linux-kernel - Re: [PATCH] perf/core: Emit PERF_RECORD

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Date:   Tue, 19 Jan 2021 10:42:34 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        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>,
        Alexey Alexandrov <aalexand@...gle.com>
Subject: Re: [PATCH] perf/core: Emit PERF_RECORD_LOST for pinned events

On Mon, Jan 18, 2021 at 9:56 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Jan 18, 2021 at 08:44:20PM +0900, Namhyung Kim wrote:
> > Hi Peter,
> >
> > On Mon, Jan 18, 2021 at 7:11 PM Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > On Mon, Jan 18, 2021 at 12:43:23PM +0900, Namhyung Kim wrote:
> > > > As of now we silently ignore pinned events when it's failed to be
> > > > scheduled and make it error state not try to schedule it again.
> > > > That means we won't get any samples for the event.
> > > >
> > > > But there's no way for users to notice and respond to it.  Let's
> > > > emit a lost event with a new misc bit to indicate this situation.
> > >
> > > Users should get a read(2) error IIRC, does that not work?
> >
> > Ah, right.  maybe I'm too specific to perf record's perspective.
> >
> > In perf record, it doesn't use read(2) so I thought it should
> > have the information in the stream of sample data.
>
> perf-record could of course do a read() at the end, to detect this.

OK, will add that.

>
> I don't think I object to having an even in the stream, but your LOST
> event is unfortunate in that it itself can get lost when there's no
> space in the buffer (which arguably is unlikely, but still).
>
> So from that point of view, I think overloading LOST is not so very nice
> for this.

But anything can get lost in case of no space.
Do you want to use something other than the LOST event?

Thanks,
Namhyung