Re: [PATCH] perf: Update event buffer tail when overwriting old events

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 09, 2013 at 09:52:17PM +0800, Yan, Zheng wrote:
> On 07/09/2013 04:05 PM, Peter Zijlstra wrote:
> > On Tue, Jul 09, 2013 at 03:05:41PM +0800, Yan, Zheng wrote:
> > 
> >> Thank you for your help. I ran the same test, the results for regular case
> >> are much better. But it still has about 1% overhead, probably because we
> >> enlarge the ring_buffer structure, make it less cache friendly.
> >>
> >>       origin    with the patch
> >> AVG    1000      1013
> >> STDEV  13.4      15.0
> > 
> > And this is the !overwrite case, right? I don't suppose you cured the logic
> > Namhyung Kim pointed out? That should affect the overwrite case I suppose since
> > it won't switch to perf_event_output_overwrite().
> 
> yes, it's the overwrite case.

So the most common case is the !overwrite one; we should ensure no significant
regression there. The overwrite case isn't used that much because as you've
found its really hard to use without a valid tail pointer. So I'm not too
bothered about making the overwrite case a _little_ more expensive if that
makes it far more usable.