linux-kernel - Re: [PATCH 2/5] sched: Add Lazy preemption model

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Message-ID: <20241025104259.NHrExSxO@linutronix.de>
Date: Fri, 25 Oct 2024 12:42:59 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, ankur.a.arora@...cle.com,
	efault@....de, tglx@...utronix.de, mingo@...nel.org
Subject: Re: [PATCH 2/5] sched: Add Lazy preemption model

On 2024-10-15 20:07:26 [+0530], Shrikanth Hegde wrote:
> 
> 
> On 10/7/24 13:16, Peter Zijlstra wrote:
> > Change fair to use resched_curr_lazy(), which, when the lazy
> > preemption model is selected, will set TIF_NEED_RESCHED_LAZY.
> > 
> > This LAZY bit will be promoted to the full NEED_RESCHED bit on tick.
> > As such, the average delay between setting LAZY and actually
> > rescheduling will be TICK_NSEC/2.
> 
> I didn't understand the math here. How?

If you set the LAZY bit you wait until sched_tick() which fires and this
happens every TICK_NSEC. In extreme case the timer fires either
immediately (right after setting the bit) or after TICK_NSEC (because it
just fired so it takes another TICK_NSEC). Given those two, assuming the
average would be in the middle.

> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1103,6 +1106,32 @@ void resched_curr(struct rq *rq)
> >   	__resched_curr(rq, TIF_NEED_RESCHED);
> >   }
> > +#ifdef CONFIG_PREEMPT_DYNAMIC
> > +static DEFINE_STATIC_KEY_FALSE(sk_dynamic_preempt_lazy);
> > +static __always_inline bool dynamic_preempt_lazy(void)
> > +{
> > +	return static_branch_unlikely(&sk_dynamic_preempt_lazy);
> > +}
> > +#else
> > +static __always_inline bool dynamic_preempt_lazy(void)
> > +{
> > +	return IS_ENABLED(PREEMPT_LAZY);
> 
> 
> This should be CONFIG_PREEMPT_LAZY no?

Correct.

Sebastian