Messages in this thread | | | Date | Thu, 11 Jan 2018 21:00:09 +0900 | From | Namhyung Kim <> | Subject | Re: [PATCH V3 04/12] perf mmap: introduce perf_mmap__read_done |
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Hi,
On Tue, Jan 09, 2018 at 03:12:28PM +0000, Liang, Kan wrote: > > > > > > > > > > > > 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. > > > > The only scenario which may cause the different 'head' may be as below. > The 'rb->head' is updated in __perf_output_begin(), but haven’t been > assigned to 'pc->data_head' for perf tool. During this period, the 'paused' > is set and __read_init() reads head. > But this scenario never happens because of the ringbuffer lock.
Which lock did you say?
> > Otherwise, I cannot imagine any other scenarios which may causes the > different 'head' in __read_init() and __read_done() with ringbuffer > paused. Please let me know if there is an example.
Maybe I'm missing something. But I don't know what makes it guarantee to see the updated data_head written by another cpu before the pause.
> > There would be some records miss. But it's only because the ringbuffer > is paused. The head should keep the same.
Hmm.. yes. It's gonna miss some records anyway, then I don't care about it anymore.
> > I also did some tests and dump the 'head' in __read_init() and > __read_done() with ringbuffer paused. I didn't see any difference either.
Maybe on a weak-ordered machine? Not sure though.
But as I said, I don't care anymore since we cannot avoid some missing records.
Thanks, Namhyung
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