Re: [PATCH V3 04/12] perf mmap: introduce perf_mmap__read_done

[Namhyung Kim]

Hi Kan,

On Wed, Jan 03, 2018 at 02:15:38PM +0000, Liang, Kan wrote:
> > Hello,
> > 
> > On Thu, Dec 21, 2017 at 10:08:46AM -0800, kan.liang@xxxxxxxxx wrote:
> > > From: Kan Liang <kan.liang@xxxxxxxxx>
> > >
> > > The direction of overwrite mode is backward. The last mmap__read_event
> > > will set tail to map->prev. Need to correct the map->prev to head
> > > which is the end of next read.
> > 
> > Why do you update the map->prev needlessly then?  I think we don't need it
> > for overwrite/backward mode, right?
> 
> The map->prev is needless only when the overwrite does really happen in ringbuffer.
> In a light load system or with big ringbuffer, the unprocessed data will not be
> overwritten. So it's necessary to keep an pointer to indicate the last position.
> 
> Overwrite mode is backward, but the event processing is always forward.
> So map->prev has to be updated in __read_done().

Yep, I meant that updating map->prev in every perf_mmap__read_event()
is unnecessary for the overwrite mode.  It only needs to be set in
perf_mmap__read_done(), right?


> 
> > 
> > Also I guess the current code might miss some events since the head can be
> > different between _read_init() and _read_done(), no?
> > 
> 
> The overwrite mode requires the ring buffer to be paused during processing.
> The head is unchanged between __read_init() and __read_done().

Ah, ok then.  Maybe we could read the head once, and use it during processing.

Thanks,
Namhyung


> 
> The event during the pause should be missed. But I think it has little impact for
> the accuracy of the snapshot and can be tolerant for perf top.
> I mentioned it in the change log of patch 11/12.
> I also removed the lost events checking for perf top.