Re: [PATCH] sched/core: An optimization of pick_next_task() not sure

[Tao Zhou]

On Wed, Aug 18, 2021 at 12:45:17AM +0800, Tao Zhou wrote:

> Hi Peter,
> 
> On Mon, Aug 16, 2021 at 08:52:39PM +0200, Peter Zijlstra wrote:
> 
> > On Mon, Aug 16, 2021 at 11:44:01PM +0800, Tao Zhou wrote:
> > > When find a new candidate max, wipe the stale and start over.
> > > Goto again: and use the new max to loop to pick the the task.
> > > 
> > > Here first want to get the max of the core and use this new
> > > max to loop once to pick the task on each thread.
> > > 
> > > Not sure this is an optimization and just stop here a little
> > > and move on..
> > > 
> > 
> > Did you find this retry was an issue on your workload? Or was this from
> > reading the source?
> 
> Thank you for your reply. Sorry for my late reply.
> This was from reading the source..
> 
> > 
> > > ---
> > >  kernel/sched/core.c | 52 +++++++++++++++++----------------------------
> > >  1 file changed, 20 insertions(+), 32 deletions(-)
> > > 
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index 20ffcc044134..bddcd328df96 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -5403,7 +5403,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> > >  	const struct sched_class *class;
> > >  	const struct cpumask *smt_mask;
> > >  	bool fi_before = false;
> > > -	int i, j, cpu, occ = 0;
> > > +	int i, cpu, occ = 0;
> > >  	bool need_sync;
> > >  
> > >  	if (!sched_core_enabled(rq))
> > > @@ -5508,11 +5508,27 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> > >  	 * order.
> > >  	 */
> > >  	for_each_class(class) {
> > > -again:
> > > +		struct rq *rq_i;
> > > +		struct task_struct *p;
> > > +
> > >  		for_each_cpu_wrap(i, smt_mask, cpu) {
> > > -			struct rq *rq_i = cpu_rq(i);
> > > -			struct task_struct *p;
> > > +			rq_i = cpu_rq(i);
> > > +			p = pick_task(rq_i, class, max, fi_before);
> > > +			/*
> > > +			 * If this new candidate is of higher priority than the
> > > +			 * previous; and they're incompatible; pick_task makes
> > > +			 * sure that p's priority is more than max if it doesn't
> > > +			 * match max's cookie. Update max.
> > > +			 *
> > > +			 * NOTE: this is a linear max-filter and is thus bounded
> > > +			 * in execution time.
> > > +			 */
> > > +			if (!max || !cookie_match(max, p))
> > > +				max = p;
> > > +		}
> > >  
> > > +		for_each_cpu_wrap(i, smt_mask, cpu) {
> > > +			rq_i = cpu_rq(i);
> > >  			if (rq_i->core_pick)
> > >  				continue;
> > >  
> > 
> > This now calls pick_task() twice for each CPU, which seems unfortunate;
> > perhaps add q->core_temp storage to cache that result. Also, since the
> > first iteration is now explicitly about the max filter, perhaps we
> > shouuld move that part of pick_task() into the loop and simplify things
> > further?
> 
> Here is my ugly patch below..
> Not compiled..
> 
> 
> >From b3de16fb6f3e6cd2a8a9f7a579e80df74fb2d865 Mon Sep 17 00:00:00 2001
> From: Tao Zhou <tao.zhou@xxxxxxxxx>
> Date: Wed, 18 Aug 2021 00:07:38 +0800
> Subject: [PATCH] optimize pick_next_task()
> 
> ---
>  kernel/sched/core.c  | 71 +++++++++++++++++++++++++++++++++-----------
>  kernel/sched/sched.h |  1 +
>  2 files changed, 54 insertions(+), 18 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 20ffcc044134..c2a403bacf99 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5380,18 +5380,32 @@ pick_task(struct rq *rq, const struct sched_class *class, struct task_struct *ma
>  	if (cookie_equals(class_pick, cookie))
