linux-kernel - Re: [PATCH] perf_counter: Add alignment-faults and emulation-faults sw events

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Message-ID: <20090718093500.GC9142@elte.hu>
Date:	Sat, 18 Jul 2009 11:35:00 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Anton Blanchard <anton@...ba.org>
Cc:	a.p.zijlstra@...llo.nl, paulus@...ba.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf_counter: Add alignment-faults and
	emulation-faults sw events


* Anton Blanchard <anton@...ba.org> wrote:

> Hi Ingo,
> 
> > Looks useful.
> > 
> > I'm wondering about the enumeration space: in other cases when 
> > some simple event was further refined we went to add a new 
> > perf_type_id and a separate enumeration space, with no limits to 
> > extensibility. We'd have a new 'enum perf_sw_fault_id' space.
> > 
> > Page faults are special anyway, because they carry a 'data' 
> > (faulting address) sample as well.
> > 
> > So i'm wondering how a good, generic enumeration of all things 
> > page faults would look like. If we extend perf_sw_ids linearly 
> > we might lose some structure.
> > 
> > For example major versus minor might be a dimension (bit) in the 
> > enumeration space, so we'd have:
> > 
> >    { major | minor } x { native, unaligned, emulated }
> > 
> > This provides an advantage already: the current 'all' page 
> > faults counter would become the 'major|minor' case in the new 
> > enumeration.
> > 
> > We could still keep around the old events as well for some time, 
> > but the tools would use the new enumeration.
> 
> My initial feeling is that emulation and alignment faults 
> shouldn't roll up into page faults, because that may cause cause 
> someone to think the problem is something to do with translation. 
> I don't have a strong opinion on it however :)

I have no strong feelings either so please pick the variant that 
feels most natural. I do think we should try to generalize it a bit 
and move it into its own enumeration space, out of the generic sw 
counters - do you agree with that?

> Since we are talking about SW events, I thought I'd bring up some 
> ideas I was discussing with Paul the other day. The hardware guys 
> like to build CPI stacks, basically breaking down the CPI into its 
> components (CPI due to TLB misses, CPI due to dcache misses etc). 
> This offers a great high level view of what needs to be fixed in 
> order to improve performance.
> 
> Taking a step back, it would be great if we could have enough SW 
> events and counters to be able to do this at the kernel level. A 
> few events/counters that come to mind are cputime lost due to 
> swap, IO initiated by the process, interrupts and other processes 
> being scheduled. I wonder if the delay accounting code has 
> anything we can reuse for this.
> 
> With these events we could simply run perf stat and instantly see 
> what needs fixing at both the cpu level (via CPI analysis) and at 
> the kernel level (via SW counters).

Yeah, i like this direction.

Mind extending your patch-set in this way, and also do the more 
structured pagefault enumeration thing? (unless you have better 
suggestions)

Thanks,

	Ingo
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