linux-kernel - Re: [PATCH v7 21/23] sched: Add find_exec

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Date: Thu, 4 Jan 2024 19:12:36 -0800
From: John Stultz <jstultz@...gle.com>
To: Metin Kaya <metin.kaya@....com>
Cc: LKML <linux-kernel@...r.kernel.org>, Joel Fernandes <joelaf@...gle.com>, 
	Qais Yousef <qyousef@...gle.com>, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Valentin Schneider <vschneid@...hat.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Zimuzo Ezeozue <zezeozue@...gle.com>, 
	Youssef Esmat <youssefesmat@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Daniel Bristot de Oliveira <bristot@...hat.com>, Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>, 
	Boqun Feng <boqun.feng@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>, 
	Xuewen Yan <xuewen.yan94@...il.com>, K Prateek Nayak <kprateek.nayak@....com>, 
	Thomas Gleixner <tglx@...utronix.de>, kernel-team@...roid.com
Subject: Re: [PATCH v7 21/23] sched: Add find_exec_ctx helper

On Fri, Dec 22, 2023 at 3:57 AM Metin Kaya <metin.kaya@....com> wrote:
> On 20/12/2023 12:18 am, John Stultz wrote:
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 0c212dcd4b7a..77a79d5f829a 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3896,6 +3896,48 @@ static void activate_blocked_entities(struct rq *target_rq,
> >       }
> >       raw_spin_unlock_irqrestore(&owner->blocked_lock, flags);
> >   }
> > +
> > +static inline bool task_queued_on_rq(struct rq *rq, struct task_struct *task)
> > +{
> > +     if (!task_on_rq_queued(task))
> > +             return false;
> > +     smp_rmb();
> > +     if (task_rq(task) != rq)
> > +             return false;
> > +     smp_rmb();
> > +     if (!task_on_rq_queued(task))
> > +             return false;
>
> * Super-nit: we may want to have empty lines between `if` blocks and
> before/after `smp_rmb()` calls.

Done.

> * I did not understand why we call `task_on_rq_queued(task)` twice.
> Should we have an explanatory comment before the function definition?

Yeah. I'll put a better comment on my todo there.

> > diff --git a/kernel/sched/cpupri.c b/kernel/sched/cpupri.c
> > index 15e947a3ded7..53be78afdd07 100644
> > --- a/kernel/sched/cpupri.c
> > +++ b/kernel/sched/cpupri.c
> > @@ -96,12 +96,17 @@ static inline int __cpupri_find(struct cpupri *cp, struct task_struct *p,
> >       if (skip)
> >               return 0;
> >
> > -     if (cpumask_any_and(&p->cpus_mask, vec->mask) >= nr_cpu_ids)
> > +     if ((p && cpumask_any_and(&p->cpus_mask, vec->mask) >= nr_cpu_ids) ||
> > +         (!p && cpumask_any(vec->mask) >= nr_cpu_ids))
> >               return 0;
> >
> >       if (lowest_mask) {
> > -             cpumask_and(lowest_mask, &p->cpus_mask, vec->mask);
> > -             cpumask_and(lowest_mask, lowest_mask, cpu_active_mask);
> > +             if (p) {
> > +                     cpumask_and(lowest_mask, &p->cpus_mask, vec->mask);
> > +                     cpumask_and(lowest_mask, lowest_mask, cpu_active_mask);
> > +             } else {
> > +                     cpumask_copy(lowest_mask, vec->mask);
> > +             }
>
> I think changes in `cpupri.c` should be part of previous (`sched: Push
> execution and scheduler context split into deadline and rt paths`)
> patch. Because they don't seem to be related with find_exec_ctx()?

So, it's here only because find_exec_ctx() can return null, so we have
to have the null p checks.

I'll think a bit if we can avoid it here.

> > @@ -2169,12 +2175,17 @@ static int find_later_rq(struct task_struct *sched_ctx, struct task_struct *exec
> >   /* Locks the rq it finds */
> >   static struct rq *find_lock_later_rq(struct task_struct *task, struct rq *rq)
> >   {
> > +     struct task_struct *exec_ctx;
> >       struct rq *later_rq = NULL;
> >       int tries;
> >       int cpu;
> >
> >       for (tries = 0; tries < DL_MAX_TRIES; tries++) {
> > -             cpu = find_later_rq(task, task);
> > +             exec_ctx = find_exec_ctx(rq, task);
> > +             if (!exec_ctx)
> > +                     break;
> > +
> > +             cpu = find_later_rq(task, exec_ctx);
> >
>
> Super-nit: this empty line should be removed to keep logically connected
> lines closer.

Done.


> > +#ifdef CONFIG_SCHED_PROXY_EXEC
> > +struct task_struct *find_exec_ctx(struct rq *rq, struct task_struct *p);
> > +#else /* !CONFIG_SCHED_PROXY_EXEC */
> > +static inline
> > +struct task_struct *find_exec_ctx(struct rq *rq, struct task_struct *p)
> > +{
> > +     return p;
> > +}
> > +#endif /* CONFIG_SCHED_PROXY_EXEC */
> >   #endif
>
> Nit: `#ifdef CONFIG_SMP` block becomes bigger after this hunk. We should
> append `/* CONFIG_SMP */` to this line, IMHO.
>

Done.

Thanks for the feedback!
-john