> 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.li...@intel.com wrote: > > > > From: Kan Liang <kan.li...@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. + 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.
