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? > > > > > 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.
