linux-kernel - Re: [PATCH 1/2] sched: Optimise task_mm_cid

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Message-ID: <6b7b30528893b091b21a06ead610709219cd9ba0.camel@redhat.com>
Date: Mon, 02 Dec 2024 15:56:07 +0100
From: Gabriele Monaco <gmonaco@...hat.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.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>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched: Optimise task_mm_cid_work duration

Hi Mathieu,

thanks for the quick reply.

> Thanks for looking into this. I understand that you are after
> minimizing the
> latency introduced by task_mm_cid_work on isolated cores. I think
> we'll need
> to think a bit harder, because the proposed solution does not work:
> 
>   * for_each_cpu_from - iterate over CPUs present in @mask, from @cpu
> to the end of @mask.
> 
> cpu is uninitialized. So this is completely broken. 

My bad, wrong macro.. Should be for_each_cpu

> Was this tested
> against a workload that actually uses concurrency IDs to ensure it
> does
> not break the whole thing ? Did you run the rseq selftests ?
> 

I did run the stress-ng --rseq command for a while and didn't see any
error reported, but it's probably not bulletproof. I'll use the
selftests for the next iterations.

> Also, the mm_cidmask is a mask of concurrency IDs, not a mask of
> CPUs. So
> using it to iterate on CPUs is wrong.
> 

Mmh I get it, during my tests I was definitely getting better results
than using the mm_cpus_allowed mask, but I guess that was a broken test
so it just doesn't count..
Do you think using mm_cpus_allowed would make more sense, with the
/risk/ of being a bit over-cautious?


Gabriele