Re: [PATCH V5 1/1] bpf: control events stored in PERF_EVENT_ARRAY maps trace data output when perf sampling

From: Peter Zijlstra
Date: Wed Oct 21 2015 - 05:12:59 EST


On Tue, Oct 20, 2015 at 03:53:02PM -0700, Alexei Starovoitov wrote:
> On 10/20/15 12:22 AM, Kaixu Xia wrote:
> >diff --git a/kernel/events/core.c b/kernel/events/core.c
> >index b11756f..5219635 100644
> >--- a/kernel/events/core.c
> >+++ b/kernel/events/core.c
> >@@ -6337,6 +6337,9 @@ static int __perf_event_overflow(struct perf_event *event,
> > irq_work_queue(&event->pending);
> > }
> >
> >+ if (unlikely(!atomic_read(&event->soft_enable)))
> >+ return 0;
> >+
> > if (event->overflow_handler)
> > event->overflow_handler(event, data, regs);
> > else
>
> Peter,
> does this part look right or it should be moved right after
> if (unlikely(!is_sampling_event(event)))
> return 0;
> or even to other function?
>
> It feels to me that it should be moved, since we probably don't
> want to active throttling, period adjust and event_limit for events
> that are in soft_disabled state.

Depends on what its meant to do. As long as you let the interrupt
happen, I think we should in fact do those things (maybe not the
event_limit), but period adjustment and esp. throttling are important
when the event is enabled.

If you want to actually disable the event: pmu->stop() will make it
stop, and you can restart using pmu->start().

And I suppose you can wrap that with a counter if you need nesting.

I'm not sure if any of that is a viable solution, because the patch
description is somewhat short on the problem statement.

As is, I'm not too charmed with the patch, but lacking a better
understanding of what exactly we're trying to achieve I'm struggling
with proposing alternatives.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/