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sched_ext: Fix SCX_KICK_WAIT to work reliably

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SCX_KICK_WAIT is used to synchronously wait for the target CPU to complete
a reschedule and can be used to implement operations like core scheduling.

This used to be implemented by scx_next_task_picked() incrementing pnt_seq,
which was always called when a CPU picks the next task to run, allowing
SCX_KICK_WAIT to reliably wait for the target CPU to enter the scheduler and
pick the next task.

However, commit b999e365c2 ("sched_ext: Replace scx_next_task_picked()
with switch_class()") replaced scx_next_task_picked() with the
switch_class() callback, which is only called when switching between sched
classes. This broke SCX_KICK_WAIT because pnt_seq would no longer be
reliably incremented unless the previous task was SCX and the next task was
not.

This fix leverages commit 4c95380701 ("sched/ext: Fold balance_scx() into
pick_task_scx()") which refactored the pick path making put_prev_task_scx()
the natural place to track task switches for SCX_KICK_WAIT. The fix moves
pnt_seq increment to put_prev_task_scx() and also increments it in
pick_task_scx() to handle cases where the same task is re-selected, whether
by BPF scheduler decision or slice refill. The semantics: If the current
task on the target CPU is SCX, SCX_KICK_WAIT waits until the CPU enters the
scheduling path. This provides sufficient guarantee for use cases like core
scheduling while keeping the operation self-contained within SCX.

v2: - Also increment pnt_seq in pick_task_scx() to handle same-task
      re-selection (Andrea Righi).
    - Use smp_cond_load_acquire() for the busy-wait loop for better
      architecture optimization (Peter Zijlstra).

Reported-by: Wen-Fang Liu <liuwenfang@honor.com>
Link: http://lkml.kernel.org/r/228ebd9e6ed3437996dffe15735a9caa@honor.com
Cc: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Tejun Heo
2025-10-22 10:56:28 -10:00
parent a9c1fbbd6d
commit a379fa1e2c
2 changed files with 30 additions and 22 deletions
Download Patch File Download Diff File Expand all files Collapse all files

46
kernel/sched/ext.c
View File

@@ -2260,12 +2260,6 @@ static void switch_class(struct rq *rq, struct task_struct *next)
struct scx_sched *sch = scx_root;
const struct sched_class *next_class = next->sched_class;
/*
* Pairs with the smp_load_acquire() issued by a CPU in
* kick_cpus_irq_workfn() who is waiting for this CPU to perform a
* resched.
*/
smp_store_release(&rq->scx.pnt_seq, rq->scx.pnt_seq + 1);
if (!(sch->ops.flags & SCX_OPS_HAS_CPU_PREEMPT))
return;
@@ -2305,6 +2299,10 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p,
struct task_struct *next)
{
struct scx_sched *sch = scx_root;
/* see kick_cpus_irq_workfn() */
smp_store_release(&rq->scx.pnt_seq, rq->scx.pnt_seq + 1);
update_curr_scx(rq);
/* see dequeue_task_scx() on why we skip when !QUEUED */
@@ -2358,6 +2356,9 @@ do_pick_task_scx(struct rq *rq, struct rq_flags *rf, bool force_scx)
bool keep_prev, kick_idle = false;
struct task_struct *p;
/* see kick_cpus_irq_workfn() */
smp_store_release(&rq->scx.pnt_seq, rq->scx.pnt_seq + 1);
rq_modified_clear(rq);
rq_unpin_lock(rq, rf);
@@ -5144,8 +5145,12 @@ static bool kick_one_cpu(s32 cpu, struct rq *this_rq, unsigned long *pseqs)
}
if (cpumask_test_cpu(cpu, this_scx->cpus_to_wait)) {
pseqs[cpu] = rq->scx.pnt_seq;
should_wait = true;
if (cur_class == &ext_sched_class) {
pseqs[cpu] = rq->scx.pnt_seq;
should_wait = true;
} else {
cpumask_clear_cpu(cpu, this_scx->cpus_to_wait);
}
}
resched_curr(rq);
@@ -5206,18 +5211,19 @@ static void kick_cpus_irq_workfn(struct irq_work *irq_work)
for_each_cpu(cpu, this_scx->cpus_to_wait) {
unsigned long *wait_pnt_seq = &cpu_rq(cpu)->scx.pnt_seq;
if (cpu != cpu_of(this_rq)) {
/*
* Pairs with smp_store_release() issued by this CPU in
* switch_class() on the resched path.
*
* We busy-wait here to guarantee that no other task can
* be scheduled on our core before the target CPU has
* entered the resched path.
*/
while (smp_load_acquire(wait_pnt_seq) == pseqs[cpu])
cpu_relax();
}
/*
* Busy-wait until the task running at the time of kicking is no
* longer running. This can be used to implement e.g. core
* scheduling.
*
* smp_cond_load_acquire() pairs with store_releases in
* pick_task_scx() and put_prev_task_scx(). The former breaks
* the wait if SCX's scheduling path is entered even if the same
* task is picked subsequently. The latter is necessary to break
* the wait when $cpu is taken by a higher sched class.
*/
if (cpu != cpu_of(this_rq))
smp_cond_load_acquire(wait_pnt_seq, VAL != pseqs[cpu]);
cpumask_clear_cpu(cpu, this_scx->cpus_to_wait);
}

6
kernel/sched/ext_internal.h
View File

@@ -997,8 +997,10 @@ enum scx_kick_flags {
SCX_KICK_PREEMPT = 1LLU << 1,
/*
* Wait for the CPU to be rescheduled. The scx_bpf_kick_cpu() call will
* return after the target CPU finishes picking the next task.
* The scx_bpf_kick_cpu() call will return after the current SCX task of
* the target CPU switches out. This can be used to implement e.g. core
* scheduling. This has no effect if the current task on the target CPU
* is not on SCX.
*/
SCX_KICK_WAIT = 1LLU << 2,
};