linux-kernel - Re: [PATCHv2 00/25] perf tool: Add support for multiple data file storage

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Date:	Tue, 10 Sep 2013 10:29:20 -0700
From:	David Ahern <dsahern@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Jiri Olsa <jolsa@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Namhyung Kim <namhyung@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCHv2 00/25] perf tool: Add support for multiple data file
 storage

On 9/9/13 9:06 AM, Ingo Molnar wrote:
>> Aren't you losing potentially important events by doing that -- FORK,
>> COMM, MMAP?
>
> I suspect these could/should be tracked and emitted fully (in bulk) when a
> new data file is opened, so that each partial data file is fully
> consistent?

In my case I am not saving task events, but processing them. In Jiri's 
case where events are written to a file it should be possible to stash 
the unprocessed events on a list, when the exit happens move them to a 
dead threads list which can be cleaned up from time to time and then on 
file dump requests dump the task events followed by the sample events.

>
>> I have a flight recorder style command that address this problem
>> (long-running/daemons) by processing task events and then stashing the
>> sample events on a time-ordered list with chopping to maintain the time
>> window.
>
> Could this be used to emit currently relevant task context?

sure.

>
> Btw., I also think it would be useful to have kernel support for that -
> the 'collections' stuff I talked about a good while ago: the kernel would
> work with user-space to iterate over all MMAPs and all running COMMs at
> the opening of a tracing session.
>
> That way we could avoid racy access to /proc, we could make sure that all
> information that is emitted by FORK/COMM/MMAP is also emitted for the
> 'bulk' data, etc.

Walking the task list and emitting events would be better but wouldn't 
that be a performance hit holding the task lock (tasklist_lock?)? (I 
thought that one is needed when walking the task list.)

David

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