Re: [PATCH 03/10] perf tool: new iterfaces to read event from ring buffer

[Wangnan (F)]

On 2017/10/11 3:55, Liang, Kan wrote:
> > On 2017/10/11 3:17, Arnaldo Carvalho de Melo wrote:
> > > Em Wed, Oct 11, 2017 at 03:10:37AM +0800, Wangnan (F) escreveu:
> > > > On 2017/10/11 3:00, Arnaldo Carvalho de Melo wrote:
> > > > > Em Tue, Oct 10, 2017 at 03:36:28PM -0300, Arnaldo Carvalho de Melo
> > escreveu:
> > > > > > Em Tue, Oct 10, 2017 at 03:34:55PM -0300, Arnaldo Carvalho de Melo
> > escreveu:
> > > > > > > Em Tue, Oct 10, 2017 at 06:28:18PM +0000, Liang, Kan escreveu:
> > > > > > > > > Em Tue, Oct 10, 2017 at 10:20:16AM -0700, kan.liang@xxxxxxxxx
> > escreveu:
> > > > > > > > > > From: Kan Liang <kan.liang@xxxxxxxxx>
> > > > > > > > > >
> > > > > > > > > > The perf_evlist__mmap_read only support forward mode. It needs
> > > > > > > > > > a common function to support both forward and backward mode.
> > > > > > > > > > The perf_evlist__mmap_read_backward is buggy.
> > > > > > > > > So, what is the bug? You state that it is buggy, but don't spell
> > > > > > > > > out the bug, please do so.
> > > > > > > > union perf_event *perf_evlist__mmap_read_backward(struct
> > > > > > > > perf_evlist *evlist, int idx) {
> > > > > > > > 	struct perf_mmap *md = &evlist->mmap[idx];  <--- it should
> > be
> > > > > > > > backward_mmap
> > > > > > > >
> > > > > > > > > If it fixes an existing bug, then it should go separate from this
> > patchkit, right?
> > > > > > > > There is no one use perf_evlist__mmap_read_backward. So it
> > doesn't trigger any issue.
> > > > > > > There is no one at the end of your patchkit? Or no user _right
> > > > > > > now_? If there is a user now, lemme see... yeah, no user right
> > > > > > > now, so _that_ is yet another bug, i.e. it should be used, no? If
> > > > > > > this is just a left over, then we should just throw it away, now, its a
> > cleanup.
> > > > > > Wang, can you take a look at these two issues?
> > > > > So it looks leftover that should've been removed by the following cset,
> > right Wang?
> > > > > commit a0c6f451f90204847ce5f91c3268d83a76bde1b6
> > > > > Author: Wang Nan <wangnan0@xxxxxxxxxx>
> > > > > Date:   Thu Jul 14 08:34:41 2016 +0000
> > > > >        perf evlist: Drop evlist->backward
> > > > >        Now there's no real user of evlist->backward. Drop it. We are going to
> > > > >        use evlist->backward_mmap as a container for backward ring buffer.
> > > > Yes, it should be removed, but then there will be no corresponding
> > > > function to perf_evlist__mmap_read(), which read an record from
> > > > forward ring buffer.
> > > > I think Kan wants to become the first user of this function because
> > > > he is trying to make 'perf top' utilizing backward ring buffer. It
> > > > needs perf_evlist__mmap_read_backward(), and he triggers the bug use
> > > > his unpublished patch set.
> > > > I think we can remove it now, let Kan fix and add it back in his 'perf top'
> > > > patch set.
> > > Well, if there will be a user, perhaps we should fix it, as it seems
> > > interesting to have now for, as you said, a counterpart for the
> > > forward ring buffer, and one that we have plans for using soon, right?
> > Right if I understand Kan's patch 00/10 correctly. He said:
> >
> > ...
> >      But perf top need to switch to overwrite backward mode for good
> >      performance.
> > ...
> Yes, it will be used for perf top optimization.
> That will be great if you can fix it.

You can fix it and post your fix together with your perf top
patch set. I can write the code but I can't test it because
there's no user now.

> I still think it's a good idea to have common interfaces for both perf record
> and other tools like perf top.
> rb_find_range/backward_rb_find_range could be shared.
> The mmap read codes are similar.

Agree. Please keep going.

> Thanks,
> Kan