linux-kernel - Re: [PATCH 0/3] tracing/perf: perf_trace_buf/perf

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Date:	Thu, 20 Jun 2013 12:53:01 -0600
From:	David Ahern <dsahern@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] tracing/perf: perf_trace_buf/perf_xxx hacks.

On 6/20/13 12:47 PM, Steven Rostedt wrote:
> On Thu, 2013-06-20 at 12:35 -0600, David Ahern wrote:
>> On 6/20/13 12:23 PM, Steven Rostedt wrote:
>>>
>>> I ran this:
>>>
>>>    perf stat --repeat 100 -- perf bench sched pipe > /tmp/perf-bench-sched.{before, after}
>>
>> You want to compare:
>>     perf stat --repeat 100 -p 1 -- perf bench sched pipe
>>
>> so that event is tagged to pid 1 and not the perf-bench workload.
>
> I guess I'm a bit confused. What's the significance of measuring pid 1
> (init)?

I believe Oleg's point is the overhead for tasks without events 
associated with them. To show that create an event tagged to the init 
task and then run some workload -- like perf bench shed pipe. It shows 
that all tasks take a hit, not just the one getting profiled.

David
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