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sched/psi: Fix mistaken CPU pressure indication after corrupted task state bug

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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.

It's not just the warning in dmesg, the task state corruption causes a
permanent CPU pressure indication, which messes with workload/machine
health monitoring.

Debugged-by-and-original-fix-by: K Prateek Nayak <kprateek.nayak@amd.com>
Fixes: 152e11f6df ("sched/fair: Implement delayed dequeue")
Closes: https://lore.kernel.org/lkml/20240830123458.3557-1-spasswolf@web.de/
Closes: https://lore.kernel.org/all/cd67fbcd-d659-4822-bb90-7e8fbb40a856@molgen.mpg.de/
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Link: https://lkml.kernel.org/r/20241010193712.GC181795@cmpxchg.org
This commit is contained in:
Johannes Weiner 2024-10-11 10:49:33 +02:00 committed by Ingo Molnar
parent f5aaff7bfa
commit c650812419
2 changed files with 39 additions and 21 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
kernel/sched/core.c
View File

@ -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));
}
/*

48
kernel/sched/stats.h
View File

@ -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,