linux-kernel - Re: [PATCH] perf_events: fix bogus warn_on(_once) in perf_prepare

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Date:	Thu, 8 Apr 2010 23:08:34 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, fweisbec@...il.com, robert.richter@....com,
	perfmon2-devel@...ts.sf.net, eranian@...il.com
Subject: Re: [PATCH] perf_events: fix bogus warn_on(_once) in 
	perf_prepare_sample()

On Thu, Apr 8, 2010 at 10:55 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, 2010-04-08 at 22:45 +0200, Stephane Eranian wrote:
>>       There is a warn_on_once() check for PERF_SAMPLE_RAW which trips
>>       when using PEBS on both Core and Nehalem. Core PEBS sample size is 144
>>       bytes and 176 bytes for Nehalem. Both are multiples of 8, but the size
>>       field is encoded as int, thus the total is never a multiple of 8 which
>>       trips the check. I think the size should have been u64, but now it is
>>       too late to change given it is ABI.
>
> PEBS hasn't seen -linus yet, so we can fix that.
>
Are you suggesting you add some padding the PEBS raw sample you
return as PERF_SAMPLE_RAW? Then you need to define what RAW
actually means? Seems here, it would mean more than what the
HW returns.

> There's various things that do indeed rely on the perf buffer to always
> be u64 aligned, so this warning isn't bogus at all.
>
I assume this has to do with the wrap-around detection.
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