Re: sched/fair: which tasks should nr_pref_llc_running be compared against?

[Chen Yu <chen.yu@linux.dev>]

On Wed, Sep 09, 2026 at 10:47:11AM -0700, Tim Chen wrote:
> On Wed, 2026-09-09 at 19:40 +0800, Chen Yu wrote:
> > Yes, I think this version looks good now. While looking back at Xusheng's proposal,
> > I noticed there is another option:
> > if (env->src_rq->nr_pref_llc_running == env->src_rq->cfs.h_nr_queued) {
> > 	...
> > }
> > May I know why we did not choose this approach, is it because of the following
> > scenario?
> 
> The reason is that a common condition we are trying to avoid in
> alb_break_llc() is the following: We have one task T1 running on cpu preferring
> src LLC and another delay queued task T2 not preferring src LLC and delayed queued.
> 
> - runnable domain (current fix): h_nr_runnable == 1, nr_pref_llc_running == 1 → equal → alb_break_llc() true → suppress.
>   Correct: the only thing actually running here wants to be here; don't rip it away.
> - h_nr_queued alternative: h_nr_queued == 2, nr_pref(queued) == 1 → not equal → alb_break_llc() false → proceed to active balance, 
>   which would then break T1's locality to relieve an "imbalance"
>   that is really just a sleeping T2.
> 
>

Got it, I see.
 
> > Suppose there are 3 queued tasks: p1 and p2 prefer the src_rq, while p3 is a delayed
> > task that also prefers src_rq. In the current implementation, nr_pref_llc_running is 3
> > and h_nr_runnable is 2, so alb_break_llc() might return false. As a result, active load
> > balance would be triggered, and p1 or p2 might be migrated away, which is undesirable.
> > However, would this still be a problem after Lu Wang's active load balance guard patch
> > has been applied?
> > https://lore.kernel.org/lkml/20260903020656.3793626-1-wanglu.priv@gmail.com/
> 
> Lu Wang's patch only mitigate the migrate_llc case but not other migration reasons.
> We shouldn't have done active balance in the example I gave if we are doing
> migration for other non migrate_llc reasons.
>

Indeed. Got it. I've launched some santify tests, will report back later.

thanks,
Chenyu