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

[Ian Rogers]

On Tue, Nov 18, 2025 at 3:19 PM Andi Kleen <ak@xxxxxxxxxxxxxxx> 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