linux-kernel - Re: [RFC] perf/perf_events: misleading number of samples due to mmap()

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Date:	Wed, 16 Jun 2010 19:43:34 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	eranian@...il.com
Cc:	Stephane Eranian <eranian@...gle.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, fweisbec@...il.com,
	perfmon2-devel@...ts.sf.net
Subject: Re: [RFC] perf/perf_events: misleading number of samples due to 
 mmap()

On Wed, 2010-06-16 at 18:41 +0200, stephane eranian wrote:
> On Wed, Jun 16, 2010 at 4:52 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Wed, 2010-06-16 at 16:40 +0200, Stephane Eranian wrote:
> >> This leads me to another point. For per-thread sampling, why
> >> do we need to record mmap() events happening *outside* of
> >> the process? I can understand the exception of kernel modules.
> >
> > How does that happen? The per-thread events should be on the per-task
> > context, so another task's mmap() events should never end up there.
> >
> 
> I don't see the test that says the vma does not belong to the current task.
> I also don't see anything in perf_event_mmap_match().
> 
> It does seem to work as you said in recent kernels, though. So I am certainly
> missing something here.

vma's are always part of the current task, its impossible to call mmap()
on another process's address space.

Look at the tail of perf_event_mmap_event(), it does:

        rcu_read_lock();
        cpuctx = &get_cpu_var(perf_cpu_context);
        perf_event_mmap_ctx(&cpuctx->ctx, mmap_event, vma->vm_flags & VM_EXEC);
        ctx = rcu_dereference(current->perf_event_ctxp);
        if (ctx)
                perf_event_mmap_ctx(ctx, mmap_event, vma->vm_flags & VM_EXEC);
        put_cpu_var(perf_cpu_context);
        rcu_read_unlock();

There it traverses the per-cpu context and the per-task context.


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