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

On Thu, May 27, 2021 at 04:31:51PM +0200, Peter Zijlstra wrote:

On Thu, May 27, 2021 at 04:10:16PM +0200, Peter Zijlstra wrote:

quoted

On Tue, May 25, 2021 at 04:14:26PM +0100, Will Deacon wrote:

quoted

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?

I'm not seeing how this handles tasks which weren't put in the freezer
because they have PF_FREEZER_SKIP set. For these tasks, we need to make
sure that they don't become runnable before we have onlined a core which
is capable of running them, and this could occur because of any old
wakeup (i.e. whatever it was that they blocked on).

Will

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel