com>
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On 20/05/2025 21:38, Peter Zijlstra wrote:
> On Tue, May 20, 2025 at 04:38:09PM +0200, Dietmar Eggemann wrote:
> 
>> 3840cbe24cf0 - sched: psi: fix bogus pressure spikes from aggregation race
>>
>> With CONFIG_PSI enabled we call cpu_clock(cpu) now multiple times (up to
>> 4 times per task switch in my setup) in:
>>
>> __schedule() -> psi_sched_switch() -> psi_task_switch() ->
>> psi_group_change().
>>
>> There seems to be another/other v6.12 related patch(es) later which
>> cause(s) another 4% regression I yet have to discover.
> 
> Urgh, let me add this to the pile to look at. Thanks!

Not sure how expensive 'cpu_clock(cpu)' is on bare-metal.

But I also don't get why PSI needs per group 'now' values when we
iterate over cgroup levels?