From: Tejun Heo Date: Wed, 22 Oct 2025 20:56:28 +0000 (-1000) Subject: sched_ext: Fix SCX_KICK_WAIT to work reliably X-Git-Tag: v6.19-rc1~177^2~33 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=a379fa1e2cae15d7422b4eead83a6366f2f445cb;p=thirdparty%2Fkernel%2Flinux.git sched_ext: Fix SCX_KICK_WAIT to work reliably 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 b999e365c298 ("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 4c95380701f5 ("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 Link: http://lkml.kernel.org/r/228ebd9e6ed3437996dffe15735a9caa@honor.com Cc: Peter Zijlstra Reviewed-by: Andrea Righi Signed-off-by: Tejun Heo --- diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index 12e00f77a8487..1999f6e67022a 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -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); } diff --git a/kernel/sched/ext_internal.h b/kernel/sched/ext_internal.h index 7d00a0a2456e2..fb161fc35328c 100644 --- a/kernel/sched/ext_internal.h +++ b/kernel/sched/ext_internal.h @@ -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, };