Re: [PATCH] kthread: Prevent unpark race which puts threads on thewrong cpu

[Srivatsa S. Bhat]

On 04/11/2013 04:18 PM, Srivatsa S. Bhat wrote:
> On 04/11/2013 03:49 PM, Thomas Gleixner wrote:
>> Dave,
>>
>> On Wed, 10 Apr 2013, Dave Hansen wrote:
>>
>>> I think I got a full trace this time:
>>>
>>> 	http://sr71.net/~dave/linux/bigbox-trace.1365621899.txt.gz
>>>
>>> The last timestamp is pretty close to the timestamp on the console:
>>>
>>> [ 2071.033434] smpboot_thread_fn():
>>> [ 2071.033455] smpboot_thread_fn() cpu: 22 159
>>> [ 2071.033470] td->cpu: 22
>>> [ 2071.033475] smp_processor_id(): 21
>>> [ 2071.033486] comm: migration/%u
>>
>> Yes, that's helpful. Though it makes my mind boggle:
>>
>>     migration/22-4335  [021] d.h.  2071.020530: sched_wakeup: comm=migration/21 pid=4323 prio=0 success=1 target_cpu=021^M
>>     migration/22-4335  [021] d...  2071.020541: sched_switch: prev_comm=migration/22 prev_pid=4335 prev_prio=0 prev_state=0x200 ==> next_comm=migration/21 next_pid=4323 next_prio=0^M
>>     migration/21-4323  [021] d...  2071.026110: sched_switch: prev_comm=migration/21 prev_pid=4323 prev_prio=0 prev_state=S ==> next_comm=migration/22 next_pid=4335 next_prio=0^M
>>     migration/22-4335  [021] ....  2071.033422: smpboot_thread_fn <-kthread^M
>>
>> So the migration thread schedules out with state TASK_PARKED and gets
>> scheduled back in right away without a wakeup. Srivatsa was about
>> right, that this might be related to the sched_set_stop_task() call,
>> but the changelog led completely down the wrong road.
>>
>> So the issue is:
>>
>> CPU14				CPU21
>> create_thread(for CPU22)
>>  park_thread()	
>>    wait_for_completion()	park_me()
>> 				 complete()
>>    sched_set_stop_task()
>> 				 schedule(TASK_PARKED)
>>
>> The sched_set_stop_task() call is issued while the task is on the
>> runqueue of CPU21 and that confuses the hell out of the stop_task
>> class on that cpu. So as we have to synchronize the state for the
>> bind call (the issue observed by Borislav) we need to do the same
>> before issuing sched_set_stop_task(). Delta patch below.
>>
> 
> In that case, why not just apply this 2 line patch on mainline?
> The patch I sent yesterday was more elaborate because I wrongly assumed
> that kthread_bind() can cause a wakeup. But now, I feel the patch shown
> below should work just fine too. Yeah, it binds the task during creation
> as well as during unpark, but that should be ok (see below).
> 
> Somehow, I believe we can handle this issue without introducing that
> whole TASK_PARKED thing..
> 
> 
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 691dc2e..74af4a8 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -308,6 +308,9 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
>  	to_kthread(p)->cpu = cpu;
>  	/* Park the thread to get it out of TASK_UNINTERRUPTIBLE state */
>  	kthread_park(p);
> +
> +	wait_task_inactive(p, TASK_INTERRUPTIBLE);
> +	__kthread_bind(p, cpu);
>  	return p;
>  }
>  

Oh wait a minute, its simpler than even that I believe! We don't need that
extra kthread_bind() also.

So, here is what is happening, from what I understand:
sched_set_stop_task() will make the newly created migration thread as the
stop task, during ht->create(). But looking into __sched_setscheduler(),
we observe that if p->on_rq is true (where p is the migration thread),
the task is enqueued on the rq by calling ->enqueue_task() of stop_task
sched class, which simply increments the rq->nr_running.

So, on the next call to schedule(), the pick_next_task() in core.c sees
that rq->nr_running is not equal to rq->cfs.h_nr_running and hence invokes
the stop_task sched class' ->pick_next_task(). And that will return the
stop task, which is nothing but this migration thread that we just created.
That's why the migration thread starts running prematurely, even before
it is bound, and even before its corresponding CPU is brought online.

So, if we just wait till p->on_rq is reset to 0 by __schedule(), before
calling ht->create(), the task doesn't get queued to the runqueue with
stop task priority. So that's it - we simply wait till it gets off the
rq and everything should be fine. And during the subsequent online, during
the unpark phase, the task gets bound to the appropriate cpu and only
then woken up, as desired.

So Dave, could you kindly test the below patch on mainline?


diff --git a/kernel/kthread.c b/kernel/kthread.c
index 691dc2e..9558355 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -308,6 +308,15 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
 	to_kthread(p)->cpu = cpu;
 	/* Park the thread to get it out of TASK_UNINTERRUPTIBLE state */
 	kthread_park(p);
+
+	/*
+	 * Wait for p->on_rq to be reset to 0, to ensure that the per-cpu
+	 * migration thread (which belongs to the stop_task sched class)
+	 * doesn't run until the cpu is actually onlined and the thread is
+	 * unparked.
+	 */
+	if (!wait_task_inactive(p, TASK_INTERRUPTIBLE))
+		WARN_ON(1);
 	return p;
 }
 
Regards,
Srivatsa S. Bhat

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/