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

[Arnaldo Carvalho de Melo]

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?

It doesn't need to go via perf/urgent as there is no current user, but I
could just fix it so that we have more info about its history in the git
commit logs.

- Arnaldo