Messages in this thread | | | Date | Thu, 15 Jan 2009 12:21:00 +0100 | From | Ingo Molnar <> | Subject | Re: [RFC PATCH] perf_counter: Add support for pinned and exclusive counter groups |
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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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