Re: [PATCH 0/2] sched: Remove sched_class::balance()

[K Prateek Nayak <kprateek.nayak@amd.com>]

Hi Aaron,

On 7/2/2026 5:19 PM, Aaron Lu wrote:
> Assume cpuX and cpuY are siblings, it appears the following happened:
> 
>      cpuX                                      cpuY
> 
>    pick_next_task()
>    goto restart_multi
> 
>    rqX->core_pick = pick_task(rqX)
> 
>    pick_task(rqY)
>      pick_task_fair(rqY)
>        sched_balance_newidle(rqY)
>          raw_spin_rq_unlock(rqY)  // drops core lock
> 
>                                            pick_next_task()
>                                            goto restart_multi
>                                            rqY->core_pick = pick_task(rqY)
>                                            rqX->core_pick = pick_task(rqX)
> 
>                                            if (rqX->curr == rqX->core_pick)
>                                              rqX->core_pick = NULL
> 
> 					   UNLOCK rq_lockp(rqY)
> 
>          raw_spin_rq_lock(rqY)
> 
>    rqY->core_pick = pick_task(rqY)
> 
>    p = rqX->core_pick        // NULL
>    cookie_equals(p, cookie)  // NULL deref

I also feel doing a balance before the core_cookie is finalized can
move tasks wrongly to a particular core, only to make them wait later
because the pick converted on a different core cookie. Stealing via
balance callback still seems like a good option.

Then there is the whole issue of core_pick_seq possibly being updated
by another CPU on the core by the time rq_lock is dropped and grabbed
again. Afaict, anything that drops the core-wide lock should just do
a RETRY_TASK if a new task arrives so everything is re-done withing
a single core-wide lock critical-section.

Up until this cleanup, only fair and ext used the RETRY_TASK
mechanism and fair bypassed the newidle with core-sched enabled so
it never used the RETRY_TASK mechanism with core-sched so I'm still
skeptical on balance for core-sched being done as part of pick.

Maybe Peter will have a way to sort it out after he is back from
vacation if he hasn't got it all figured out already ;-)

-- 
Thanks and Regards,
Prateek