Re: [PATCH 04/30] sched: Add sched_class->switching_to() and expose check_class_changing/changed()

Phil Auld
2024-06-21T16:53:54Z

On Tue, Jun 18, 2024 at 11:17:19AM -1000 Tejun Heo wrote:
> When a task switches to a new sched_class, the prev and new classes are
> notified through ->switched_from() and ->switched_to(), respectively, after
> the switching is done.
>
> A new BPF extensible sched_class will have callbacks that allow the BPF
> scheduler to keep track of relevant task states (like priority and cpumask).
> Those callbacks aren't called while a task is on a different sched_class.
> When a task comes back, we wanna tell the BPF progs the up-to-date state

"wanna" ? How about "want to"?

That makes me wanna stop reading right there... :)

> before the task gets enqueued, so we need a hook which is called before the
> switching is committed.
>
> This patch adds ->switching_to() which is called during sched_class switch
> through check_class_changing() before the task is restored. Also, this patch
> exposes check_class_changing/changed() in kernel/sched/sched.h. They will be
> used by the new BPF extensible sched_class to implement implicit sched_class
> switching which is used e.g. when falling back to CFS when the BPF scheduler
> fails or unloads.
>
> This is a prep patch and doesn't cause any behavior changes. The new
> operation and exposed functions aren't used yet.
>
> v3: Refreshed on top of tip:sched/core.
>
> v2: Improve patch description w/ details on planned use.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Reviewed-by: David Vernet <dvernet@meta.com>
> Acked-by: Josh Don <joshdon@google.com>
> Acked-by: Hao Luo <haoluo@google.com>
> Acked-by: Barret Rhoden <brho@google.com>
> ---
> kernel/sched/core.c | 12 ++++++++++++
> kernel/sched/sched.h | 3 +++
> kernel/sched/syscalls.c | 1 +
> 3 files changed, 16 insertions(+)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 48f9d00d0666..b088fbeaf26d 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2035,6 +2035,17 @@ inline int task_curr(const struct task_struct *p)
> return cpu_curr(task_cpu(p)) == p;
> }
>
> +/*
> + * ->switching_to() is called with the pi_lock and rq_lock held and must not
> + * mess with locking.
> + */
> +void check_class_changing(struct rq *rq, struct task_struct *p,
> + const struct sched_class *prev_class)
> +{
> + if (prev_class != p->sched_class && p->sched_class->switching_to)
> + p->sched_class->switching_to(rq, p);
> +}

Does this really need wrapper? The compiler may help but it doesn't seem to
but you're doing a function call and passing in prev_class just to do a
simple check. I guess it's not really a fast path. Just seemed like overkill.

I guess I did read past the commit message ...

Cheers,
Phil

> +
> /*
> * switched_from, switched_to and prio_changed must _NOT_ drop rq->lock,
> * use the balance_callback list if you want balancing.
> @@ -7021,6 +7032,7 @@ void rt_mutex_setprio(struct task_struct *p, struct task_struct *pi_task)
> }
>
> __setscheduler_prio(p, prio);
> + check_class_changing(rq, p, prev_class);
>
> if (queued)
> enqueue_task(rq, p, queue_flag);
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index a2399ccf259a..0ed4271cedf5 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2322,6 +2322,7 @@ struct sched_class {
> * cannot assume the switched_from/switched_to pair is serialized by
> * rq->lock. They are however serialized by p->pi_lock.
> */
> + void (*switching_to) (struct rq *this_rq, struct task_struct *task);
> void (*switched_from)(struct rq *this_rq, struct task_struct *task);
> void (*switched_to) (struct rq *this_rq, struct task_struct *task);
> void (*reweight_task)(struct rq *this_rq, struct task_struct *task,
> @@ -3608,6 +3609,8 @@ extern void set_load_weight(struct task_struct *p, bool update_load);
> extern void enqueue_task(struct rq *rq, struct task_struct *p, int flags);
> extern void dequeue_task(struct rq *rq, struct task_struct *p, int flags);
>
> +extern void check_class_changing(struct rq *rq, struct task_struct *p,
> + const struct sched_class *prev_class);
> extern void check_class_changed(struct rq *rq, struct task_struct *p,
> const struct sched_class *prev_class,
> int oldprio);
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index ae1b42775ef9..cf189bc3dd18 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -797,6 +797,7 @@ int __sched_setscheduler(struct task_struct *p,
> __setscheduler_prio(p, newprio);
> }
> __setscheduler_uclamp(p, attr);
> + check_class_changing(rq, p, prev_class);
>
> if (queued) {
> /*
> --
> 2.45.2
>
>

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