Re: [PATCH] trace "exit to userspace" event

[Gleb Natapov <gleb@redhat.com>]

On Thu, Oct 14, 2010 at 01:43:52PM +0200, Avi Kivity wrote:
>  On 10/14/2010 01:41 PM, Gleb Natapov wrote:
> >>  >>   >So what do you propose? I see no issue with my original patch.
> >>  >>
> >>  >>   Record both errno and exit_reason.  While they're never both valid
> >>  >>   at the same time, they're both necessary.
> >>  >>
> >>  >If they can't be valid at the same time, why not record one of them?
> >>
> >>  If you record just one, you don't know if the other one happened.
> >>
> >I mean to record type/value. So it can be (type error/value -EINVAL) or
> >(type exit/value HALT). The goal is to not print non-relevant info in
> >ftrace, but we can do the same with recording errno and exit_reason and
> >show only exit_reason is errno>=0 or errno otherwise.
> 
> That's fine.  As long as you don't drop information.
> 
> >>  >The
> >>  >one that happened? Also any error other then -EINTR will cause qemu to
> >>  >stop, so it is not very interesting. And ioctl return value can be
> >>  >traced by strace anyway.
> >>
> >>  You can't correlate it with ftrace.
> >>
> >True. But given that the only interesting error code in -EINTR I do not
> >know if this is useful, but potentially may generate a lot of events in
> >ftrace. Sometimes too much info is almost as bad as not enough and if we
> >will use the same event for both exit_reason and errno it will be
> >impossible to enable one without the other.
> 
> You can always filter excess information away.  If you're looking
> for exits to userspace (a major performance issue) then you want to
> see both normal exits and signal exits.
> 
How do I do it via /debug file system? Now I just echo name of the event
I am interested in set_event, how do I add filter to that?

--
			Gleb.