Re: [PATCH 02/11] sched_ext: Refactor consume_remote_task()

David Vernet
2024-08-31T04:05:19Z

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On Fri, Aug 30, 2024 at 01:03:46AM -1000, Tejun Heo wrote:
> The tricky p->scx.holding_cpu handling was split across
> consume_remote_task() body and move_task_to_local_dsq(). Refactor such that:
>
> - All the tricky part is now in the new unlink_dsq_and_lock_task_rq() with
> consolidated documentation.
>
> - move_task_to_local_dsq() now implements straightforward task migration
> making it easier to use in other places.
>
> - dispatch_to_local_dsq() is another user move_task_to_local_dsq(). The
> usage is updated accordingly. This makes the local and remote cases more
> symmetric.
>
> No functional changes intended.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---
> kernel/sched/ext.c | 145 ++++++++++++++++++++++++---------------------
> 1 file changed, 76 insertions(+), 69 deletions(-)
>
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 5423554a11af..3facfca73337 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -2109,49 +2109,13 @@ static bool yield_to_task_scx(struct rq *rq, struct task_struct *to)
> * @src_rq: rq to move the task from, locked on entry, released on return
> * @dst_rq: rq to move the task into, locked on return
> *
> - * Move @p which is currently on @src_rq to @dst_rq's local DSQ. The caller
> - * must:
> - *
> - * 1. Start with exclusive access to @p either through its DSQ lock or
> - * %SCX_OPSS_DISPATCHING flag.
> - *
> - * 2. Set @p->scx.holding_cpu to raw_smp_processor_id().
> - *
> - * 3. Remember task_rq(@p) as @src_rq. Release the exclusive access so that we
> - * don't deadlock with dequeue.
> - *
> - * 4. Lock @src_rq from #3.
> - *
> - * 5. Call this function.
> - *
> - * Returns %true if @p was successfully moved. %false after racing dequeue and
> - * losing. On return, @src_rq is unlocked and @dst_rq is locked.
> + * Move @p which is currently on @src_rq to @dst_rq's local DSQ.
> */
> -static bool move_task_to_local_dsq(struct task_struct *p, u64 enq_flags,
> +static void move_task_to_local_dsq(struct task_struct *p, u64 enq_flags,
> struct rq *src_rq, struct rq *dst_rq)
> {
> lockdep_assert_rq_held(src_rq);
>
> - /*
> - * If dequeue got to @p while we were trying to lock @src_rq, it'd have
> - * cleared @p->scx.holding_cpu to -1. While other cpus may have updated
> - * it to different values afterwards, as this operation can't be
> - * preempted or recurse, @p->scx.holding_cpu can never become
> - * raw_smp_processor_id() again before we're done. Thus, we can tell
> - * whether we lost to dequeue by testing whether @p->scx.holding_cpu is
> - * still raw_smp_processor_id().
> - *
> - * @p->rq couldn't have changed if we're still the holding cpu.
> - *
> - * See dispatch_dequeue() for the counterpart.
> - */
> - if (unlikely(p->scx.holding_cpu != raw_smp_processor_id()) ||
> - WARN_ON_ONCE(src_rq != task_rq(p))) {
> - raw_spin_rq_unlock(src_rq);
> - raw_spin_rq_lock(dst_rq);
> - return false;
> - }
> -
> /* the following marks @p MIGRATING which excludes dequeue */
> deactivate_task(src_rq, p, 0);

Not a functional change from the prior patch, but it occurred to me that
if we just deactivate like this then we'll also fire the ops.quiescent()
callback in dequeue_task_scx(). Should we add a check to skip the
dequeue callbacks if p->scx.holding_cpu >= 0?

Cleanup looks great otherwise.

