linux-kernel - Re: [PATCH 3/4] trace: add ability to collect call chain of non-current task.

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Date:	Wed, 28 Sep 2011 17:53:02 +0400
From:	Andrew Vagin <avagin@...il.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Vagin <avagin@...nvz.org>, linux-kernel@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCH 3/4] trace: add ability to collect call chain of non-current
 task.

On 09/28/2011 12:55 AM, Frederic Weisbecker wrote:
> On Tue, Sep 27, 2011 at 04:05:51PM +0200, Peter Zijlstra wrote:
>> On Mon, 2011-09-26 at 19:55 +0400, Andrew Vagin wrote:
>>> Know issues:
>>> * Now call chains for non-current tasks are collected on x86 only,
>>>    but it may be done for other architectures simply.
>>> * It collects only kernel call chains, because we can't get direct
>>>    access to memory of other processes and this operation should be
>>>    fast enough.
>> Also, it changes the semantics of tracepoint, normally you return the
>> callchain leading to the tracepoint, this changes that.
>>
>>
>> I'm not entirely against it, as I can see the use, but I would like to
>> solicit other opinions.
> There is also a problem in perf tools because we are dealing with a
> callchain that doesn't belong to the current thread memory mapping and symbol
> space. I believe that's a problem once we deal with the userspace part of
> the callchain.
>
> That and the fact there are other ways to get that callchain like taking
> the one of the last sched_switch event from the waked thread.
>
> Perhaps we should use the sched_switch event and a post_sched_switch
> event, compute the time difference as a weight and use the callchain of any of
> those. Perhaps as a plugin in perf report?
>
> In any case that rather sounds like something that should be done in post-processing
> in userspace, in perf tools.
>
> We should probably avoid the remote callchains, sounds like asking for complications
> everywhere.
Thank you for comments and suggestions. I take timeout to think over them.
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