linux-kernel - Re: [PATCH] perf: Fix inconsistency between IP and callchain sampling

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Date:	Thu, 21 Jan 2010 14:20:18 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Anton Blanchard <anton@...ba.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mundt <lethal@...ux-sh.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: Fix inconsistency between IP and callchain
	sampling

On Mon, Jan 18, 2010 at 04:47:07PM +1100, Anton Blanchard wrote:
> 
> When running perf across all cpus with backtracing (-a -g), sometimes we
> get samples without associated backtraces:
> 
>     23.44%         init  [kernel]                     [k] restore
>     11.46%         init                       eeba0c  [k] 0x00000000eeba0c
>      6.77%      swapper  [kernel]                     [k] .perf_ctx_adjust_freq
>      5.73%         init  [kernel]                     [k] .__trace_hcall_entry
>      4.69%         perf  libc-2.9.so                  [.] 0x0000000006bb8c
>                        |          
>                        |--11.11%-- 0xfffa941bbbc
> 
> It turns out the backtrace code has a check for the idle task and the IP
> sampling does not. This creates problems when profiling an interrupt
> heavy workload (in my case 10Gbit ethernet) since we get no backtraces
> for interrupts received while idle (ie most of the workload).
> 
> Right now x86 and sh check that current is not NULL, which should never
> happen so remove that too.
> 
> Signed-off-by: Anton Blanchard <anton@...ba.org>



I'm queuing it. Thanks.

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