linux-kernel - Re: [PATCH] perf_events: fix errors path in perf_output

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Message-ID: <AANLkTilJH8RnWjBIY7vrOnXi7w1_z8j9_abD-8tuwL-u@mail.gmail.com>
Date:	Mon, 17 May 2010 14:04:43 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, acme@...radead.org,
	perfmon2-devel@...ts.sf.net, eranian@...il.com
Subject: Re: [PATCH] perf_events: fix errors path in perf_output_begin()

On Mon, May 17, 2010 at 2:03 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, 2010-05-17 at 13:56 +0200, Stephane Eranian wrote:
>> >> +     if (!data->nr_pages) {
>> >> +             atomic_inc(&data->lost);
>> >> +             goto out;
>> >> +     }
>
>
>> Well, nr_pages = 0 means all you have is the sampling buffer
>> header page. You cannot save any sample, so you  actually
>> loose everything. Perf reports a dozen samples but those
>> are synthetic. The kernel cannot reject an mmap() for
>> just one page because of the remapped counters, thus it
>> has to handle this gracefully and at least report the lost
>> samples.
>
> So you want to preserve this state for when you munmap() and mmap()
> again? The only user of data->lost is writing the PERF_RECORD_LOST
> event, which only ever happens when you have pages, so counting it when
> there's no pages seems futile.

You're right. But couldn't you report lost samples in the buffer header as well?
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