Messages in this thread | | | Date | Tue, 25 Feb 2014 12:29:36 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 4/7] perf: be less pessimistic when scheduling events |
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On Tue, Feb 11, 2014 at 05:48:05PM +0000, Mark Rutland wrote: > On Mon, Feb 10, 2014 at 05:58:54PM +0000, Peter Zijlstra wrote: > > On Mon, Feb 10, 2014 at 05:44:21PM +0000, Mark Rutland wrote: > > > This patch makes ctx_flexible_sched_in attempt to schedule every group > > > in the flexible_groups list even when earlier groups failed to schedule, > > > enabling more groups to be scheduled simultaneously. > > > > Since you're basically free to create as many events as you want, you can > > (as an unprivilidged user) make the context switch to and from your task > > arbitrarily expensive. > > Not that it makes the approach any better, but surely this is already > the case for software events?
Yeah, software events are a problem; I realized this the moment I send that email. However, software events do not have the O(n!) programming fail hw events have.
AMD Fam15h has a O(n^4) termination of the O(n*n!) algorithm, the rest of x86 has a normal O(n^2) bound.
> I can create as many as I want, and they'll always try to schedule. > > I'll take a look into other ways to stop disparate PMUs' events from > adversely affecting each other. I guess it might not be possible to > solve the general problem without allowing said expensive context > switch.
So the problem you're facing is that you'll attach events for each PMU available on the system to a particular task? And simply want to continue scheduling even though a particular event will never fit on the CPUs PMU its currently running on?
So the trivial work-around would be to create an event per cpu and set event->cpu. That way you'll get the event_filter_match() exception in ctx_flexible_sched_in() and we'll skip over all events that cannot be programmed on this CPU.
The other approach is adding a cpumask to events, which would drastically cut down on the amount of events you'd need to create; then again, ARM doesn't have a silly amount of CPUs x86 has just quite yet.
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