linux-kernel - Re: [PATCH v5 3/3] perf stat: Add no-affinity flag

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Message-ID: <CAP-5=fWSXboT2jEYtua_N8AnaXc0SUeoC2iBgxSz5QRF09bfoQ@mail.gmail.com>
Date: Tue, 18 Nov 2025 16:58:29 -0800
From: Ian Rogers <irogers@...gle.com>
To: Andi Kleen <ak@...ux.intel.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, "Dr. David Alan Gilbert" <linux@...blig.org>, 
	Yang Li <yang.lee@...ux.alibaba.com>, James Clark <james.clark@...aro.org>, 
	Thomas Falcon <thomas.falcon@...el.com>, Thomas Richter <tmricht@...ux.ibm.com>, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Dapeng Mi <dapeng1.mi@...ux.intel.com>
Subject: Re: [PATCH v5 3/3] perf stat: Add no-affinity flag

On Tue, Nov 18, 2025 at 3:19 PM Andi Kleen <ak@...ux.intel.com> wrote:
>
> On Tue, Nov 18, 2025 at 01:13:26PM -0800, Ian Rogers wrote:
> > Add flag that disables affinity behavior. Using sched_setaffinity to
> > place a perf thread on a CPU can avoid certain interprocessor
> > interrupts but may introduce a delay due to the scheduling,
> > particularly on loaded machines. Add a command line option to disable
> > the behavior. This behavior is less present in other tools like `perf
> > record`, as it uses a ring buffer and doesn't make repeated system
> > calls.
>
> Like i wrote earlier a much better fix for starvation is to use
> real-time priority instead of the old IPI storms this flag
> is bringing back.

Ack. This is only adding the flag to perf stat, are the storms as much
of an issue there? Patch 2 of 3 changes it so that for a single event
we still use affinities, where a dummy and an event count as >1 event.
We have specific examples of loaded machines where the scheduling
latency causes broken metrics - the flag at least allows investigation
of issues like this. I don't mind reviewing a patch adding real time
priorities as an option.

Thanks,
Ian