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

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Message-ID: <4EC51636.3030001@redhat.com>
Date:	Thu, 17 Nov 2011 16:12:06 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Gleb Natapov <gleb@...hat.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, Jason Baron <jbaron@...hat.com>,
	rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH RFC] remove jump_label optimization for perf sched events

On 11/17/2011 03:47 PM, Peter Zijlstra wrote:
> 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.

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