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

[Avi Kivity <avi@redhat.com>]

  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.

-- 
error compiling committee.c: too many arguments to function