Re: [PATCH 1/4] sched/cache: Keep nr_pref_llc_running in the runnable domain

[Kayra Cizmeci]

Hello :>,

> alb_break_llc() decides whether to break LLC preference during active
> load balance. It does so by testing that every runnable fair task on the
> source rq prefers its LLC:
> 
> 	env->src_rq->nr_pref_llc_running == env->src_rq->cfs.h_nr_runnable
> 
> But the two counters cover different sets. nr_pref_llc_running is updated
> in account_llc_enqueue()/account_llc_dequeue(), next to cfs_rq->nr_queued,
> so it follows queued tasks. h_nr_runnable is updated in set_delayed()/
> clear_delayed() and drops delay-dequeued tasks.

> So under DELAY_DEQUEUE, a preferring task that goes to sleep stays counted
> in nr_pref_llc_running while h_nr_runnable falls. The equality then breaks,
> alb_break_llc() returns false, and active balance is free to pull a task
> off its preferred LLC. Active balance only moves runnable tasks, and this
> is the only LLC check it consults: once the stopper runs, LBF_ACTIVE_LB
> skips the per-task test in can_migrate_task(). The runnable set is the one
> we want.

> Fix it on the counter side. A task should be counted in
> nr_pref_llc_running exactly while it is both queued on its preferred LLC
> (pref_llc_queued) and runnable (!sched_delayed). Define that membership
> once in task_pref_llc_runnable(), and adjust the counter only through
> pref_llc_running_inc()/pref_llc_running_dec() from the four sites that
> change either input: account_llc_enqueue(), account_llc_dequeue(),
> set_delayed() and clear_delayed(). Gating every update on the same
> predicate keeps the delay, wake and dequeue paths from double-counting
> or underflowing; see the comments at those sites for the ordering.

> nr_llc_running and sd->llc_counts are not touched and stay on queued
> semantics.

I have one question tho, can't we combine the checks with h_nr_runnable? On the paper
if we are updating h_nr_runnable we could check if the nr_pref_llc_running can be 
updated and update it if the condition is right. Because, every nr_pref_llc_running enters
h_nr_runnable while not every h_nr_runnable enters nr_pref_llc_running. 

Why instead we just check the nr_pref_llc_running's conditions on task_pref_llc_runnable()
and call these dec and inc functions after the h_nr_runnable updates. Wouldn't it be clear that way?
If possible?

Thanks,
Kayra :_: