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Date:   Wed, 4 Oct 2023 18:02:24 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Namhyung Kim <namhyung@kernel.org>
Cc:     Ingo Molnar <mingo@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Stephane Eranian <eranian@google.com>,
        Kan Liang <kan.liang@linux.intel.com>,
        Ravi Bangoria <ravi.bangoria@amd.com>, stable@vger.kernel.org
Subject: Re: [PATCH] perf/core: Introduce cpuctx->cgrp_ctx_list
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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.