linux-kernel - Re: [patch] x86, perf: avoid spamming kernel log for bts buffer failure

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Message-ID: <20140701093442.GN6758@twins.programming.kicks-ass.net>
Date:	Tue, 1 Jul 2014 11:34:42 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Stephane Eranian <eranian@...gle.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] x86, perf: avoid spamming kernel log for bts buffer
 failure

On Mon, Jun 30, 2014 at 04:04:08PM -0700, David Rientjes wrote:
> It's unnecessary to excessively spam the kernel log anytime the BTS buffer 
> cannot be allocated, so make this allocation __GFP_NOWARN.
> 
> The user probably will want to at least find some artifact that the 
> allocation has failed in the past, probably due to fragmentation because 
> of its large size, when it's not allocated at bootstrap.  Thus, add a 
> WARN_ONCE() so something is left behind for them to understand why perf 
> commnads that require PEBS is not working properly.

Can you elaborate a bit under which conditions this triggered? Typically
we should be doing fairly well allocating such buffers with GFP_KERNEL,
that should allow things like compaction to run and create higher order
pages.

And the BTS (branch trace store) isn't _that_ large.

That said, the patch is reasonable; although arguably we should maybe do
the same to alloc_pebs_buffer().

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