Re: [PATCH v7 2/2] sched/fair: Introduce SIS_CURRENT to wake up short task on current CPU

[Chen Yu]

On 2023-04-26 at 16:03:24 +0200, Peter Zijlstra wrote:
> On Sat, Apr 22, 2023 at 12:08:18AM +0800, Chen Yu wrote:
> 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 4af5799b90fc..46c1321c0407 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6501,6 +6501,46 @@ static int wake_wide(struct task_struct *p)
> >  	return 1;
> >  }
> >  
> > +/*
> > + * Wake up the task on current CPU, if the following conditions are met:
> > + *
> > + * 1. waker A is the only running task on this_cpu
> > + * 3. A is a short duration task (waker will fall asleep soon)
> > + * 4. wakee B is a short duration task (impact of B on A is minor)
> > + * 5. A and B wake up each other alternately
> > + */
> > +static bool
> > +wake_on_current(int this_cpu, struct task_struct *p)
> > +{
> > +	if (!sched_feat(SIS_CURRENT))
> > +		return false;
> > +
> > +	if (cpu_rq(this_cpu)->nr_running > 1)
> > +		return false;
> > +
> > +	/*
> > +	 * If a task switches in and then voluntarily relinquishes the
> > +	 * CPU quickly, it is regarded as a short duration task. In that
> > +	 * way, the short waker is likely to relinquish the CPU soon, which
> > +	 * provides room for the wakee. Meanwhile, a short wakee would bring
> > +	 * minor impact to the target rq. Put the short waker and wakee together
> > +	 * bring benefit to cache-share task pairs and avoid migration overhead.
> > +	 */
> > +	if (!current->se.dur_avg || ((current->se.dur_avg * 8) >= sysctl_sched_min_granularity))
> > +		return false;
> > +
> > +	if (!p->se.dur_avg || ((p->se.dur_avg * 8) >= sysctl_sched_min_granularity))
> > +		return false;
> > +
> > +	if (current->wakee_flips || p->wakee_flips)
> > +		return false;
> > +
> > +	if (current->last_wakee != p || p->last_wakee != current)
> > +		return false;
> > +
> > +	return true;
> > +}
> 
> So I was going to play with this and found I needed to change things up
> since these sysctl's no longer exist in my EEVDF branch.
> 
> And while I can easily do
> 's/sysctl_sched_min_granularity/sysctl_sched_base_slice/', it did make
> me wonder if that's the right value to use.
> 
> min_gran/base_slice is related to how long we want a task to run before
> switching, but that is not related to how long it needs to run to
> establish a cache footprint.
> 
> Would not sched_migration_cost be a better measure to compare against?
> That is also used in task_hot() to prevent migrations.
Yes, thanks for the suggestion, this looks more reasonable. I'll tune on this
and check the result.

thanks,
Chenyu