linux-kernel - Re: [PATCH] perf tools: fix processing of pid/tid for mmap records

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Date:	Tue, 22 Apr 2014 15:40:31 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	linux-kernel@...r.kernel.org, acme@...hat.com, jolsa@...hat.com,
	peterz@...radead.org, mingo@...e.hu, namhyung@...nel.org,
	dsahern@...il.com
Subject: Re: [PATCH] perf tools: fix processing of pid/tid for mmap records

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.

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).

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.

Hopefully Jiri's thread work using pointers will solve both our problems.
:-)  But I can't ack your patch because it will break my work :-(

Cheers,
Don


> 
> The solution to the problem is to handle ALL mmaps as
> occurring in the master thread (pid = tid) and then to
> lookup for thread map using pid as the tid argument.
> This is how samples are looking up for the thread map
> already (notice pid passed twice):
>     
> int perf_event__preprocess_sample(const union perf_event *event,
>                                   struct machine *machine,
>                                   struct addr_location *al,
>                                   struct perf_sample *sample)
> {
>         struct thread *thread = machine__findnew_thread(machine, sample->pid,
>                                                         sample->pid);
> }
>     
> Without this fix, some samples in overlapping regions
> may not be symbolized.
>     
> Signed-off-by: Stephane Eranian <eranian@...gle.com>
> 
> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> index a53cd0b..43cdc0a 100644
> --- a/tools/perf/util/machine.c
> +++ b/tools/perf/util/machine.c
> @@ -1025,9 +1025,9 @@ int machine__process_mmap2_event(struct machine *machine,
>  			goto out_problem;
>  		return 0;
>  	}
> -
> +	/* only look by pid for mmap events */
>  	thread = machine__findnew_thread(machine, event->mmap2.pid,
> -					event->mmap2.tid);
> +					event->mmap2.pid);
>  	if (thread == NULL)
>  		goto out_problem;
>  
> @@ -1073,9 +1073,9 @@ int machine__process_mmap_event(struct machine *machine, union perf_event *event
>  			goto out_problem;
>  		return 0;
>  	}
> -
> +	/* only look by pid for mmap events */
>  	thread = machine__findnew_thread(machine, event->mmap.pid,
> -					 event->mmap.tid);
> +					 event->mmap.pid);
>  	if (thread == NULL)
>  		goto out_problem;
>  
> --
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