Re: [PATCH 2/2] perf_counter: optimize context switch betweenidentical inherited contexts

[Paul Mackerras]

Peter Zijlstra writes:

> On Fri, 2009-05-22 at 14:27 +1000, Paul Mackerras wrote:
> > Since we don't have individual fds for the counters in a cloned
> > context, the only thing that can make two clones of a given parent
> > different after they have been cloned is enabling or disabling all
> > counters with prctl.  To account for this, we keep a count of the
> > number of enabled counters in each context.  Two contexts must have
> > the same number of enabled counters to be considered equivalent.
> 
> Curious point that.. so prctl() can disable counters it doesn't own.
> 
> Shouldn't we instead fix that?

Well, prctl enables/disables the counters that are counting on the
current process, regardless of who or what created them.  I always
thought that was a little strange; maybe it is useful to be able to
disable all the counters that any other process might have put on to
you, but I can't think of a scenario where you'd really need to do
that, particularly since the disable is a one-shot operation, and
doesn't prevent new (enabled) counters being attached to you.

On the other hand, what does "all the counters I own" mean?  Does it
mean all the ones that I have fds open for?  Or does it mean all the
ones that I created?  Either way we don't have a good way to enumerate
them.

Paul.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/