[PATCH v6 15/21] sched: Defer wakeup in ttwu() for unschedulable frozen tasks

[Will Deacon]

Asymmetric systems may not offer the same level of userspace ISA support
across all CPUs, meaning that some applications cannot be executed by
some CPUs. As a concrete example, upcoming arm64 big.LITTLE designs do
not feature support for 32-bit applications on both clusters.

Although we take care to prevent explicit hot-unplug of all 32-bit
capable CPUs on such a system, this is required when suspending on some
SoCs where the firmware mandates that the suspend/resume operation is
handled by CPU 0, which may not be capable of running 32-bit tasks.

Consequently, there is a window on the resume path where no 32-bit
capable CPUs are available for scheduling and waking up a 32-bit task
will result in a scheduler BUG() due to failure of select_fallback_rq():

  | kernel BUG at kernel/sched/core.c:2858!
  | Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
  | ...
  | Call trace:
  |  select_fallback_rq+0x4b0/0x4e4
  |  try_to_wake_up.llvm.4388853297126348405+0x460/0x5b0
  |  default_wake_function+0x1c/0x30
  |  autoremove_wake_function+0x1c/0x60
  |  __wake_up_common.llvm.11763074518265335900+0x100/0x1b8
  |  __wake_up+0x78/0xc4
  |  ep_poll_callback+0x20c/0x3fc

Prevent wakeups of unschedulable frozen tasks in ttwu() and instead
defer the wakeup to __thaw_tasks(), which runs only once all the
secondary CPUs are back online.

Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
 kernel/freezer.c    | 10 +++++++++-
 kernel/sched/core.c | 13 +++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

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 d7d058fc012e..f5ff55786344 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3525,6 +3525,19 @@ 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: */
-- 
2.31.1.751.gd2f1c929bd-goog