linux-kernel - RE: [PATCH V3 04/12] perf mmap: introduce perf_mmap__read

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Date:   Thu, 11 Jan 2018 14:40:00 +0000
From:   "Liang, Kan" <kan.liang@...el.com>
To:     Namhyung Kim <namhyung@...nel.org>
CC:     "acme@...nel.org" <acme@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "wangnan0@...wei.com" <wangnan0@...wei.com>,
        "jolsa@...nel.org" <jolsa@...nel.org>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "yao.jin@...ux.intel.com" <yao.jin@...ux.intel.com>,
        "kernel-team@....com" <kernel-team@....com>
Subject: RE: [PATCH V3 04/12] perf mmap: introduce perf_mmap__read_done

> 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?
> 
The RCU lock.
> 
> >
> > 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.
>

I think it should be the kernel's responsibility. It's described in the changelog of
Commit 86e7972f690c "perf/ring_buffer: Introduce new ioctl options to
pause and resume the ring-buffer"
 
> 
> >
> > 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.
>

OK.
Thanks for the review.

Thanks,
Kan