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 :) > > But like stated, perf_mmap() seems to avoid that issue entirely by not > allowing inherited events that aren't cpu bound. > > Someone please double check this.. :-) -- 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/
