linux-kernel - Re: [PATCH 1/3] perf/core: Update perf_adjust_freq_unthr

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Message-ID: <ZV024X0VKUwExQio@google.com>
Date:   Tue, 21 Nov 2023 23:01:53 +0000
From:   Mingwei Zhang <mizhang@...gle.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Ian Rogers <irogers@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kan Liang <kan.liang@...ux.intel.com>
Subject: Re: [PATCH 1/3] perf/core: Update perf_adjust_freq_unthr_context()

On Tue, Nov 21, 2023, Namhyung Kim wrote:

Hi Namhyung,

> Hi Mingwei,
> 
> On Mon, Nov 20, 2023 at 3:24 PM Mingwei Zhang <mizhang@...gle.com> wrote:
> >
> > On Mon, Nov 20, 2023, Ian Rogers wrote:
> > > On Mon, Nov 20, 2023 at 2:19 PM Namhyung Kim <namhyung@...nel.org> wrote:
> > > >
> > > > It was unnecessarily disabling and enabling PMUs for each event.  It
> > > > should be done at PMU level.  Add pmu_ctx->nr_freq counter to check it
> > > > at each PMU.  As pmu context has separate active lists for pinned group
> > > > and flexible group, factor out a new function to do the job.
> > > >
> > > > Another minor optimization is that it can skip PMUs w/ CAP_NO_INTERRUPT
> > > > even if it needs to unthrottle sampling events.
> > > >
> > > > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > >
> > > Series:
> > > Reviewed-by: Ian Rogers <irogers@...gle.com>
> > >
> > > Thanks,
> > > Ian
> > >
> >
> > Can we have "Cc: stable@...r.kernel.org" for the whole series? This
> > series should have a great performance improvement for all VMs in which
> > perf sampling events without specifying period.
> 
> I was not sure if it's ok to have this performance fix in the stable series.
> 

Critical performance bug fix is ok to be added to stable tree, as the
requirements are mentioned here:

https://www.kernel.org/doc/Documentation/process/stable-kernel-rules.rst

In particular, this patch satisfies the 2nd sub-bullet of the forth
bullet.

But let me step back. Only this patch is needed with stable tag instead
of the whole series. This patch impact 69 lines of code. It satisfies
the rule of within 100 lines (bullet 3).

I will give a try and test it today or tomorrow and make sure we satisfy
bullet 2.

Once it gets in, bullet 1 will be satisfied as well.

Overall, the intention is to improve PMU performance in VM as early as
we can since we don't control the schedule of distro kernel upgrade and
we don't control when end customers upgrade their running kernel. So I
presume even adding to the stable tree may take years to see the result
change. But if we don't do it, it may take way longer (since it does not
contain a "Fixes" tag as well).

Thanks.
-Mingwei

> Thanks,
> Namhyung