On Wed, Oct 02, 2013 at 01:27:30PM +0200, Peter Zijlstra 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. > > So counters belonging to prev will then belong to next and vice versa. > This avoids having to read hardware counters, update stats, removes > counters from hardware, and re-program hardware with possible the exact > same set. > > When a child context changes its context (eg, inserts or removes a > counter) we break this swapping because now the contexts don't match > anymore and we have to take the slow and painful way of prodding > hardware.
Ah right, I remember that now. This caused me quite some headaches a few years ago :) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/