On Fri, May 15, 2020 at 01:17:32PM +0200, Peter Zijlstra wrote:
> On Fri, May 15, 2020 at 09:47:40AM +0100, Mel Gorman wrote:
>
> > However, the wakeups are so rapid that the wakeup
> > happens while the server is descheduling. That forces the waker to spin
> > on smp_cond_load_acquire for longer. In this case, it can be cheaper to
> > add the task to the rq->wake_list even if that potentially requires an IPI.
>
> Right, I think Rik ran into that as well at some point. He wanted to
> make ->on_cpu do a hand-off, but simply queueing the wakeup on the prev
> cpu (which is currently in the middle of schedule()) should be an easier
> proposition.
>
> Maybe something like this untested thing... could explode most mighty,
> didn't thing too hard.
>
Mel pointed out that that patch got mutilated somewhere (my own .Sent
copy was fine), let me try again.
---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3b64ffd6c728..df588ac75bf0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2330,7 +2330,7 @@ void scheduler_ipi(void)
irq_exit();
}
-static void ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags)
+static void __ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags)
{
struct rq *rq = cpu_rq(cpu);
@@ -2372,6 +2372,17 @@ bool cpus_share_cache(int this_cpu, int that_cpu)
{
return per_cpu(sd_llc_id, this_cpu) == per_cpu(sd_llc_id, that_cpu);
}
+
+static bool ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags)
+{
+ if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(),
cpu)) {
+ sched_clock_cpu(cpu); /* Sync clocks across CPUs */
+ __ttwu_queue_remote(p, cpu, wake_flags);
+ return true;
+ }
+
+ return false;
+}
#endif /* CONFIG_SMP */
static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
@@ -2380,11 +2391,8 @@ static void ttwu_queue(struct task_struct *p, int cpu,
int wake_flags)
struct rq_flags rf;
#if defined(CONFIG_SMP)
- if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(),
cpu)) {
- sched_clock_cpu(cpu); /* Sync clocks across CPUs */
- ttwu_queue_remote(p, cpu, wake_flags);
+ if (ttwu_queue_remote(p, cpu, wake_flags))
return;
- }
#endif
rq_lock(rq, &rf);
@@ -2568,7 +2576,15 @@ try_to_wake_up(struct task_struct *p, unsigned int
state, int wake_flags)
if (p->on_rq && ttwu_remote(p, wake_flags))
goto unlock;
+ if (p->in_iowait) {
+ delayacct_blkio_end(p);
+ atomic_dec(&task_rq(p)->nr_iowait);
+ }
+
#ifdef CONFIG_SMP
+ p->sched_contributes_to_load = !!task_contributes_to_load(p);
+ p->state = TASK_WAKING;
+
/*
* Ensure we load p->on_cpu _after_ p->on_rq, otherwise it would be
* possible to, falsely, observe p->on_cpu == 0.
@@ -2599,15 +2615,10 @@ try_to_wake_up(struct task_struct *p, unsigned int
state, int wake_flags)
* This ensures that tasks getting woken will be fully ordered against
* their previous state and preserve Program Order.
*/
- smp_cond_load_acquire(&p->on_cpu, !VAL);
-
- p->sched_contributes_to_load = !!task_contributes_to_load(p);
- p->state = TASK_WAKING;
+ if (READ_ONCE(p->on_cpu) && ttwu_queue_remote(p, cpu, wake_flags))
+ goto unlock;
- if (p->in_iowait) {
- delayacct_blkio_end(p);
- atomic_dec(&task_rq(p)->nr_iowait);
- }
+ smp_cond_load_acquire(&p->on_cpu, !VAL);
cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags);
if (task_cpu(p) != cpu) {
@@ -2615,14 +2626,6 @@ try_to_wake_up(struct task_struct *p, unsigned int
state, int wake_flags)
psi_ttwu_dequeue(p);
set_task_cpu(p, cpu);
}
-
-#else /* CONFIG_SMP */
-
- if (p->in_iowait) {
- delayacct_blkio_end(p);
- atomic_dec(&task_rq(p)->nr_iowait);
- }
-
#endif /* CONFIG_SMP */
ttwu_queue(p, cpu, wake_flags);
