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

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Message-ID: <54097793.4050201@intel.com>
Date:	Fri, 05 Sep 2014 11:42:59 +0300
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
CC:	Jiri Olsa <jolsa@...hat.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 09/04/2014 06:19 PM, Arnaldo Carvalho de Melo wrote:
> Em Thu, Sep 04, 2014 at 03:32:08PM +0300, Adrian Hunter escreveu:
>> On 09/04/2014 12:59 AM, Arnaldo Carvalho de Melo wrote:
>>> So that we don't continue polling on vanished file descriptors, i.e.
>>> file descriptors for events monitoring threads that exited.
> 
>>> I.e. the following 'perf record' command now exits as expected, instead
>>> of staying in an eternal loop:
> 
>>>   $ sleep 5s &
>>>   $ perf record -p `pidof sleep`
> 
>>> Reported-by: Jiri Olsa <jolsa@...hat.com>
> <SNIP>
>>> +++ b/tools/perf/builtin-record.c
>>> @@ -467,6 +467,9 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
>>>  			if (err > 0 || (err < 0 && errno == EINTR))
>>>  				err = 0;
>>>  			waking++;
>>> +			if (perf_evlist__filter_pollfd(rec->evlist, POLLERR | POLLHUP) == 0)
>>
>> If the poll fds only include the ones mmapped, then mightn't it filter out
>> ones still in use? e.g. what if you record two processes and one exits?
> 
> Humm, without looking at the code, we would be needlessly looking at the
> mmaps for the threads that exited.
> 
> I.e. we need to associate at the fdarray level the mmaps for the
> descriptors, so that we can, at fdarray__filter() time, munmap the
> POLLHUPed descriptors, right?
> 
> I'll take a look at the code to see what is needed.
> 
> But then this makes the patch set good for a class of problems while
> maintaining existing behaviour for the case you outlined, i.e. this is a
> partial fix and should go now, with further patches on top of it?

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.

I think Jiri's patchset changed pollfds to include all fds for that reason.

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