linux-kernel - Re: perf: Translating mmap2 ids into socket info?

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Date:	Wed, 22 Oct 2014 21:55:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	Don Zickus <dzickus@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, eranian@...gle.com,
	Andi Kleen <andi@...stfloor.org>, jolsa@...hat.com,
	jmario@...hat.com, rfowles@...hat.com
Subject: Re: perf:  Translating mmap2 ids into socket info?

On Wed, Oct 22, 2014 at 03:15:53PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Wed, Oct 22, 2014 at 02:42:26PM -0300, Arnaldo Carvalho de Melo escreveu:
> > Em Wed, Oct 22, 2014 at 06:45:10PM +0200, Peter Zijlstra escreveu:
> > > On Wed, Oct 22, 2014 at 12:20:26PM -0400, Don Zickus wrote:
> > > > Our cache-to-cache tool noticed the slowdown but we couldn't understand
> > > > why because we had falsely assumed the memory was allocated on the local
> > > > node but instead it was on the remote node.
>   
> > > But in general, you can never say for user memory, since that has the
> > > process page table mapping in between, the user virtual address is
> > > unrelated to backing (and can change frequently and without
> > > notification).
>   
> > > Therefore the mmap(2) information is useless for this, it only concerns
> > > user memory.
>  
> > So what you are saying is that it is difficult to have some sort of
> > mechanism that an mmap moved from one node to another, when that
> > happens, i.e. a new tracepoint for that?
> 
> Humm, so, after reading a bit more, I see that we can figure out which
> ranges are in which NUMA nodes (like shown in dmesg), and we have the
> phys addr, so is this just a matter of perf tooling complement the
> information the kernel provides on mmap2 (and the hardware provides on
> those events)?

We explicitly do not expose the phys address of user space pages. So
nope.
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