Re: [PATCH 1/6] sched/proxy: Remove superfluous clear_task_blocked_in()

[Peter Zijlstra]

On Fri, May 29, 2026 at 10:47:12AM +0200, Peter Zijlstra wrote:
> On Fri, May 29, 2026 at 12:15:09PM +0530, K Prateek Nayak wrote:
> 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index a125e65c35bb..fe903976fd09 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3764,28 +3764,28 @@ static inline void proxy_reset_donor(struct rq *rq)
> >   */
> >  static inline bool proxy_needs_return(struct rq *rq, struct task_struct *p)
> >  {
> > -	if (!p->is_blocked)
> > -		return false;
> > -
> > -	/*
> > -	 * Typically per __set_task_cpu(), task_cpu(p) == p->wake_cpu.
> > -	 *
> > -	 * However, proxy_set_task_cpu() is such that it preserves the
> > -	 * original cpu in p->wake_cpu while migrating p for proxy reasons
> > -	 * (possibly outside of the allowed p->cpus_ptr).
> > -	 *
> > -	 * Furthermore, migration_cpu_stop() / __migrate_swap_task(), will
> > -	 * only set p->wake_cpu when !p->on_rq, and since here p->on_rq, this
> > -	 * will not apply. But if it did, this check is the safe way around
> > -	 * and would migrate.
> > -	 */
> > -	if (task_cpu(p) == p->wake_cpu)
> > +	if (!task_is_blocked(p))
> >  		return false;
> >  
> >  	scoped_guard(raw_spinlock, &p->blocked_lock) {
> >  		/* Task is waking up; clear any blocked_on relationship */
> >  		__clear_task_blocked_on(p, NULL);
> >  
> > +		/*
> > +		 * Typically per __set_task_cpu(), task_cpu(p) == p->wake_cpu.
> > +		 *
> > +		 * However, proxy_set_task_cpu() is such that it preserves the
> > +		 * original cpu in p->wake_cpu while migrating p for proxy reasons
> > +		 * (possibly outside of the allowed p->cpus_ptr).
> > +		 *
> > +		 * Furthermore, migration_cpu_stop() / __migrate_swap_task(), will
> > +		 * only set p->wake_cpu when !p->on_rq, and since here p->on_rq, this
> > +		 * will not apply. But if it did, this check is the safe way around
> > +		 * and would migrate.
> > +		 */
> > +		if (task_cpu(p) == p->wake_cpu)
> > +			return false;
> > +
> >  		/* If already current, don't need to return migrate */
> >  		if (task_current(rq, p))
> >  			return false;
> 
> Egads, this is terrible. This means all is_blocked tasks always end up
> taking blocked_lock.
> 
> The other suggestion (in the other subthread) was to simply delay this
> patch until the end. That seems far more sensible.
> 
> Anyway, let me go try and find your git tree and see what you ended up
> with.

Argh, github is such an absolute terrible piece of shit :-( It won't
even let me browse git trees because of rate-limiting or something
stupid -- I've done less than 10 clicks on the site and it says I hit a
limit and should wait and/or log in.

Fuckers.