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sched/fair: Fix external p->on_rq users

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Sean noted that ever since commit 152e11f6df ("sched/fair: Implement
delayed dequeue") KVM's preemption notifiers have started
mis-classifying preemption vs blocking.

Notably p->on_rq is no longer sufficient to determine if a task is
runnable or blocked -- the aforementioned commit introduces tasks that
remain on the runqueue even through they will not run again, and
should be considered blocked for many cases.

Add the task_is_runnable() helper to classify things and audit all
external users of the p->on_rq state. Also add a few comments.

Fixes: 152e11f6df ("sched/fair: Implement delayed dequeue")
Reported-by: Sean Christopherson <seanjc@google.com>
Tested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20241010091843.GK33184@noisy.programming.kicks-ass.net
This commit is contained in:
Peter Zijlstra
2024-10-10 11:38:10 +02:00
committed by Ingo Molnar
parent c650812419
commit cd9626e9eb
8 changed files with 38 additions and 7 deletions
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kernel/freezer.c
+6 -1
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@@ -109,7 +109,12 @@ static int __set_task_frozen(struct task_struct *p, void *arg)
{
unsigned int state = READ_ONCE(p->__state);
if (p->on_rq)
/*
* Allow freezing the sched_delayed tasks; they will not execute until
* ttwu() fixes them up, so it is safe to swap their state now, instead
* of waiting for them to get fully dequeued.
*/
if (task_is_runnable(p))
return 0;
if (p != current && task_curr(p))