linux-kernel - Re: [Question] How does perf still record the stack of a specified pid even when that process is interrupted and CPU is scheduled to other process

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Date:	Tue, 05 May 2015 16:24:54 -0600
From:	David Ahern <dsahern@...il.com>
To:	Rabin Vincent <rabin@....in>,
	Yunlong Song <yunlong.song@...wei.com>
CC:	a.p.zijlstra@...llo.nl, paulus@...ba.org, mingo@...hat.com,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, wangnan0@...wei.com
Subject: Re: [Question] How does perf still record the stack of a specified
 pid even when that process is interrupted and CPU is scheduled to other process

On 5/5/15 3:53 PM, Rabin Vincent wrote:
> The commands above will identify the sys_write which takes time but only
> provide the stacktrace at the entry and exit of the syscall, but this do
> not show why the process blocked or what it did inside the system call.
>
> So a way to get what is required for the use case would be to make the
> following changes to the above sequence:
>
> (1) include the sched:* events when perf trace record is run
>
> (2) around the time of interest, look at the kernel stack st the sched:switch
>      events between the entry and the exit.  This will show what the process was
>      waiting for when it when it blocked.  The stacktraces at the
>      stat_runtime events in the process may also be useful to understand what
>      was going on.

agreed.


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