Re: perf overlapping maps...

[Don Zickus <dzickus@redhat.com>]

On Mon, Oct 22, 2018 at 06:16:13PM +0200, Jiri Olsa wrote:
> On Mon, Oct 22, 2018 at 10:07:38AM -0400, Don Zickus wrote:
> > (adding Jiri)
> > 
> > On Fri, Oct 19, 2018 at 09:44:01PM -0700, David Miller wrote:
> > > From: David Miller <davem@davemloft.net>
> > > Date: Fri, 19 Oct 2018 21:05:49 -0700 (PDT)
> > > 
> > > > One solution I've come up with is:
> > > > 
> > > > 1) When synthesizing a fork event, set PERF_RECORD_MISC_COMM_EXEC in
> > > >    header->misc.
> > > > 
> > > > 2) Use this to elide the map groups clone in
> > > >    thread__clone_map_groups().
> > > 
> > > Looking into code history, I notice:
> > > 
> > > commit 363b785f3805a2632eb09a8b430842461c21a640
> > > Author: Don Zickus <dzickus@redhat.com>
> > > Date:   Fri Mar 14 10:43:44 2014 -0400
> > > 
> > >     perf tools: Speed up thread map generation
> > > 
> > > and the subsequent:
> > > 
> > > commit 4aa5f4f7bb8bc41cba15bcd0d80c4fb085027d6b
> > > Author: Arnaldo Carvalho de Melo <acme@redhat.com>
> > > Date:   Fri Feb 27 19:52:10 2015 -0300
> > > 
> > >     perf tools: Fix FORK after COMM when synthesizing records for pre-existing threads
> > > 
> > > If Don wanted to have the map cloning to happen for processes without
> > > CLONE_VM, I'm not sure that's right.
> > > 
> > > For real threads, we just take a reference to the map group from
> > > the parent.
> > > 
> > > Don, a quick summary.  If we synthesize a fork event, let's say for an
> > > emacs process.  perf will clone the map groups of the parent bash
> > > shell which invoked emacs.  Via:
> > > 
> > > thread__fork(thread, parent, timestamp)
> > > {
> > >  ...
> > >  thread__clone_map_groups(thread, parent)
> > >  {
> > >    ...
> > >    map_groups__clone(thread, parent->mg)
> > > 
> > > Which is completely bogus.  It brings all of the bash process maps
> > > into the emacs thread map group.  Then we process the emacs mmap2
> > > events, which overlap the bash process maps already cloned into the
> > > emacs map group.  And this make all kinds of erroneous things happen.
> > > 
> > > I'm suggesting to elide the map groups clone in this situation where
> > > we are synthesizing the fork.
> 
> right, this seems correct.. we should only clone parent maps
> for kernel fork event, not when we synthesize.. I'll check
> the solution you proposed and try to come with a patch

Thanks Jiri for helping me out on this!

Cheers,
Don

> 
> > 
> > Hi David,
> > 
> > Honestly, I remember very little of this change other than we ran specjbb
> > which created thousands of threads and we wanted a better way to handle that
> > situation (waiting 15 minutes seemed wrong).
> > 
> > Jiri Olsa is probably more knowledgable about this then I am these days and
> > can work with Joe to re-do the test to verify any suggested changes does not
> > break our intended use case.
> > 
> > Thinking about it more, I am wondering if we did this because we ran the
> > test and it takes about 20 minutes to 'warm up' then we attached perf to the
> > test.  This implies we had to handle the situation where 10K threads already
> > existed hence our optimization.  But I can be wrong.
> > 
> > Your suggestion is probably right and I am sure we can reproduce the
> > scenario to verify things didn't regress.
> 
> I think the fix might actualy speed things up,
> but yes, there could be other report regressions
> 
> thanks,
> jirka