Re: [PATCH] sched_ext: Fix missing rq lock in scx_bpf_cpuperf_set()

[Tejun Heo <tj@kernel.org>]

Hello,

On Thu, Mar 27, 2025 at 10:53:39AM +0100, Andrea Righi wrote:
...
> > Hm... that's right, it looks like this requires a bit more work than
> > expected, but saving the currently locked rq might be helpful also for
> > other kfuncs, I'll take a look at this.
> 
> What if we lock the rq in the scx_kf_allowed_if_unlocked() case, and for
> all the other cases we ignore locking if rq == this_rq(). If we need to
> operate on a different rq than the current one we could either defer the
> work or just trigger an ops error. Something like:
> 
> 	if (scx_kf_allowed_if_unlocked()) {
> 		rq_lock_irqsave(rq, &rf);
> 		update_rq_clock(rq);
> 	} else if (rq != this_rq()) {
> 		// defer work or ops error
> 		return;
> 	}
> 
> 	lockdep_assert_rq_held(rq);
> 	rq->scx.cpuperf_target = perf;
> 	cpufreq_update_util(rq, 0);
> 
> 	if (scx_kf_allowed_if_unlocked())
> 		rq_unlock_irqrestore(rq, &rf);
> 
> AFAICS all the current scx schedulers call scx_bpf_cpuperf_set() from
> ops.running(), ops.tick() or ops.init(), so even with the ops error we
> should cover all the existent cases.
> 
> The only unsupported scenario is calling scx_bpf_cpuperf_set() from
> ops.enqueue() / ops.select_cpu(), but maybe we could add the deferred work
> later to handle that if needed.

balance_one() can be called from a sibling CPU when core sched is enabled,
so ttwu isn't the only path where this_rq() test wouldn't work. Even if we
plug all the existing holes and make it work, it feels a bit too fragile to
me. It's something which can easily break inadvertently and cause subtle
failures.

If we don't want to do locked rq tracking, we can always use
schedule_deferred() when any rq is locked too. That's a bit more expensive
tho.

Thanks.

-- 
tejun