linux-kernel - Re: [PATCH] perf/core: Introduce cpuctx->cgrp_ctx

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Message-ID: <20231004160224.GB6307@noisy.programming.kicks-ass.net>
Date:   Wed, 4 Oct 2023 18:02:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     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>,
        Stephane Eranian <eranian@...gle.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Ravi Bangoria <ravi.bangoria@....com>, stable@...r.kernel.org
Subject: Re: [PATCH] perf/core: Introduce cpuctx->cgrp_ctx_list

On Tue, Oct 03, 2023 at 09:08:44PM -0700, Namhyung Kim wrote:

> But after the change, it ended up iterating all pmus/events in the cpu
> context if there's a cgroup event somewhere on the cpu context.
> Unfortunately it includes uncore pmus which have much longer latency to
> control.

Can you describe the problem in more detail please?

We have cgrp as part of the tree key: {cpu, pmu, cgroup, idx},
so it should be possible to find a specific cgroup for a cpu and or skip
to the next cgroup on that cpu in O(log n) time.

> To fix the issue, I restored a linked list equivalent to cgrp_cpuctx_list
> in the perf_cpu_context and link perf_cpu_pmu_contexts that have cgroup
> events only.  Also add new helpers to enable/disable and does ctx sched
> in/out for cgroups.

Adding a list and duplicating the whole scheduling infrastructure seems
'unfortunate' at best.