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

[Wangnan (F)]

On 2017/10/11 3:50, Liang, Kan wrote:
> > 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.

The existing code never trigger that warning because the size computed
by rb_find_range is never larger than size of ring buffer. After applying
your patch, I believe it will trigger this WARN_ONCE and drop the whole
ring buffer. Please set a smaller ring buffer and try again.

> 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.


This is my test result: add a '-m 1' for 'perf record' for shrinking 
ring buffer,
start a while loop on CPU 0 to increase data rate.

It stops increasing after the ring buffer is full:

$:~/linux/tools/perf$ sudo ./perf record -m1 -e cycles:P -C0 --overwrite 
--switch-output=1s
  Warning: File /home/w00229757/.perfconfig not owned by current user 
or root, ignoring it.
[ perf record: dump data: Woken up 1 times ]
[ perf record: Dump perf.data.2017101212165072 ]
[ perf record: dump data: Woken up 1 times ]
[ perf record: Dump perf.data.2017101212165175 ]
[ perf record: dump data: Woken up 1 times ]
[ perf record: Dump perf.data.2017101212165278 ]
[ perf record: dump data: Woken up 1 times ]
[ perf record: Dump perf.data.2017101212165381 ]
[ perf record: dump data: Woken up 1 times ]
[ perf record: Dump perf.data.2017101212165484 ]
[ perf record: dump data: Woken up 1 times ]
[ perf record: Dump perf.data.2017101212165586 ]
^C[ perf record: Woken up 1 times to write data ]
[ perf record: Dump perf.data.2017101212165653 ]
[ perf record: Captured and wrote 1.013 MB perf.data.<timestamp> ]

$ ls -l ./perf.data*
-rw------- 1 root root  538988 Oct 12 12:16 ./perf.data.2017101212165072
-rw------- 1 root root  538988 Oct 12 12:16 ./perf.data.2017101212165175
-rw------- 1 root root  538988 Oct 12 12:16 ./perf.data.2017101212165278
-rw------- 1 root root  538988 Oct 12 12:16 ./perf.data.2017101212165381
-rw------- 1 root root  538988 Oct 12 12:16 ./perf.data.2017101212165484
-rw------- 1 root root  538988 Oct 12 12:16 ./perf.data.2017101212165586
-rw------- 1 root root 1067812 Oct 12 12:16 ./perf.data.2017101212165653

You see the result keep getting larger because the ring buffer
is never full in your case.

Thank you.