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

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jul 02, 2026 at 07:49:19PM +0800, 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

Well, damn :/ That's a nice race. So while we did a lock-break, it does
not trigger RETRY_TASK and continues.

The 'easy' fix is taking a local copy of core_task_seq when we increase
it for the pick, and double checking that is still valid at the end and
then restarting if not.

Except that is susceptible to live-locks. It doesn't have forward
progress guarantees. For that we need to limit the amount of
lock-breaks/newidle invocations.

Bah, let me go poke at this for a bit.