Messages in this thread | | | Date | Tue, 9 Jan 2018 16:12:58 +0900 | From | Namhyung Kim <> | Subject | Re: [PATCH V3 04/12] perf mmap: introduce perf_mmap__read_done |
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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@intel.com wrote: > > > > > From: Kan Liang <kan.liang@intel.com> > > > > > > > > > > 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);
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