linux-kernel - Re: perf: behavior of poll() changed in 3.18

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Message-ID: <20150122133455.GC2473@krava.brq.redhat.com>
Date:	Thu, 22 Jan 2015 14:34:55 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Vince Weaver <vincent.weaver@...ne.edu>
Cc:	linux-kernel@...r.kernel.org, Jiri Olsa <jolsa@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: perf: behavior of poll() changed in 3.18

On Thu, Jan 22, 2015 at 01:53:35AM -0500, Vince Weaver wrote:
> On Wed, 21 Jan 2015, Jiri Olsa wrote:
> > 
> > However if we revert this code, we'll loose nice (and standard) way
> > to check if the event is still valid.. not sure how to handle this.
> 
> there's likely no need to revert as my code wasn't really released and 
> I've already fixed it to work with the new interface.
> 
> I was mostly asking just so I could update the manpage to explain the new 
> behavior, as tools that expect to be backwards compatible will have to 
> handle both ways of detecting a process dieing.
> 
> > > Part of why my code doesn't just exit on POLLHUP is because you can
> > > get that result for reasons other than a process exit (for example,
> > > if you are using ioctl(PERF_EVENT_IOC_REFRESH)
> > 
> > Nope, this is related to POOL_HUP (notice the '_') which you'll get
> > accompanied with SIGIO if you setup this.
> 
> So what happens if you are using a signal handler to monitor a child and 
> the child exits?

AFAICS wrt to SIGIO, we notify only with POLL_IN if there's new data
and POLL_HUP if we reached the event_limit - the one you set with
PERF_EVENT_IOC_REFRESH ioctl

> 
> It's a shame the poll and signal handler interfaces are subtly different, 
> though I guess some of that is probably due to historical reasons.

I've actually never used the SIGIO interface in perf other than
when I was checking your gi repo with test code ;-)

so I'm not sure what the correct behaviour should be when monitored
process dies.. I'd say we should send SIGIO with POLL_HUP, but that
clashes with that 'event_limit' thing.. I'll check on that

thanks,
jirka
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