linux-kernel - Re: [PATCH 1/2] perf mmap: Fix perf backward recording

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Date:   Thu, 2 Nov 2017 14:34:05 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     "Liang, Kan" <kan.liang@...el.com>
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 Kan,

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

Thanks,
Namhyung