linux-kernel - RE: [PATCH 02/10] perf tool: fix: Don't discard prev in backward mode

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Date:   Fri, 13 Oct 2017 12:55:34 +0000
From:   "Liang, Kan" <kan.liang@...el.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
CC:     "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jolsa@...nel.org" <jolsa@...nel.org>,
        "wangnan0@...wei.com" <wangnan0@...wei.com>,
        "hekuang@...wei.com" <hekuang@...wei.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "Hunter, Adrian" <adrian.hunter@...el.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: RE: [PATCH 02/10] perf tool: fix: Don't discard prev in backward
 mode

> Em Tue, Oct 10, 2017 at 10:20:15AM -0700, kan.liang@...el.com escreveu:
> > From: Kan Liang <kan.liang@...el.com>
> >
> > Perf record can switch output. The new output should only store the
> > data after switching. However, in overwrite backward mode, the new
> > output still have the data from old output.
> >
> > At the end of mmap_read, the position of processed ring buffer is
> > saved in md->prev. Next mmap_read should be end in md->prev.
> > However, the md->prev is discarded. So next mmap_read has to process
> > whole valid ring buffer, which definitely include the old processed
> > data.
> >
> > Set the prev as the end of the range in backward mode.
> 
> Do you think this should go together with the rest of this patchkit?
> Probably it should be processed ASAP, i.e. perf/urgent, no?
>

Hi Arnaldo,

After some discussion, Wang Nan proposed a new patch to fix this issue.
https://lkml.org/lkml/2017/10/12/441
I did some tests. It fixed the issue.

Could you please take a look?
If it's OK for you, I think it could be merged separately.

Thanks,
Kan
 
> - Arnaldo
> 
> > Signed-off-by: Kan Liang <kan.liang@...el.com>
> > ---
> >  tools/perf/util/evlist.c | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c index
> > 33b8837..7d23cf5 100644
> > --- a/tools/perf/util/evlist.c
> > +++ b/tools/perf/util/evlist.c
> > @@ -742,13 +742,25 @@ static int
> >  rb_find_range(void *data, int mask, u64 head, u64 old,
> >  	      u64 *start, u64 *end, bool backward)  {
> > +	int ret;
> > +
> >  	if (!backward) {
> >  		*start = old;
> >  		*end = head;
> >  		return 0;
> >  	}
> >
> > -	return backward_rb_find_range(data, mask, head, start, end);
> > +	ret = backward_rb_find_range(data, mask, head, start, end);
> > +
> > +	/*
> > +	 * The start and end from backward_rb_find_range is the range for
> all
> > +	 * valid data in ring buffer.
> > +	 * However, part of the data is processed previously.
> > +	 * Reset the end to drop the processed data
> > +	 */
> > +	*end = old;
> > +
> > +	return ret;
> >  }
> >
> >  /*
> > --
> > 2.5.5