linux-kernel - Re: [PATCH v2 2/2] kernel/events: Add option to enable counting sideband events in wakeup

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Date:   Fri, 4 Aug 2017 23:43:08 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jiri Olsa <jolsa@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vince Weaver <vincent.weaver@...ne.edu>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] kernel/events: Add option to enable counting
 sideband events in wakeup_events

On 2017/08/04 12:59PM, Peter Zijlstra wrote:
> On Tue, Aug 01, 2017 at 08:14:04PM +0530, Naveen N. Rao wrote:
> > @@ -5974,19 +5976,8 @@ void perf_output_sample(struct perf_output_handle *handle,
> >  		}
> >  	}
> >  
> > +	if (!event->attr.count_sb_events)
> > +		rb_handle_wakeup_events(event, handle->rb);
> >  }
> 
> > +void __always_inline
> > +rb_handle_wakeup_events(struct perf_event *event, struct ring_buffer *rb)
> > +{
> > +	int wakeup_events = event->attr.wakeup_events;
> > +
> > +	if (!event->attr.watermark && wakeup_events) {
> > +		int events = local_inc_return(&rb->events);
> > +
> > +		if (events >= wakeup_events) {
> > +			local_sub(wakeup_events, &rb->events);
> > +			local_inc(&rb->wakeup);
> > +		}
> > +	}
> > +}
> > +
> >  static int __always_inline
> >  __perf_output_begin(struct perf_output_handle *handle,
> >  		    struct perf_event *event, unsigned int size,
> > @@ -197,6 +212,9 @@ __perf_output_begin(struct perf_output_handle *handle,
> >  	 * none of the data stores below can be lifted up by the compiler.
> >  	 */
> >  
> > +	if (unlikely(event->attr.count_sb_events))
> > +		rb_handle_wakeup_events(event, rb);
> > +
> >  	if (unlikely(head - local_read(&rb->wakeup) > rb->watermark))
> >  		local_add(rb->watermark, &rb->wakeup);
> >  
> 
> I'm still slightly uneasy over this.. Yes most of our events are
> samples, so we'd pay the overhead already. But could you still look at
> performance of this, see for example this commit:
> 
>   9ecda41acb97 ("perf/core: Add ::write_backward attribute to perf event")
> 
> we went through a lot of variants to not hurt performance.

Sure. I'll run the tests and get back.

Thanks,
Naveen