[tip: sched/urgent] Since sched_delayed tasks remain queued even after blocking, the load
From: tip-bot2 for Johannes Weiner
Date: Sat Oct 12 2024 - 10:16:24 EST
The following commit has been merged into the sched/urgent branch of tip:
Commit-ID: f2c9767170bead8d0ceb9c01d074c6916350310d
Gitweb: https://git.kernel.org/tip/f2c9767170bead8d0ceb9c01d074c6916350310d
Author: Johannes Weiner <hannes@xxxxxxxxxxx>
AuthorDate: Fri, 11 Oct 2024 10:49:33 +02:00
Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 11 Oct 2024 10:49:33 +02:00
Since sched_delayed tasks remain queued even after blocking, the load
balancer can migrate them between runqueues while PSI considers them
to be asleep. As a result, it misreads the migration requeue followed
by a wakeup as a double queue:
psi: inconsistent task state! task=... cpu=... psi_flags=4 clear=. set=4
First, call psi_enqueue() after p->sched_class->enqueue_task(). A
wakeup will clear p->se.sched_delayed while a migration will not, so
psi can use that flag to tell them apart.
Then teach psi to migrate any "sleep" state when delayed-dequeue tasks
are being migrated.
Delayed-dequeue tasks can be revived by ttwu_runnable(), which will
call down with a new ENQUEUE_DELAYED. Instead of further complicating
the wakeup conditional in enqueue_task(), identify migration contexts
instead and default to wakeup handling for all other cases.
Debugged-by-and-original-fix-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Fixes: 152e11f6df29 ("sched/fair: Implement delayed dequeue")
Closes: https://lore.kernel.org/lkml/20240830123458.3557-1-spasswolf@xxxxxx/
Closes: https://lore.kernel.org/all/cd67fbcd-d659-4822-bb90-7e8fbb40a856@xxxxxxxxxxxxx/
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Link: https://lkml.kernel.org/r/20241010193712.GC181795@xxxxxxxxxxx
---
kernel/sched/core.c | 12 +++++------
kernel/sched/stats.h | 48 +++++++++++++++++++++++++++++--------------
2 files changed, 39 insertions(+), 21 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9e09140..71232f8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2012,11 +2012,6 @@ void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
if (!(flags & ENQUEUE_NOCLOCK))
update_rq_clock(rq);
- if (!(flags & ENQUEUE_RESTORE)) {
- sched_info_enqueue(rq, p);
- psi_enqueue(p, (flags & ENQUEUE_WAKEUP) && !(flags & ENQUEUE_MIGRATED));
- }
-
p->sched_class->enqueue_task(rq, p, flags);
/*
* Must be after ->enqueue_task() because ENQUEUE_DELAYED can clear
@@ -2024,6 +2019,11 @@ void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
*/
uclamp_rq_inc(rq, p);
+ if (!(flags & ENQUEUE_RESTORE)) {
+ sched_info_enqueue(rq, p);
+ psi_enqueue(p, flags & ENQUEUE_MIGRATED);
+ }
+
if (sched_core_enabled(rq))
sched_core_enqueue(rq, p);
}
@@ -2041,7 +2041,7 @@ inline bool dequeue_task(struct rq *rq, struct task_struct *p, int flags)
if (!(flags & DEQUEUE_SAVE)) {
sched_info_dequeue(rq, p);
- psi_dequeue(p, flags & DEQUEUE_SLEEP);
+ psi_dequeue(p, !(flags & DEQUEUE_SLEEP));
}
/*
diff --git a/kernel/sched/stats.h b/kernel/sched/stats.h
index 237780a..767e098 100644
--- a/kernel/sched/stats.h
+++ b/kernel/sched/stats.h
@@ -119,45 +119,63 @@ static inline void psi_account_irqtime(struct rq *rq, struct task_struct *curr,
/*
* PSI tracks state that persists across sleeps, such as iowaits and
* memory stalls. As a result, it has to distinguish between sleeps,
- * where a task's runnable state changes, and requeues, where a task
- * and its state are being moved between CPUs and runqueues.
+ * where a task's runnable state changes, and migrations, where a task
+ * and its runnable state are being moved between CPUs and runqueues.
+ *
+ * A notable case is a task whose dequeue is delayed. PSI considers
+ * those sleeping, but because they are still on the runqueue they can
+ * go through migration requeues. In this case, *sleeping* states need
+ * to be transferred.
*/
-static inline void psi_enqueue(struct task_struct *p, bool wakeup)
+static inline void psi_enqueue(struct task_struct *p, bool migrate)
{
- int clear = 0, set = TSK_RUNNING;
+ int clear = 0, set = 0;
if (static_branch_likely(&psi_disabled))
return;
- if (p->in_memstall)
- set |= TSK_MEMSTALL_RUNNING;
-
- if (!wakeup) {
+ if (p->se.sched_delayed) {
+ /* CPU migration of "sleeping" task */
+ SCHED_WARN_ON(!migrate);
if (p->in_memstall)
set |= TSK_MEMSTALL;
+ if (p->in_iowait)
+ set |= TSK_IOWAIT;
+ } else if (migrate) {
+ /* CPU migration of runnable task */
+ set = TSK_RUNNING;
+ if (p->in_memstall)
+ set |= TSK_MEMSTALL | TSK_MEMSTALL_RUNNING;
} else {
+ /* Wakeup of new or sleeping task */
if (p->in_iowait)
clear |= TSK_IOWAIT;
+ set = TSK_RUNNING;
+ if (p->in_memstall)
+ set |= TSK_MEMSTALL_RUNNING;
}
psi_task_change(p, clear, set);
}
-static inline void psi_dequeue(struct task_struct *p, bool sleep)
+static inline void psi_dequeue(struct task_struct *p, bool migrate)
{
if (static_branch_likely(&psi_disabled))
return;
/*
+ * When migrating a task to another CPU, clear all psi
+ * state. The enqueue callback above will work it out.
+ */
+ if (migrate)
+ psi_task_change(p, p->psi_flags, 0);
+
+ /*
* A voluntary sleep is a dequeue followed by a task switch. To
* avoid walking all ancestors twice, psi_task_switch() handles
* TSK_RUNNING and TSK_IOWAIT for us when it moves TSK_ONCPU.
* Do nothing here.
*/
- if (sleep)
- return;
-
- psi_task_change(p, p->psi_flags, 0);
}
static inline void psi_ttwu_dequeue(struct task_struct *p)
@@ -190,8 +208,8 @@ static inline void psi_sched_switch(struct task_struct *prev,
}
#else /* CONFIG_PSI */
-static inline void psi_enqueue(struct task_struct *p, bool wakeup) {}
-static inline void psi_dequeue(struct task_struct *p, bool sleep) {}
+static inline void psi_enqueue(struct task_struct *p, bool migrate) {}
+static inline void psi_dequeue(struct task_struct *p, bool migrate) {}
static inline void psi_ttwu_dequeue(struct task_struct *p) {}
static inline void psi_sched_switch(struct task_struct *prev,
struct task_struct *next,