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

[Liang, Kan]

> On 2017/10/11 2:23, Wangnan (F) wrote:
> >
> >
> > On 2017/10/11 1:20, kan.liang@xxxxxxxxx wrote:
> >> From: Kan Liang <kan.liang@xxxxxxxxx>
> >>
> >> 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.
> >>
> >> Signed-off-by: Kan Liang <kan.liang@xxxxxxxxx>
> >> ---
> >>   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;
> >> +
> 
> [SNIP]
> 
> >
> > If you really want to avoid record duplication, you need to changes
> > record__mmap_read()'s
> > logic. Now it complains "failed to keep up with mmap data" and avoid
> > dumping data when
> > size of newly generated data is larger than the size of the ring
> > buffer. It is reasonable
> > for forward ring buffer because in this case you lost the head of the
> > first record, the
> > whole ring buffer is unparseable. However, it is wrong in backward
> > case. What you
> > should do in this case is dumping the whole ring buffer.
> >
> 
> I think what you want should be something like this: (not tested)
>

No. That's not what I want.
My test code never trigger the WARN_ONCE.

I think you will see the problem, if you simply run the command as below.
sudo ./perf record -e cycles:P -C0 --overwrite --switch-output=1s

The output size keep increasing. Because the new output always include the old outputs.
What I want is the 'start' and 'end' for the increase, not everything.

> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index ee7d0a8..f621a8e 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -173,7 +173,9 @@ rb_find_range(void *data, int mask, u64 head, u64
> old,
>           return 0;
>       }
> 
> -    return backward_rb_find_range(data, mask, head, start, end);
> +    *start = head;
> +    *end = old;
> +    return 0;
>   }
> 
>   static int
> @@ -199,10 +201,15 @@ record__mmap_read(struct record *rec, struct
> perf_mmap *md,
> 
>       size = end - start;
>       if (size > (unsigned long)(md->mask) + 1) {
> -        WARN_ONCE(1, "failed to keep up with mmap data. (warn only
> once)\n");
> +        if (!backward) {
> +            WARN_ONCE(1, "failed to keep up with mmap data. (warn only
> once)\n");
> 
> -        md->prev = head;
> -        perf_mmap__consume(md, overwrite || backward);
> +            md->prev = head;
> +            perf_mmap__consume(md, overwrite || backward);
> +        } else {
> +            backward_rb_find_range(data, md->mask, head, start, end);
> +            /* FIXME: error processing */
> +        }
>           return 0;
>       }
> 
> 
> Use 'head' and 'old' to locate data position in ring buffer by default.
> If overwrite happen, use
> backward_rb_find_range() to fetch the last available record and dump the
> whole ring buffer.
> 
> Thank you.
>