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

[Liang, Kan]

> 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?

Right, for overwrite, only updating the map->prev in perf_mmap__read_done()
is enough.

But for non-overwrite, we have to update map->prev.
It will be used by perf_mmap__consume() later to write the ring buffer tail.
So I specially handle the non-overwrite as below in patch 5/12.
+	event = perf_mmap__read(map, start, end);
+
+	if (!overwrite)
+		map->prev = *start;

> >
> > >
> > > 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.

Yes, it only needs to read head once for overwrite mode.
But for non-overwrite, we have to read the head in every
perf_mmap__read_event(). Because the head is floating.
The non-overwrite is specially handled in patch 5/12 as well.
+	/* non-overwirte doesn't pause the ringbuffer */
+	if (!overwrite)
+		end = perf_mmap__read_head(map);



Thanks,
Kan

> 
> 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.