Re: [PATCH v7 16/22] sched: Defer wakeup in ttwu() for unschedulable frozen tasks

Peter Zijlstra
2021-05-27T14:58:38Z

On Thu, May 27, 2021 at 04:10:16PM +0200, Peter Zijlstra wrote:
> On Tue, May 25, 2021 at 04:14:26PM +0100, Will Deacon wrote:
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 42e2aecf087c..6cb9677d635a 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3529,6 +3529,19 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> > if (!(p->state & state))
> > goto unlock;
> >
> > +#ifdef CONFIG_FREEZER
> > + /*
> > + * If we're going to wake up a thread which may be frozen, then
> > + * we can only do so if we have an active CPU which is capable of
> > + * running it. This may not be the case when resuming from suspend,
> > + * as the secondary CPUs may not yet be back online. See __thaw_task()
> > + * for the actual wakeup.
> > + */
> > + if (unlikely(frozen_or_skipped(p)) &&
> > + !cpumask_intersects(cpu_active_mask, task_cpu_possible_mask(p)))
> > + goto unlock;
> > +#endif
> > +
> > trace_sched_waking(p);
> >
> > /* We're going to change ->state: */
>
> OK, I really hate this. This is slowing down the very hot wakeup path
> for the silly freezer that *never* happens. Let me try and figure out if
> there's another option.

How's something *completely* untested like this?

---
diff --git a/include/linux/freezer.h b/include/linux/freezer.h
index 0621c5f86c39..44ece41c3db3 100644
--- a/include/linux/freezer.h
+++ b/include/linux/freezer.h
@@ -24,7 +24,7 @@ extern unsigned int freeze_timeout_msecs;
*/
static inline bool frozen(struct task_struct *p)
{
- return p->flags & PF_FROZEN;
+ return p->state == TASK_FROZEN;
}

extern bool freezing_slow_path(struct task_struct *p);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2982cfab1ae9..7e7775c5b742 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -95,7 +95,8 @@ struct task_group;
#define TASK_WAKING 0x0200
#define TASK_NOLOAD 0x0400
#define TASK_NEW 0x0800
-#define TASK_STATE_MAX 0x1000
+#define TASK_FROZEN 0x1000
+#define TASK_STATE_MAX 0x2000

/* Convenience macros for the sake of set_current_state: */
#define TASK_KILLABLE (TASK_WAKEKILL | TASK_UNINTERRUPTIBLE)
@@ -1579,7 +1580,6 @@ extern struct pid *cad_pid;
#define PF_USED_MATH 0x00002000 /* If unset the fpu must be initialized before use */
#define PF_USED_ASYNC 0x00004000 /* Used async_schedule*(), used by module init */
#define PF_NOFREEZE 0x00008000 /* This thread should not be frozen */
-#define PF_FROZEN 0x00010000 /* Frozen for system suspend */
#define PF_KSWAPD 0x00020000 /* I am kswapd */
#define PF_MEMALLOC_NOFS 0x00040000 /* All allocation requests will inherit GFP_NOFS */
#define PF_MEMALLOC_NOIO 0x00080000 /* All allocation requests will inherit GFP_NOIO */
diff --git a/kernel/freezer.c b/kernel/freezer.c
index dc520f01f99d..be6c86078510 100644
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -63,18 +63,13 @@ bool __refrigerator(bool check_kthr_stop)
pr_debug("%s entered refrigerator\n", current->comm);

for (;;) {
- set_current_state(TASK_UNINTERRUPTIBLE);
-
spin_lock_irq(&freezer_lock);
- current->flags |= PF_FROZEN;
- if (!freezing(current) ||
- (check_kthr_stop && kthread_should_stop()))
- current->flags &= ~PF_FROZEN;
+ if (freezing(current) && (check_kthr_stop && kthread_should_stop())) {
+ set_current_state(TASK_FROZEN);
+ was_frozen = true;
+ }
spin_unlock_irq(&freezer_lock);

- if (!(current->flags & PF_FROZEN))
- break;
- was_frozen = true;
schedule();
}

@@ -149,7 +144,7 @@ void __thaw_task(struct task_struct *p)

spin_lock_irqsave(&freezer_lock, flags);
if (frozen(p))
- wake_up_process(p);
+ wake_up_state(p, TASK_FROZEN);
spin_unlock_irqrestore(&freezer_lock, flags);
}

diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 396ebaebea3f..71a6509f8d4f 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -92,8 +92,8 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
* Ensure the task is not frozen.
* Also, skip vfork and any other user process that freezer should skip.
*/
- if (unlikely(t->flags & (PF_FROZEN | PF_FREEZER_SKIP)))
- return;
+ if (unlikely((t->flags & PF_FREEZER_SKIP) || frozen(t)))
+ return;

/*
* When a freshly created task is scheduled once, changes its state to
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 401f012349d1..1eedf6a044f3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5889,7 +5889,7 @@ static void __sched notrace __schedule(bool preempt)
prev->sched_contributes_to_load =
(prev_state & TASK_UNINTERRUPTIBLE) &&
!(prev_state & TASK_NOLOAD) &&
- !(prev->flags & PF_FROZEN);
+ !(prev_state & TASK_FROZEN);

if (prev->sched_contributes_to_load)
rq->nr_uninterruptible++;