Re: [patch 0/3] [Announcement] Performance Counters for Linux

From: Ingo Molnar
Date: Fri Dec 05 2008 - 05:41:57 EST

* Paul Mackerras <paulus@xxxxxxxxx> wrote:

> Ingo Molnar writes:
> > The 'target' task does not have to be stopped to offer counter
> > virtualization (counter overcommit or counter scheduling) - or to offer
> > any of the other performance counter features. Please let us know why it
> > needs the task to be stopped - we asked about that on lkml in the perfmon
> > thread and no technical answer was given, and couldnt find any such
> > technical reason while implementing it ourselves.
> I like this feature of your patchset, in fact, and the code looks
> pretty clean (as I would expect :). What I don't like (as I have
> already said) is having to use an API that splits up the PMU into
> pieces, plus the requirement that flows from that to have the kernel
> know about the event selection logic on every CPU model we support.
> One thing I haven't figured out yet is what happens if you have a
> counter on a task and the task dies. Can I still use the counter fd
> after the task has died, and read out the total count?

yes, it will work just the way you'd expect it to work: the counter is
attached to the fd of the monitoring task, so it does not go away. The
counter simply stops counting but otherwise can be read even after the
monitored task has exited.

We are also planning a natural 'the task has died' notification: a -EPIPE
returned by read(), after the final count has been allowed to be read
out. With blocking counters this will behave quite smoothly: instead of
blocking indefinitely, we'd get back -EPIPE. Hm?

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