linux-kernel - Re: [PATCH v4 00/16] perf, persistent: Add persistent events

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Date:	Tue, 6 May 2014 20:58:26 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Robert Richter <rric@...nel.org>
Cc:	Jean Pihet <jean.pihet@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>, linux-kernel@...r.kernel.org,
	Tomasz Nowicki <tomasz.nowicki@...aro.org>
Subject: Re: [PATCH v4 00/16] perf, persistent: Add persistent events

On Tue, May 06, 2014 at 02:39:07PM +0200, Robert Richter wrote:
> I rather would change the ioctl to
> 
>  id = ioctl(PERF_EVENT_IOC_SET_PERSIST, arg);
> 
>  arg != 0:	create persistent event (unclaim)
>  arg == 0:	delete persistent event (claim)
> 
> This has the advantage that the naming is better and arg can be used
> as parameter (e.g. event id to share a namespace).

Yep, this is better than the CLAIM/UNCLAIM thing.

> The unclaim ioctl *creates* a buffer with 512kB default size. The
> reopening process must mmap with the same buffer size. This is a
> problem as in this implementation the buffer size is fix and can not
> be adjusted.

Why do we need it to be adjustable?

> We could let create the process the buffers and make the
> event persistent including the current buffers.
> 
> Variable buffer size is a must,

Yeah, why? I'm just asking for my own understanding.

> so the reopening process also must be
> able to detect buffer size. Mmap buffer size could be detected by
> mmap'ing only the header page, reading the buffer size from the header
> and then remapping the buffer with adjusted size.
> 
> It would be good to have a perf tool option -P that puts events in
> persistent state instead of starting a command:
> 
>  # perf record -e ... -a -P <namespace>		# create pers. events
>  # perf record -e persistent/<namespace>/ -a	# read existing buffers
>  # perf record -e persistent/<namespace>/ -a -k	# read and kill existing events
> 
> Note that no args for a command are given.

Yep, looks sane to me.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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