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

[Andrea Righi]

On Thu, Mar 27, 2025 at 08:56:12AM +0100, Andrea Righi wrote:
> On Wed, Mar 26, 2025 at 02:24:16PM -1000, Tejun Heo wrote:
> > Hello, Andrea.
> > 
> > On Tue, Mar 25, 2025 at 03:00:21PM +0100, Andrea Righi wrote:
> > > @@ -7114,12 +7114,22 @@ __bpf_kfunc void scx_bpf_cpuperf_set(s32 cpu, u32 perf)
> > >  
> > >  	if (ops_cpu_valid(cpu, NULL)) {
> > >  		struct rq *rq = cpu_rq(cpu);
> > > +		struct rq_flags rf;
> > > +		bool rq_unlocked;
> > > +
> > > +		preempt_disable();
> > > +		rq_unlocked = (rq != this_rq()) || scx_kf_allowed_if_unlocked();
> > > +		if (rq_unlocked) {
> > > +			rq_lock_irqsave(rq, &rf);
> > 
> > I don't think this is correct:
> > 
> > - This is double-locking regardless of the locking order and thus can lead
> >   to ABBA deadlocks.
> > 
> > - There's no guarantee that the locked rq is this_rq(). e.g. In wakeup path,
> >   the locked rq is on the CPU that the wakeup is targeting, not this_rq().
> > 
> > Hmm... this is a bit tricky. SCX_CALL_OP*() always knows whether the rq is
> > locked or not. We might as well pass it the currently locked rq and remember
> > that in a percpu variable, so that scx_bpf_*() can always tell whether and
> > which cpu is rq-locked currently. If unlocked, we can grab the rq lock. If
> > the traget cpu is not the locked one, we can either fail the operation (and
> > trigger ops error) or bounce it to an irq work.
> 
> 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.

Thanks,
-Andrea