>  		return class_pick;
>  
> -	cookie_pick = sched_core_find(rq, cookie);
> +	return class_pick;
> +}
>  
> -	/*
> -	 * If class > max && class > cookie, it is the highest priority task on
> -	 * the core (so far) and it must be selected, otherwise we must go with
> -	 * the cookie pick in order to satisfy the constraint.
> -	 */
> -	if (prio_less(cookie_pick, class_pick, in_fi) &&
> -	    (!max || prio_less(max, class_pick, in_fi)))
> -		return class_pick;
> +static task_struct *
> +filter_max_prio(struct rq *rq, struct task_struct *class_pick,
> +				struct task_struct **cookie_pick, struct task_struct *max,
> +				bool in_fi)
> +{
> +	unsigned long cookie = rq->core->core_cookie;
>  
> -	return cookie_pick;
> +	*cookie_pick = NULL;
> +	if (cookie && !cookie_equals(class_pick, cookie)) {
> +		*cookie_pick = sched_core_find(rq, cookie);
> +		/*
> +		 * If class > max && class > cookie, it is the
> +		 * highest priority task on the core (so far)
> +		 * and it must be selected, otherwise we must
> +		 * go with the cookie pick in order to satisfy
> +		 * the constraint.
> +		 */
> +		if (prio_less(cookie_pick, class_pick, in_fi) &&
> +		    (!max || prio_less(max, class_pick, in_fi)))
> +			return class_pick;
> +	}
> +
> +	return NULL;
>  }
>  
>  extern void task_vruntime_update(struct rq *rq, struct task_struct *p, bool in_fi);
> @@ -5508,24 +5522,44 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  	 * order.
>  	 */
>  	for_each_class(class) {
> -again:
> +		struct task_struct *class_pick, *cookie_pick;
> +		struct rq *rq_i;
> +
> +		for_each_cpu_wrap(i, smt_mask, cpu) {
> +			rq_i = cpu_rq(i);
> +			class_pick = pick_task(rq_i, class, max, fi_before);
> +			rq_i->core_temp = class_pick;
> +			/*
> +			 * This sibling doesn't yet have a suitable task to
> +			 * run.
> +			 */
> +			if (!class_pick)
> +				continue;
> +
> +			if (filter_max_prio(rq_i, class_pick, &cookie_pick, max, fi_before))
> +				max = class_pick;
> +		}
> +
>  		for_each_cpu_wrap(i, smt_mask, cpu) {
> -			struct rq *rq_i = cpu_rq(i);
>  			struct task_struct *p;
> +			rq_i = cpu_rq(i);
>  
>  			if (rq_i->core_pick)
>  				continue;
>  
>  			/*
> -			 * If this sibling doesn't yet have a suitable task to
> -			 * run; ask for the most eligible task, given the
> -			 * highest priority task already selected for this
> -			 * core.
> +			 * This sibling doesn't yet have a suitable task to
> +			 * run.
>  			 */
> -			p = pick_task(rq_i, class, max, fi_before);
> -			if (!p)
> +			if (!rq_i->core_temp)
>  				continue;
>  
> +			p = class_pick = rq_i->core_temp;
> +			if (!filter_max_prio(rq_i, class_pick, &cookie_pick, max, fi_before)) {
> +				if (cookie_pick)
> +					p = cookie_pick;
> +			}
> +
>  			if (!is_task_rq_idle(p))
>  				occ++;
>  
> @@ -9024,6 +9058,7 @@ void __init sched_init(void)
>  #ifdef CONFIG_SCHED_CORE
>  		rq->core = NULL;
>  		rq->core_pick = NULL;
> +		rq->core_temp = NULL;
>  		rq->core_enabled = 0;
>  		rq->core_tree = RB_ROOT;
>  		rq->core_forceidle = false;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 14a41a243f7b..2b21a3846b8e 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1089,6 +1089,7 @@ struct rq {
>  	/* per rq */
>  	struct rq		*core;
>  	struct task_struct	*core_pick;
> +	struct task_struct	*core_temp;
>  	unsigned int		core_enabled;
>  	unsigned int		core_sched_seq;
>  	struct rb_root		core_tree;
> -- 
> 2.31.1
> 
> 
> Thanks,
> Tao


Based on the above suggestion and the source. Here is another try.
Compiled.