Thread (57 messages) 57 messages, 4 authors, 2021-06-03

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

From: Peter Zijlstra <peterz@infradead.org>
Date: 2021-05-27 14:11:26
Also in: linux-arm-kernel, lkml

On Tue, May 25, 2021 at 04:14:26PM +0100, Will Deacon wrote:
quoted hunk ↗ jump to hunk
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.
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