linux-kernel - Re: perf_counter: Track all mmaps, heap and stack extensions

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Date:	Sun, 2 Aug 2009 21:46:37 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Anton Blanchard <anton@...ba.org>, paulus@...ba.org,
	fweisbec@...il.com, rdreier@...co.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: perf_counter: Track all mmaps, heap and stack extensions


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Tue, 2009-07-28 at 18:46 +1000, Anton Blanchard wrote:
> > Hi,
> > 
> > Right now perf_counter only logs executable mmaps. While this is enough
> > for instruction profiling, at some stage we are also going to want
> > to do data profiling. This will require us to log non-executable mmaps as
> > well as stack and heap extensions.
> > 
> > Why would we care about heap and stack? A few examples:
> > 
> > 1. We can monitor TLB miss rates to suggest what regions of memory should be
> >    put into hugepages.
> >  
> > 2. We can look into various TLB miss issues. On PowerPC a data prefetch
> >    that goes to an unmapped area takes a significant amount of time (it
> >    initiates a tablewalk that may take 40+ cycles). With accurate mapping
> >    data we can catch areas of code with bad prefetch instructions.
> 
> Agreed.
> 
> > Taking it a bit further, since in some sense perf_counter is a channel for
> > getting events out to userspace, I wonder if we could solve Roland's RDMA
> > address space unmap issue with perf_counter:
> > 
> > http://patchwork.kernel.org/patch/37267/
> 
> Yeah, saw some of that, Andrew has some good points there. Haven't had
> time to see the reply yet.
> 
> Not sure its a good match, but if so, very nice.
> 
> > Below is a dodgy hack I've been using to prototype tracking of 
> > all mmaps and heap/stack extensions. Naturally we'd want a 
> > perf_counter feature to turn this on and keep instruction 
> > profiles more compact. We'd also want munmap events.
> 
> I used to have an munmap hook in there at some point. We could 
> restore that.
> 
> > Thoughts?
> 
> Seems like a good direction, go for it ;-)

Seconded! Please submit the full patch once you are happy with it, 
with some tools/perf/ variant/feature that makes use of it and we 
are golden.

	Ingo
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