Re: [PATCH updated v2] sched/fair: core wide cfs task priority comparison

[Aaron Lu]

On Thu, May 14, 2020 at 03:02:48PM +0200, Peter Zijlstra wrote:
> On Fri, May 08, 2020 at 08:34:57PM +0800, Aaron Lu wrote:
> > With this said, I realized a workaround for the issue described above:
> > when the core went from 'compatible mode'(step 1-3) to 'incompatible
> > mode'(step 4), reset all root level sched entities' vruntime to be the
> > same as the core wide min_vruntime. After all, the core is transforming
> > from two runqueue mode to single runqueue mode... I think this can solve
> > the issue to some extent but I may miss other scenarios.
> 
> A little something like so, this syncs min_vruntime when we switch to
> single queue mode. This is very much SMT2 only, I got my head in twist
> when thikning about more siblings, I'll have to try again later.

Thanks a lot for the patch, I now see that "there is no need to adjust
every se's vruntime". :-)

> This very much retains the horrible approximation of S we always do.
> 
> Also, it is _completely_ untested...

I've been testing it.

One problem below.

> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4293,10 +4281,11 @@ static struct task_struct *
>  pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  {
>  	struct task_struct *next, *max = NULL;
> +	int old_active = 0, new_active = 0;
>  	const struct sched_class *class;
>  	const struct cpumask *smt_mask;
> -	int i, j, cpu;
>  	bool need_sync = false;
> +	int i, j, cpu;
>  
>  	cpu = cpu_of(rq);
>  	if (cpu_is_offline(cpu))
> @@ -4349,10 +4338,14 @@ pick_next_task(struct rq *rq, struct tas
>  		rq_i->core_pick = NULL;
>  
>  		if (rq_i->core_forceidle) {
> +			// XXX is_idle_task(rq_i->curr) && rq_i->nr_running ??
>  			need_sync = true;
>  			rq_i->core_forceidle = false;
>  		}
>  
> +		if (!is_idle_task(rq_i->curr))
> +			old_active++;
> +
>  		if (i != cpu)
>  			update_rq_clock(rq_i);
>  	}
> @@ -4463,8 +4456,12 @@ next_class:;
>  
>  		WARN_ON_ONCE(!rq_i->core_pick);
>  
> -		if (is_idle_task(rq_i->core_pick) && rq_i->nr_running)
> -			rq_i->core_forceidle = true;
> +		if (is_idle_task(rq_i->core_pick)) {
> +			if (rq_i->nr_running)
> +				rq_i->core_forceidle = true;
> +		} else {
> +			new_active++;
> +		}
>  
>  		if (i == cpu)
>  			continue;
> @@ -4476,6 +4473,16 @@ next_class:;
>  		WARN_ON_ONCE(!cookie_match(next, rq_i->core_pick));
>  	}
>  
> +	/* XXX SMT2 only */
> +	if (new_active == 1 && old_active > 1) {

There is a case when incompatible task appears but we failed to 'drop
into single-rq mode' per the above condition check. The TLDR is: when
there is a task that sits on the sibling rq with the same cookie as
'max', new_active will be 2 instead of 1 and that would cause us missing
the chance to do a sync of core min_vruntime.

This is how it happens:
1) 2 tasks of the same cgroup with different weight running on 2 siblings,
   say cg0_A with weight 1024 bound at cpu0 and cg0_B with weight 2 bound
   at cpu1(assume cpu0 and cpu1 are siblings);
2) Since new_active == 2, we didn't trigger min_vruntime sync. For
   simplicity, let's assume both siblings' root cfs_rq's min_vruntime and
   core_vruntime are all at 0 now;
3) let the two tasks run a while;
4) a new task cg1_C of another cgroup gets queued on cpu1. Since cpu1's
   existing task has a very small weight, its cfs_rq's min_vruntime can
   be much larger than cpu0's cfs_rq min_vruntime. So cg1_C's vruntime is
   much larger than cg0_A's and the 'max' of the core wide task
   selection goes to cg0_A;
5) Now I suppose we should drop into single-rq mode and by doing a sync
   of core min_vruntime, cg1_C's turn shall come. But the problem is, our
   current selection logic prefer not to waste CPU time so after decides
   cg0_A as the 'max', the sibling will also do a cookie_pick() and
   get cg0_B to run. This is where problem asises: new_active is 2
   instead of the expected 1.
6) Due to we didn't do the sync of core min_vruntime, the newly queued
   cg1_C shall wait a long time before cg0_A's vruntime catches up.

One naive way of precisely determine when to drop into single-rq mode is
to track how many tasks of a particular tag exists and use that to
decide if the core is in compatible mode(all tasks belong to the same
cgroup, IOW, have the same core_cookie) or not and act accordingly,
except that: does this sound too complex and inefficient?...

> +		/*
> +		 * We just dropped into single-rq mode, increment the sequence
> +		 * count to trigger the vruntime sync.
> +		 */
> +		rq->core->core_sync_seq++;
> +	}
> +	rq->core->core_active = new_active;
> +
>  done:
>  	set_next_task(rq, next);
>  	return next;