Messages in this thread | | | Subject | Re: [PATCH 1/2] perf_events: add support for per-cpu per-cgroup monitoring (v5) | From | Peter Zijlstra <> | Date | Thu, 25 Nov 2010 16:02:24 +0100 |
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On Thu, 2010-11-25 at 15:51 +0100, Stephane Eranian wrote: > > > On Thu, Nov 25, 2010 at 12:20 PM, Peter Zijlstra <peterz@infradead.org> wrote: > On Thu, 2010-11-18 at 12:40 +0200, Stephane Eranian wrote: > > @@ -919,6 +945,10 @@ static inline void perf_event_task_sched_in(struct task_struct *task) > > static inline > > void perf_event_task_sched_out(struct task_struct *task, struct task_struct *next) > > { > > +#ifdef CONFIG_CGROUPS > > + atomic_t *cgroup_events = &__get_cpu_var(perf_cgroup_events); > > + COND_STMT(cgroup_events, perf_cgroup_switch(task, next)); > > +#endif > > COND_STMT(&perf_task_events, __perf_event_task_sched_out(task, next)); > > } > > > I don't think that'll actually work, the jump label stuff needs a static > address. > > I did not know that.
Yeah, its unfortunate the fallback code doesn't mandate this :/
> Why not simply: s/perf_task_events/perf_sched_events/ and > increment it > for cgroup events as well? > > But you would need to demultiplex. that's not because perf_sched_events is > set that you want BOTH perf_cgroup_switch() AND perf_event_task_sched_out().
The main purpose of the jump-label stuff is to optimize the function call and conditional into the perf code away, the moment we a function call we might as well do everything, at that point its only a single conditional.
Jump labels are supposed to work like (they don't actually work like this yet):
my_func: asm-foo addr_of_nop: nop5 after_nop: more-asm-foo iret
out_of_line: do-special-foo jmp after_nop
We then keep a section of tuples:
__jump_labels:
&perf_task_events,addr_of_nop
Then when we flip perf_task_events from 0 -> !0 we rewrite the nop5 at addr_of_nop to "jmp out_of_line" (5 bytes on x86, hence nop5), or the reverse on !0 -> 0.
So 1) we need the 'key' (&perf_task_events) to be a static address because the compiler needs to place the address in the special section -- otherwise we can never find the nop location again, this also means per-cpu variables don't make sense, there's only 1 copy of the text.
and 2) the moment we take the out-of-line branch we incur the icache hit and already set up a call, so optimizing away another conditional at the cost of an extra function call doesn't really make sense.
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