linux-kernel - Re: [PATCH 04/10] perf record: Filter out POLLHUP'ed file descriptors

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Date:	Sat, 6 Sep 2014 22:39:15 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	Adrian Hunter <adrian.hunter@...el.com>,
	linux-kernel@...r.kernel.org, David Ahern <dsahern@...il.com>,
	Don Zickus <dzickus@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 04/10] perf record: Filter out POLLHUP'ed file descriptors

On Fri, Sep 05, 2014 at 11:07:56AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Fri, Sep 05, 2014 at 11:42:59AM +0300, Adrian Hunter escreveu:
> > On 09/04/2014 06:19 PM, Arnaldo Carvalho de Melo wrote:
> > > Em Thu, Sep 04, 2014 at 03:32:08PM +0300, Adrian Hunter escreveu:
> > No I was meaning something different. For example, 'perf record' opens an
> > event for 2 processes per-cpu and gets 4 file descriptors:
>  
> > 	task1	task2
> > cpu0	fd0	fd1
> > cpu1	fd2	fd3
>  
> > Now, perf record will mmap fd0 and fd2 and set-output fd1->fd0
> > and fd3->fd2.
>  
> > pollfds includes only fd0 and fd2.
>  
> > But if task2 exits, the POLLHUP will appear on fd1 and fd3.
> 
> So? We are not interested in fd1 and fd3, since all our reading is done
> on fd0 and fd2 mmaps, no?

hm, what if task1 (fd0, fd2) exits first.. perf record will exit,
but it still has to read task2..?

> 
> I.e. when we ask the kernel to point fd B to fd A's mmap (what you
> called set-output) and fd B inserts an event into fd A's mmap ring
> buffer, we get fd A poll return as POLLRD, no?
> 
> Have to check... Otherwise we would have to poll all fds all the time,
> not just the ones mmaping, right?
> 
> > I think Jiri's patchset changed pollfds to include all fds for that reason.

hm, I did not think of that.. ;-) I needed more grained feedback
for future features like cpu hotplug

> 
> It did? I have to look again, probably went together with other changes,
> has it?

it was done by replacing 'int' with 'struct poll_item' for evsel::fd xyarray

jirka
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