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

[Sebastian Andrzej Siewior]

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