Messages in this thread | | | Date | Sat, 23 May 2009 14:38:28 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 2/2] perf_counter: optimize context switch between identical inherited contexts |
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* Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> On Fri, 2009-05-22 at 19:56 +1000, Paul Mackerras wrote: > > 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. > > I'm for all counters you created (ie have a fd for). Being able to > disable counters others created on you just sounds wrong. > > If we can settle on a semantic, I'm sure we can implement it :-) > > Ingo, Corey, any opinions?
It indeed doesnt sound correct that we can disable counters others created on us - especially if they are in a different (higher privileged) security context than us.
OTOH, enabling/disabling counters in specific functions of a library might be a valid use-case. So perhaps make this an attribute: .transparent or so, with perf stat defaulting on it to be transparent (i.e. not child context disable-able).
Ingo
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