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

From: Jiri Olsa
Date: Sat Sep 06 2014 - 16:40:05 EST

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

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