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Date:   Thu, 2 Nov 2017 13:25:08 +0000
From:   "Liang, Kan" <kan.liang@...el.com>
To:     Namhyung Kim <namhyung@...nel.org>
CC:     "Wangnan (F)" <wangnan0@...wei.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "acme@...nel.org" <acme@...nel.org>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "kernel-team@....com" <kernel-team@....com>
Subject: RE: [PATCH 1/2] perf mmap: Fix perf backward recording

Hi Namhyung,

> On Wed, Nov 01, 2017 at 04:22:53PM +0000, Liang, Kan wrote:
> > > On 2017/11/1 21:57, Liang, Kan wrote:
> > > >> On 2017/11/1 20:00, Namhyung Kim wrote:
> > > >>> On Wed, Nov 01, 2017 at 06:32:50PM +0800, Wangnan (F) wrote:
> > > > There are only four test cases which set overwrite,
> > > > sw-clock,task-exit, mmap-basic, backward-ring-buffer.
> > > > Only backward-ring-buffer is 'backward overwrite'.
> > > > The rest three are all 'forward overwrite'. We just need to set
> > > > write_backward to convert them to 'backward overwrite'.
> > > > I think it's not hard to clean up.
> > >
> > > If we add a new rule that overwrite ring buffers are always backward
> > > then it is not hard to cleanup. However, the support of forward
> > > overwrite ring buffer has a long history and the code is not written
> > > by me. I'd like to check if there is some reason to keep support this
> configuration?
> > >
> >
> > As my observation, currently, there are no perf tools which support
> > 'forward overwrite'.
> > There are only three test cases (sw-clock, task-exit, mmap-basic)
> > which is set to 'forward overwrite'. I don’t see any reason it cannot
> > be changed to 'backward overwrite'
> >
> > Arnaldo? Jirka? Kim?
> >
> > What do you think?
> 
> I think sw-clock, task-exit and mmap-basic test cases can be changed to use
> the forward non-overwrite mode.
> 
> The forward overwrite might be used by externel applications accessing the
> ring buffer directly but not needed for perf tools IMHO.  

The proposal is only for perf tool, not kernel. So external applications can still
use forward overwrite to access the ring buffer.

> Let's keep the code simpler as much as possible.

Agree.
Now, there are too many options to access the ring buffer. Not all of them are
supported.
I think we should only keep the crucial options (overwrite/non-overwrite), clearly
define them in the document and cleanup the code.

Also, perf record doesn't use the generic interface (e.g. perf_evlist__mmap*) as other
tools to access ring buffer. Because the generic interface is hardcoded to only support
forward non-overwrite. We should cleanup it as well. But that could be done later
separately.

Thanks,
Kan

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