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SubjectRe: [PATCH 5/6] perf_counter: add more context information

* Peter Zijlstra <> wrote:

> On Thu, 2009-04-02 at 20:18 +0200, Ingo Molnar wrote:
> > * Peter Zijlstra <> wrote:
> >
> > > On Thu, 2009-04-02 at 13:36 +0200, Ingo Molnar wrote:
> > >
> > > > > -#define MAX_STACK_DEPTH 255
> > > > > +#define MAX_STACK_DEPTH 254
> > > > >
> > > > > struct perf_callchain_entry {
> > > > > - u64 nr;
> > > > > + u32 nr, hv, kernel, user;
> > > > > u64 ip[MAX_STACK_DEPTH];
> > > > > };
> > >
> > > Oh, and Paul suggested using u16s right after I send it out. So
> > > I'll either send an update or send a incremental in case you
> > > already applied it.
> >
> > yes, that's probably a good idea. Although u8 might be even better -
> > do we ever want to do more than 256 deep stack vectors? Even those
> > would take quite some time to construct and pass down.
> We'd have to pad it with 4 more bytes to remain u64 aligned,

ok, indeed.

> [...] also, why restrict ourselves. That MAX_STACK_DEPTH limit is
> trivially fixable if indeed someone finds its insufficient.

well .. think about it: walking more than 256 stack frames for every
IRQ event? Getting backtraces like:


does that make much sense _per event_? How do you visualize it?

But yeah ... i could imagine some user-space craziness and since we
want to align to u64 i guess that pretty much settles it to u16.


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