Re: [PATCH 1/2] perf_events: add support for per-cpu per-cgroup monitoring (v5)

[Peter Zijlstra <peterz@infradead.org>]

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.