Messages in this thread | | | Date | Wed, 2 Oct 2013 14:29:01 +0200 | From | Ingo Molnar <> | Subject | Re: PERF_EVENT_IOC_SET_OUTPUT |
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* Peter Zijlstra <peterz@infradead.org> wrote:
> On Wed, Oct 02, 2013 at 12:29:56PM +0200, Frederic Weisbecker wrote: > > On Wed, Oct 02, 2013 at 12:03:50PM +0200, Peter Zijlstra wrote: > > > On Tue, Oct 01, 2013 at 10:11:56PM +0300, Adrian Hunter wrote: > > > > Hi > > > > > > > > It does not seem possible to use set-output between > > > > task contexts of different types (e.g. a software event > > > > to a hardware event) > > > > > > > > If you look at perf_event_set_output(): > > > > > > > > /* > > > > * If its not a per-cpu rb, it must be the same task. > > > > */ > > > > if (output_event->cpu == -1 && output_event->ctx != event->ctx) > > > > goto out; > > > > > > > > ctx (perf_event_context) won't be the same for events > > > > of different types. Is this restriction necessary? > > > > > > Hmm.. so last night I wrote me a big reply saying we couldn't do it; > > > then this morning I reconsidered and thing that something like: > > > > > > output_event->ctx->task != event->ctx->task > > > > > > should actually work. > > > > > > The reason it should be OK I think is because perf_mmap() will refuse to > > > create a buffer for inherited events that have ->cpu == -1. > > > > > > My initial response was going to say that it wouldn't be possible > > > because __perf_event_task_sched_out() could 'break' one ctx while still > > > swapping the other, at which point the buffer would have to service two > > > different tasks, potentially from different CPUs and with the buffers > > > not actually being SMP safe that's a problem. > > > > I don't get what you mean with breaking or swapping a ctx. But I can > > confirm that perf_mmap() won't allow a buffer to be remotely accessed > > from another CPU. Now there may be other issues than locality which > > I'm missing :) > > The way we 'optimize' context switches between tasks with identical > contexts is to simply swap the context and leave the hardware alone.
Btw., this does not seem to be working very well when the perf context is inherited:
Baseline kernel with no perf context:
aldebaran:~> taskset 1 perf stat --null perf bench sched pipe # Running sched/pipe benchmark... # Executed 1000000 pipe operations between two tasks
Total time: 5.024 [sec]
5.024951 usecs/op 199006 ops/sec
with inherited perf contexts:
aldebaran:~> taskset 1 perf stat perf bench sched pipe # Running sched/pipe benchmark... # Executed 1000000 pipe operations between two tasks
Total time: 17.869 [sec]
17.869061 usecs/op 55962 ops/sec
+12.8 usecs of perf switching fat per context switch, on a non-debug kernel on a 2.8GHz CPU :-/
We should declare a hard feature stop until that is improved.
Thanks,
Ingo
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