Thanks,
David

> set_task_cpu(p, cpu_of(dst_rq));
> @@ -2170,8 +2134,6 @@ static bool move_task_to_local_dsq(struct task_struct *p, u64 enq_flags,
> dst_rq->scx.extra_enq_flags = enq_flags;
> activate_task(dst_rq, p, 0);
> dst_rq->scx.extra_enq_flags = 0;
> -
> - return true;
> }
>
> #endif /* CONFIG_SMP */
> @@ -2236,28 +2198,69 @@ static bool task_can_run_on_remote_rq(struct task_struct *p, struct rq *rq,
> return true;
> }
>
> -static bool consume_remote_task(struct rq *rq, struct scx_dispatch_q *dsq,
> - struct task_struct *p, struct rq *task_rq)
> +/**
> + * unlink_dsq_and_lock_task_rq() - Unlink task from its DSQ and lock its task_rq
> + * @p: target task
> + * @dsq: locked DSQ @p is currently on
> + * @task_rq: @p's task_rq, stable with @dsq locked
> + *
> + * Called with @dsq locked but no rq's locked. We want to move @p to a different
> + * DSQ, including any local DSQ, but are not locking @task_rq. Locking @task_rq
> + * is required when transferring into a local DSQ. Even when transferring into a
> + * non-local DSQ, it's better to use the same mechanism to protect against
> + * dequeues and maintain the invariant that @p->scx.dsq can only change while
> + * @task_rq is locked, which e.g. scx_dump_task() depends on.
> + *
> + * We want to grab @task_rq but that can deadlock if we try while locking @dsq,
> + * so we want to unlink @p from @dsq, drop its lock and then lock @task_rq. As
> + * this may race with dequeue, which can't drop the rq lock or fail, do a little
> + * dancing from our side.
> + *
> + * @p->scx.holding_cpu is set to this CPU before @dsq is unlocked. If @p gets
> + * dequeued after we unlock @dsq but before locking @task_rq, the holding_cpu
> + * would be cleared to -1. While other cpus may have updated it to different
> + * values afterwards, as this operation can't be preempted or recurse, the
> + * holding_cpu can never become this CPU again before we're done. Thus, we can
> + * tell whether we lost to dequeue by testing whether the holding_cpu still
> + * points to this CPU. See dispatch_dequeue() for the counterpart.
> + *
> + * On return, @dsq is unlocked and @task_rq is locked. Returns %true if @p is
> + * still valid. %false if lost to dequeue.
> + */
> +static bool unlink_dsq_and_lock_task_rq(struct task_struct *p,
> + struct scx_dispatch_q *dsq,
> + struct rq *task_rq)
> {
> - lockdep_assert_held(&dsq->lock); /* released on return */
> + s32 cpu = raw_smp_processor_id();
> +
> + lockdep_assert_held(&dsq->lock);
>
> - /*
> - * @dsq is locked and @p is on a remote rq. @p is currently protected by
> - * @dsq->lock. We want to pull @p to @rq but may deadlock if we grab
> - * @task_rq while holding @dsq and @rq locks. As dequeue can't drop the
> - * rq lock or fail, do a little dancing from our side. See
> - * move_task_to_local_dsq().
> - */
> WARN_ON_ONCE(p->scx.holding_cpu >= 0);
> task_unlink_from_dsq(p, dsq);
> dsq_mod_nr(dsq, -1);
> - p->scx.holding_cpu = raw_smp_processor_id();
> - raw_spin_unlock(&dsq->lock);
> + p->scx.holding_cpu = cpu;
>
> - raw_spin_rq_unlock(rq);
> + raw_spin_unlock(&dsq->lock);
> raw_spin_rq_lock(task_rq);
>
> - return move_task_to_local_dsq(p, 0, task_rq, rq);
> + /* task_rq couldn't have changed if we're still the holding cpu */
> + return likely(p->scx.holding_cpu == cpu) &&
> + !WARN_ON_ONCE(task_rq != task_rq(p));
> +}
> +
> +static bool consume_remote_task(struct rq *this_rq, struct scx_dispatch_q *dsq,
> + struct task_struct *p, struct rq *task_rq)
> +{
> + raw_spin_rq_unlock(this_rq);
> +
> + if (unlink_dsq_and_lock_task_rq(p, dsq, task_rq)) {
> + move_task_to_local_dsq(p, 0, task_rq, this_rq);
> + return true;
> + } else {
> + raw_spin_rq_unlock(task_rq);
> + raw_spin_rq_lock(this_rq);
> + return false;
> + }
> }
> #else /* CONFIG_SMP */
> static inline bool task_can_run_on_remote_rq(struct task_struct *p, struct rq *rq, bool trigger_error) { return false; }
> @@ -2361,7 +2364,8 @@ dispatch_to_local_dsq(struct rq *rq, u64 dsq_id, struct task_struct *p,
> * As DISPATCHING guarantees that @p is wholly ours, we can
> * pretend that we're moving from a DSQ and use the same
> * mechanism - mark the task under transfer with holding_cpu,
> - * release DISPATCHING and then follow the same protocol.
> + * release DISPATCHING and then follow the same protocol. See
> + * unlink_dsq_and_lock_task_rq().
> */
> p->scx.holding_cpu = raw_smp_processor_id();
>
> @@ -2374,28 +2378,31 @@ dispatch_to_local_dsq(struct rq *rq, u64 dsq_id, struct task_struct *p,
> raw_spin_rq_lock(src_rq);
> }
>
> - if (src_rq == dst_rq) {
> + /* task_rq couldn't have changed if we're still the holding cpu */
> + dsp = p->scx.holding_cpu == raw_smp_processor_id() &&
> + !WARN_ON_ONCE(src_rq != task_rq(p));
> +
> + if (likely(dsp)) {
> /*
> - * As @p is staying on the same rq, there's no need to
> + * If @p is staying on the same rq, there's no need to
> * go through the full deactivate/activate cycle.
> * Optimize by abbreviating the operations in
> * move_task_to_local_dsq().
> */
> - dsp = p->scx.holding_cpu == raw_smp_processor_id();
> - if (likely(dsp)) {
> + if (src_rq == dst_rq) {
> p->scx.holding_cpu = -1;
> - dispatch_enqueue(&dst_rq->scx.local_dsq, p,
> - enq_flags);
> + dispatch_enqueue(&dst_rq->scx.local_dsq,
> + p, enq_flags);
> + } else {
> + move_task_to_local_dsq(p, enq_flags,
> + src_rq, dst_rq);
> }
> - } else {
> - dsp = move_task_to_local_dsq(p, enq_flags,
> - src_rq, dst_rq);
> - }
>
> - /* if the destination CPU is idle, wake it up */
> - if (dsp && sched_class_above(p->sched_class,
> - dst_rq->curr->sched_class))
> - resched_curr(dst_rq);
> + /* if the destination CPU is idle, wake it up */
> + if (sched_class_above(p->sched_class,
> + dst_rq->curr->sched_class))
> + resched_curr(dst_rq);
> + }
>
> /* switch back to @rq lock */
> if (rq != dst_rq) {
> --
> 2.46.0
>

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