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Subject: Re: [tip:perfcounters/core] perf record/report: Add call graph /
 call chain profiling
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Ingo Molnar <mingo@elte.hu>
Cc: Frederic Weisbecker <fweisbec@gmail.com>, mingo@redhat.com, hpa@zytor.com,
       paulus@samba.org, acme@redhat.com, linux-kernel@vger.kernel.org,
       penberg@cs.helsinki.fi, efault@gmx.de, arjan@infradead.org,
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       Corey Ashford <cjashfor@linux.vnet.ibm.com>
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Date: Wed, 17 Jun 2009 09:37:59 +0200
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On Tue, 2009-06-16 at 10:09 +0200, Ingo Molnar wrote:
> * Frederic Weisbecker <fweisbec@gmail.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.)

Right, that works too, but should we use (u64)-1..-4095 for that? We
already use that range for things like ERR_PTR() so its very unlikely we
have something sensible mapped there.