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

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Message-ID: <YbHn6JaaOo3b5GLO@hirez.programming.kicks-ass.net>
Date:   Thu, 9 Dec 2021 12:26:32 +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 Sun, Dec 05, 2021 at 02:48:43PM -0800, Namhyung Kim wrote:

> Actually 18446744069443110306 is 0xffffffff01b345a2 so it seems to
> have a negative enabled time.  In fact, bperf keeps values returned by
> bpf_perf_event_read_value() which calls perf_event_read_local(), and
> accumulates delta between two calls.  When event->shadow_ctx_time is
> not set, it'd return invalid enabled time which is bigger than normal.

*that*, how does it happen that shadow_time isn't set? It should be last
set when the event switches to INACTIVE, no? At which point the logic in
perf_event_read_local() should make @enabled move forward while @running
stays put.

Let me go rummage around a bit... either I'm missing something obvious
or something's smelly.