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

Peter Zijlstra
2021-05-27T14:47:50Z

On Tue, May 25, 2021 at 04:14:26PM +0100, Will Deacon wrote:
> diff --git a/kernel/freezer.c b/kernel/freezer.c
> index dc520f01f99d..8f3d950c2a87 100644
> --- a/kernel/freezer.c
> +++ b/kernel/freezer.c
> @@ -11,6 +11,7 @@
> #include <linux/syscalls.h>
> #include <linux/freezer.h>
> #include <linux/kthread.h>
> +#include <linux/mmu_context.h>
>
> /* total number of freezing conditions in effect */
> atomic_t system_freezing_cnt = ATOMIC_INIT(0);
> @@ -146,9 +147,16 @@ bool freeze_task(struct task_struct *p)
> void __thaw_task(struct task_struct *p)
> {
> unsigned long flags;
> + const struct cpumask *mask = task_cpu_possible_mask(p);
>
> spin_lock_irqsave(&freezer_lock, flags);
> - if (frozen(p))
> + /*
> + * Wake up frozen tasks. On asymmetric systems where tasks cannot
> + * run on all CPUs, ttwu() may have deferred a wakeup generated
> + * before thaw_secondary_cpus() had completed so we generate
> + * additional wakeups here for tasks in the PF_FREEZER_SKIP state.
> + */
> + if (frozen(p) || (frozen_or_skipped(p) && mask != cpu_possible_mask))
> wake_up_process(p);
> spin_unlock_irqrestore(&freezer_lock, flags);
> }
> 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.