linux-kernel - Re: [PATCH 27/53] perf/core: Put size of a sample at the end of it by PERF_SAMPLE

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Message-ID: <20160112141430.GH6357@twins.programming.kicks-ass.net>
Date:	Tue, 12 Jan 2016 15:14:30 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Wang Nan <wangnan0@...wei.com>
Cc:	acme@...nel.org, linux-kernel@...r.kernel.org, pi3orama@....com,
	lizefan@...wei.com, netdev@...r.kernel.org, davem@...emloft.net,
	Adrian Hunter <adrian.hunter@...el.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	David Ahern <dsahern@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Yunlong Song <yunlong.song@...wei.com>
Subject: Re: [PATCH 27/53] perf/core: Put size of a sample at the end of it
 by PERF_SAMPLE_TAILSIZE

On Mon, Jan 11, 2016 at 01:48:18PM +0000, Wang Nan wrote:
> Before reading such ring buffer, perf must ensure all events which may
> output to it is already stopped, so the 'head' pointer it get is the
> end of the last record.

We could add an extra ioctl() to pause/resume ring-buffer output to make
this more convenient to achieve.

Say replace the !rb->nr_pages test in perf_output_begin() with a control
variable load and ensure its set to pause/fail when !nr_pages.

That way we only have to issue a single ioctl to freeze the buffer,
without adding extra instructions/loads to the fast path.