Re: PERF_EVENT_IOC_SET_OUTPUT

[Frederic Weisbecker <fweisbec@gmail.com>]

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..  :-)