Date: Thu, 15 Jan 2009 12:21:00 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Paul Mackerras <paulus@samba.org>
Cc: linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [RFC PATCH] perf_counter: Add support for pinned and exclusive
	counter groups
Message-ID: <20090115112100.GC22850@elte.hu>
References: <18797.44875.470975.535175@cargo.ozlabs.ibm.com> <20090114095743.GA1389@elte.hu> <18798.44766.734357.560006@cargo.ozlabs.ibm.com>
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* Paul Mackerras <paulus@samba.org> wrote:

> Ingo Molnar writes:
> 
> > (btw., the percpu allocation code seems to have bitrotten a bit - the 
> > logic around perf_reserved_percpu looks wrong and somewhat complicated.)
> 
> Yes, it has.  I don't think it was ever fixed to apply only to hardware 
> counters when software counters were added.
> 
> Side question - were you intending to make the various software counters 
> able to act as interrupting counters?  That will mean e.g. that anything 
> that increments current->maj_flt or current->min_flt (i.e. 
> do_page_fault, __get_user_pages) will need to check if that causes any 
> counter to overflow, which could be a bit invasive.

Yes, eventually my intention was to bring all the sw counters up to that 
level, and allow system sampling via sw counter overflows. It's an 
arguably powerful concept.

I first wanted to see where they all go though, and how many of them we 
want, and how nuanced we want to make them.

Initially we can sample via __builtin_return_address(0) addresses, or 
pt_regs(current)->rip or so. Later on we could use save_stack_trace() 
perhaps, for a more vectored sample.

It would result in some rather cool kerneltop output: we could see a 
profile of which functions generate pagefaults, or which places generate 
context-switches, which places migrate a lot, etc.

> > Again, which restrictions users/developers are more willing to live 
> > with will be shown in actual usage of these facilities.
> 
> Indeed.

So my approach was always to have a good guess about what the best usage 
pattern is, but also to not be rigid and ignore/exclude other usecases. 
The main principle of this subsystem is to be maximally useful in 
performance analysis, via modern and flexible abstractions. Pinning and 
exclusivity fits into that just fine - they are mechanism to improve the 
quality of the statistics.

	Ingo
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