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


    >
    > >
    > > 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.

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