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Subject[RFC][PATCH 6/7] sched: Add rq::ttwu_pending
In preparation of removing rq->wake_list, replace the
!list_empty(rq->wake_list) with rq->ttwu_pending. This is not fully
equivalent as this new variable is racy.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/sched/core.c | 13 +++++++++++--
kernel/sched/debug.c | 1 -
kernel/sched/fair.c | 2 +-
kernel/sched/sched.h | 4 +++-
4 files changed, 15 insertions(+), 5 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2275,13 +2275,21 @@ static int ttwu_remote(struct task_struc
void sched_ttwu_pending(void)
{
struct rq *rq = this_rq();
- struct llist_node *llist = llist_del_all(&rq->wake_list);
+ struct llist_node *llist;
struct task_struct *p, *t;
struct rq_flags rf;

+ llist = llist_del_all(&rq->wake_list);
if (!llist)
return;

+ /*
+ * rq::ttwu_pending racy indication of out-standing wakeups.
+ * Races such that false-negatives are possible, since they
+ * are shorter lived that false-positives would be.
+ */
+ WRITE_ONCE(rq->ttwu_pending, 0);
+
rq_lock_irqsave(rq, &rf);
update_rq_clock(rq);

@@ -2312,6 +2320,7 @@ static void ttwu_queue_remote(struct tas

p->sched_remote_wakeup = !!(wake_flags & WF_MIGRATED);

+ WRITE_ONCE(rq->ttwu_pending, 1);
if (llist_add(&p->wake_entry, &rq->wake_list)) {
if (!set_nr_if_polling(rq->idle))
smp_call_function_single_async(cpu, &rq->wake_csd);
@@ -4668,7 +4677,7 @@ int idle_cpu(int cpu)
return 0;

#ifdef CONFIG_SMP
- if (!llist_empty(&rq->wake_list))
+ if (rq->ttwu_pending)
return 0;
#endif

--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -638,7 +638,6 @@ do { \

P(nr_running);
P(nr_switches);
- P(nr_load_updates);
P(nr_uninterruptible);
PN(next_balance);
SEQ_printf(m, " .%-30s: %ld\n", "curr->pid", (long)(task_pid_nr(rq->curr)));
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8590,7 +8590,7 @@ static int idle_cpu_without(int cpu, str
*/

#ifdef CONFIG_SMP
- if (!llist_empty(&rq->wake_list))
+ if (rq->ttwu_pending)
return 0;
#endif

--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -895,7 +895,9 @@ struct rq {
atomic_t nohz_flags;
#endif /* CONFIG_NO_HZ_COMMON */

- unsigned long nr_load_updates;
+#ifdef CONFIG_SMP
+ unsigned int ttwu_pending;
+#endif
u64 nr_switches;

#ifdef CONFIG_UCLAMP_TASK

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