linux-kernel - Re: [PATCH] kthread/smpboot: Serialize kthread parking against wakeup

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Date:   Tue, 24 Apr 2018 20:05:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Kohli, Gaurav" <gkohli@...eaurora.org>
Cc:     tglx@...utronix.de, mpe@...erman.id.au, dzickus@...hat.com,
        mingo@...nel.org, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        Neeraj Upadhyay <neeraju@...eaurora.org>
Subject: Re: [PATCH] kthread/smpboot: Serialize kthread parking against wakeup

On Tue, Apr 24, 2018 at 08:12:49PM +0530, Kohli, Gaurav wrote:
> @@ -157,6 +156,7 @@ static int smpboot_thread_fn(void *data)
> 
>                 if (!ht->thread_should_run(td->cpu)) {
> 
>                         preempt_enable_no_resched();
> 
> +                       set_current_state(TASK_INTERRUPTIBLE);
> 
>                         schedule();
> 
>                 } else {
> 
>                         __set_current_state(TASK_RUNNING);
> 
> Please suggest if this approach is better.

Bah, my brain isn't working... see below for the 'correct' version of
your second patch.

But this violates the normal pattern; see the comment near
set_current_state(). That pattern ensures the thread either sees the
wakeup condition or the actual wakeup.

I'm thinking that with this patch there is a scenario where we'll miss
both the kthread_should_park() and the actual wakeup and end up not
doing anything.

I do the like the end result, but I suspect it's buggy.


---
 kernel/smpboot.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 5043e7433f4b..5bdf57f2ce68 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -109,10 +109,8 @@ static int smpboot_thread_fn(void *data)
 	struct smp_hotplug_thread *ht = td->ht;
 
 	while (1) {
-		set_current_state(TASK_INTERRUPTIBLE);
 		preempt_disable();
 		if (kthread_should_stop()) {
-			__set_current_state(TASK_RUNNING);
 			preempt_enable();
 			/* cleanup must mirror setup */
 			if (ht->cleanup && td->status != HP_THREAD_NONE)
@@ -122,7 +120,6 @@ static int smpboot_thread_fn(void *data)
 		}
 
 		if (kthread_should_park()) {
-			__set_current_state(TASK_RUNNING);
 			preempt_enable();
 			if (ht->park && td->status == HP_THREAD_ACTIVE) {
 				BUG_ON(td->cpu != smp_processor_id());
@@ -139,7 +136,6 @@ static int smpboot_thread_fn(void *data)
 		/* Check for state change setup */
 		switch (td->status) {
 		case HP_THREAD_NONE:
-			__set_current_state(TASK_RUNNING);
 			preempt_enable();
 			if (ht->setup)
 				ht->setup(td->cpu);
@@ -147,7 +143,6 @@ static int smpboot_thread_fn(void *data)
 			continue;
 
 		case HP_THREAD_PARKED:
-			__set_current_state(TASK_RUNNING);
 			preempt_enable();
 			if (ht->unpark)
 				ht->unpark(td->cpu);
@@ -156,10 +151,10 @@ static int smpboot_thread_fn(void *data)
 		}
 
 		if (!ht->thread_should_run(td->cpu)) {
+			set_current_state(TASK_IDLE);
 			preempt_enable_no_resched();
 			schedule();
 		} else {
-			__set_current_state(TASK_RUNNING);
 			preempt_enable();
 			ht->thread_fn(td->cpu);
 		}