On Thu, 2010-11-25 at 15:51 +0100, Stephane Eranian wrote:
>
>
> On Thu, Nov 25, 2010 at 12:20 PM, Peter Zijlstra <[email protected]> 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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