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

Peter Zijlstra
2009-06-17T07:38:09Z

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.