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Date:   Wed, 20 Jan 2021 09:16:21 -0300
From:   Arnaldo Carvalho de Melo <acme@kernel.org>
To:     Namhyung Kim <namhyung@kernel.org>
Cc:     Andi Kleen <ak@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.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>
Subject: Re: [PATCH] perf/core: Emit PERF_RECORD_LOST for pinned events
Message-ID: <20210120121621.GQ12699@kernel.org>
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Em Wed, Jan 20, 2021 at 08:53:48PM +0900, Namhyung Kim escreveu:
> On Tue, Jan 19, 2021 at 12:11 PM Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > 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.
> 
> Stephane suggested emitting an event for poll() like EPOLLERR or
> EPOLLHUP.  I'll take a look at that.

Looks sane, that way the poll returns immediately when we start seeing
lost events, so tools can warn the user and then, if/when space becomes
available, tell how many events were lost.

- Arnaldo