Messages in this thread |  | | | Date | Wed, 31 Mar 2010 16:13:46 +0200 | | Subject | Re: [RFC] perf_events: support for uncore a.k.a. nest units | | From | stephane eranian <> |
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On Wed, Mar 31, 2010 at 4:01 PM, Peter Zijlstra <peterz@infradead.org> wrote: > On Tue, 2010-03-30 at 15:12 -0700, Corey Ashford wrote: >> >> > Initially I'd not allow per-pmu-per-task contexts >> > because then things like perf_event_task_sched_out() would get rather >> > complex. >> >> Definitely. I don't think it makes sense to have per-task context on >> nest/uncore PMUs. At least we haven't found any justification for it. > > For uncore no, but there is also the hw-breakpoint stuff that is being > presented as a pmu, for those it would make sense to have a separate > per-task context. > > But doing multiple per-task contexts is something for a next step > indeed. > >> > For RR we can move away from perf_event_task_tick and let the pmu >> > install a (hr)timer for this on their own. >> >> This is necessary I think, because of the access time for some of the PMU's. I >> wonder though if it should, perhaps optionally, be off-loaded to a high-priority >> task to do the switching so that access latency to the PMU can be controlled. >> >> As I mentioned when we met, some of the Wire-Speed processor nest PMU control >> registers are accessed via SCOM, which is an internal, 200 MHz serial bus. We >> are being quoted ~525 SCOM bus ticks to do a PMU control register access, which >> comes out to about 2.5 microseconds. If you figure 5 accesses to rotate the >> events on a PMU, that's a minimum of 12.5 microseconds. > > Yeah, you mentioned that.. for those things we need some changes anyway, > since currently we install per-cpu counters using IPIs and expect the > pmu::enable() method to be synchronous (it has a return value). It would > be totally unacceptable to do 2.5ms pokes with IRQs disabled. > > The RR thing would be the easiest to solve, just let the timer wake up a > thread instead of doing the work itself, that's fully isolated to how > the pmu chooses to implement that. The above mentioned issue however > would be much more challenging to fix nicely. > > Also some of perf_enable()/perf_disable() would have to be per PMU and not global like they are today. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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