linux-kernel - Re: [PATCH v3] perf/core: Set event shadow time for inactive events too

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Message-ID: <YcBPZXtu0OOesleq@hirez.programming.kicks-ass.net>
Date:   Mon, 20 Dec 2021 10:39:49 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>,
        Song Liu <songliubraving@...com>
Subject: Re: [PATCH v3] perf/core: Set event shadow time for inactive events
 too

On Fri, Dec 10, 2021 at 10:59:49AM -0800, Namhyung Kim wrote:

> > You're doing that bpf-cgroup crud, right? Where exactly do you hook into
> > to do the counter reads?
> 
> That's true but it doesn't use cgroup events actually.  They are plain cpu
> events and BPF is called from a separate 'cgroup-switches' event to
> read out the counters.

Oh, right.

> > > Maybe because the event is enabled from the beginning.
> > > Then it might miss set_state/update_time at all.
> >
> > Even then, it's set to INACTIVE and any state change thereafter needs to
> > go through perf_event_set_state() and update the relevant timestamps.
> 
> Right, but the problem happens when you read the event *before*
> any state change.

But the per-cpu event should be the simplest case ever, the cpu context
is *always* active, enabled time always runs.