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

[Namhyung Kim]

Hi,

On Thu, Jan 04, 2018 at 02:00:03PM +0000, Liang, Kan wrote:
> > 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.

Oh I can see it in the patch 5, thanks.


> +	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.

Right, I understand it for the non-overwrite mode.

But, for the overwrite mode, my concern was that it might be possible
that it reads a stale head in __read_init() (even after it paused the
ring buffer) and reads an update head in __read_done().  Then it's
gonna miss some records.  I'm not sure whether it reads the same head
in __read_init() and __read_done() by the pause.

Thanks,
Namhyung


> +	/* non-overwirte doesn't pause the ringbuffer */
> +	if (!overwrite)
> +		end = perf_mmap__read_head(map);