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

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Date:	Mon, 8 Sep 2014 16:04:54 +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 Mon, Sep 08, 2014 at 10:46:16AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Sat, Sep 06, 2014 at 10:39:15PM +0200, Jiri Olsa escreveu:
> > 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..?
> 
> Ok, what happens in that case, i.e. when the fds that were set to be the
> ones to be polled, gets nuked, does the set-output command gets just
> undone? Or does the mmap stands, receiving the events from the remaining
> fds and the polling notifications get sent to, in this case, fd3 and
> fd1?

mmaps stays for fd1 and fd3.. and they get poll notifications as well,
we just do not check/poll them now

> 
> I'll look at the kernel code for that...
>  
> > > 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
> 
> So this is good for something you didn't tried to fix (and document) but
> good for something that may be nice in the future? Grumpf, we have
> already way too much stuff that will be eventually used but is not used
> right now :-\
>  

IMO it's more clear to poll pm all event FDs.. and now with the
case Adrian described it seems necessary anyway

jirka
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