Date:	Wed, 23 Apr 2014 14:52:09 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Don Zickus <dzickus@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	Namhyung Kim <namhyung@...nel.org>,
	David Ahern <dsahern@...il.com>
Subject: Re: [PATCH] perf tools: fix processing of pid/tid for mmap records

On Tue, Apr 22, 2014 at 10:45 PM, Don Zickus <dzickus@...hat.com> wrote:
> On Tue, Apr 22, 2014 at 09:50:17PM +0200, Stephane Eranian wrote:
>> On Tue, Apr 22, 2014 at 9:40 PM, Don Zickus <dzickus@...hat.com> wrote:
>> > On Mon, Apr 21, 2014 at 06:06:55PM +0200, Stephane Eranian wrote:
>> >> perf tools: fix processing of pid/tid for mmap records
>> >>
>> >> Mmaps are global to a process (always). Processing them
>> >> per-thread was causing some serious issues in case mmaps
>> >> would overlap. The overlap fixups would only occur in the
>> >> context of the thread which generated the overlapping
>> >> mmap. But that was cause issues later on when a sample
>> >> from another thread would fall into that overlapping
>> >> mmap.
>> >
>> > eh?  You are basically reverting my patch for a similar problem. :-)
>> >
>> > I was running a large multi-threading program (specjbb) and the threads
>> > were not being seperated into their own map'd areas.  So either I had to
>> > lump all threads in to the same pid space or make the change you are
>> > reverting.
>> >
>> I don't understand your problem. The address space is shared by ALL
>> threads of a process. The mmaps are always shared by all threads?
>
> Sure.
>
>
>>
>> > The problem I had with the double pid (as you propose), I would later look
>> > up samples based on pid/tid and there would be _no_ map available because
>> > it was created as a pid/pid pair.  As a result, our c2c program would drop
>> > thousands of samples on the floor (because there was no mapping for the
>> > data address to get the major/minor/inode info).
>>
>> That is your problem. You should only lookup mapping baseds on pid only
>> not pid/tid. Why do you need tid?
>
> Because the function is machine__findnew_thread not
> machine__findnew_map.  I want the thread info.  That includes the tid,
> comm and any other thread specific info.  :-)
>
But then, the two call should be separated: one to get the mapping, one to get
the thread info. What's wrong with this approach?

> The problem was the thread maps were not being shared internally, leading
> to your problem and my problem.
>
> Jiri fixed that.  So now I can request a thread struct based on a pid/tid
> and the map groups all point to the same one created by the pid.  As it
> should.
>
>
>>
>> >
>> > Now I modified our c2c program to lookup samples as pid/pid but now the
>> > maps lost tid info, and I had to hack around that by carrying the tid info
>> > in a private struct.
>> >
>> If you need the tid, then it is okay to carry it on the side. I believe mmap
>> lookups should ONLY use pid. That is how the address space of a process
>> is contructed.
>
> The tid is stored in the thread struct.  So if I have a pointer to the
> thread struct, I shouldn't have to carry the tid on the side.  That was
> the point. :-)  In fact the thread struct is inside the hist_entry which
> is referenced by struct hist.  So now I can easily sort and display tid
> without carrying anything on the side.
>
> But that only works if the pid/tid mappings are setup correctly.  Which I
> believe Jiri has done with his recent patchset.
>
> Cheers,
> Don
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