Re: [PATCH RFC] remove jump_label optimization for perf sched events

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Thu, 2011-11-17 at 15:24 +0200, Avi Kivity wrote:
> On 11/17/2011 03:10 PM, Peter Zijlstra wrote:
> > On Thu, 2011-11-17 at 15:00 +0200, Gleb Natapov wrote:
> >
> > > > That said, I'd much rather throttle this particular jump label than
> > > > remove it altogether, some people really don't like all this scheduler
> > > > hot path crap.
> > > What about moving perf_event_task_sched() to sched_(in|out)_preempt_notifiers?
> > > preempt notifiers checking is already on the scheduler hot path, so no
> > > additional overhead for perf case.
> >
> > Same problem really, some people complain about the overhead of preempt
> > notifiers, also not all kernels have those in. 
> 
> We could combine the two, sort-circuit preempt notifiers with jump
> labels if empty && not much activity on them.

Jump-labels are still more efficient, also I don't much like preempt
notifiers.

> > Futhermore I loathe notifier lists because they obscure wtf is done.
> 
> That's life in a general purpose kernel, if everyone gets their hook in
> to keep their code clean, the scheduler will bloat.

Uhm, no. The bloat isn't different, the only difference is you can
actually see it. So I very much prefer direct hooks.

> An advantage of preempt notifiers is that you can make the perf code
> modular.

Yeah, and you know I loathe modules even more.