linux-kernel - Re: [tip:perfcounters/core] perf record/report: Add call graph / call chain profiling

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Message-ID: <20090616080924.GB16229@elte.hu>
Date:	Tue, 16 Jun 2009 10:09:24 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	mingo@...hat.com, hpa@...or.com, paulus@...ba.org, acme@...hat.com,
	linux-kernel@...r.kernel.org, penberg@...helsinki.fi,
	a.p.zijlstra@...llo.nl, efault@....de, arjan@...radead.org,
	tglx@...utronix.de, linux-tip-commits@...r.kernel.org,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>
Subject: Re: [tip:perfcounters/core] perf record/report: Add call graph /
	call chain profiling

* Frederic Weisbecker <fweisbec@...il.com> wrote:

> > @@ -43,11 +44,19 @@ static int		full_paths;
> >  static unsigned long	page_size;
> >  static unsigned long	mmap_window = 32;
> >  
> > +struct ip_chain_event {
> > +	__u16 nr;
> 
> Is it needed to have the nr encoded in the ip_chain? We can 
> already find it by doing kernel + user.

That's a good observation. Since we havent exposed the call-chain 
bits in upstream version of the tools, we could still improve on 
this a little bit.

I think the best would be context separators which occupy a special 
address in some quiet corner of the 64-bit address space.

That way we'd have streams of u64 entries:

   ip-1
   ip-2
   CONTEXT_IRQ
   ip-3
   ip-4
   CONTEXT_SYSCALL
   ip-5
   ip-6

The following contexts IDs would be useful:

  CONTEXT_NMI
  CONTEXT_HARDIRQ
  CONTEXT_SOFTIRQ
  CONTEXT_KERNEL
  CONTEXT_USER
  CONTEXT_GUEST_NMI
  CONTEXT_GUEST_HARDIRQ
  CONTEXT_GUEST_SOFTIRQ
  CONTEXT_GUEST_KERNEL
  CONTEXT_GUEST_USER

The context IDs would occupy some rare and 
unlikely-to-be-allocated-soon corner of the address space - say 
startig at 0x8765432112345000. (and real RIPs would be filtered and 
nudged just outside that space of a handful IDs.)

The advantage would be that this is infinitely flexible and 
extensible - any level of nesting can be expressed without having 
separate fields for nr_hv_guest_irq, etc. It's also pretty fast to 
parse.

Hm?

	Ingo
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