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From:   Namhyung Kim <namhyung@kernel.org>
Date:   Tue, 19 Jan 2021 12:11:58 +0900
Message-ID: <CAM9d7chk5qC7er5EVmmrHZtP2PB6Y9EikfGYxYibE4HZpCOmbQ@mail.gmail.com>
Subject: Re: [PATCH] perf/core: Emit PERF_RECORD_LOST for pinned events
To:     Andi Kleen <ak@linux.intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Stephane Eranian <eranian@google.com>,
        Ian Rogers <irogers@google.com>,
        Alexey Alexandrov <aalexand@google.com>
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Hi Andi,

On Tue, Jan 19, 2021 at 11:47 AM Andi Kleen <ak@linux.intel.com> wrote:
>
> > > 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?
>
> Could always reserve the last entry in the ring buffer for a LOST event,
> that would guarantee you can always get one out.

A problem is that we can have more than one event that failed.

In my understanding, we keep the lost count and add a LOST event
when there's a space later.  So probably we can keep a list of the
failed events and do similar for each event.  Or just use a single
event to notify some number of events were failed.

Thanks,
Namhyung