Re: [RFC PATCH 6/8] kthread: Enable parking requests from setup() and unpark() callbacks

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 05/21/2013 02:28 PM, anish singh wrote:
> On Tue, May 21, 2013 at 1:19 PM, Srivatsa S. Bhat
> <srivatsa.bhat@linux.vnet.ibm.com> wrote:
>> On 05/21/2013 11:04 AM, anish singh wrote:
>>> On Mon, May 20, 2013 at 9:31 PM, Frederic Weisbecker <fweisbec@gmail.com> wrote:
>>>> When the watchdog code is boot-disabled by the user, for example
>>>> through the 'nmi_watchdog=0' boot option, the setup() callback of
>>>> the watchdog kthread requests to park the task, and that until the
>>>> user later re-enables the watchdog through sysctl or procfs.
>>>>
>>>> However the parking request is not well handled when done from
>>>> the setup() callback. After ->setup() is called, the generic smpboot
>>>> kthread loop directly tries to call the thread function or wait
>>>> for some event if ->thread_should_run() is false.
>>>>
>>>> In the case of the watchdog kthread, ->thread_should_run() returns
>>>> false and the kthread goes to sleep and wait for the watchdog timer
>>>> to wake it up. But the timer is not enabled since the user requested
>>>> to disable the watchdog. We want the kthread to park instead of waiting
>>>> for events that can't happen.
>>>>
>>>> As a result, later unpark requests after sysctl write through
>>>> 'sysctl -w kernel.watchdog=1' won't wake up/unpark the task as
>>>> expected, since it's not parked anyway, leaving the value modified
>>>> without triggering any action.
>>> Out of curiosity, this can happen only for short period of time right?After
>>> some time this will work right as the thread get back in action
>>> after the schedule call.Then sysctl and procfs will work I think.
>>
>> kthread_unpark() can wake up a task only if the task is in TASK_PARKED
>> state. But since the above task would be in TASK_INTERRUPTIBLE state
>> (since it is not parked), kthread_unpark() will be powerless to do anything.
> Yes but this will happen only for a short period of time right?
> After the schdule() call the below code gets executed in while() loop.
> 

schedule() is not just another function call from which you'll simply
return "after a short period of time". We have set the task's state to
TASK_INTERRUPTIBLE at the beginning of the loop. So the call to schedule()
will kick the task out of the runqueue, since it is not runnable any more.

You need somebody to do a wake_up_process() or something similar to make
the task runnable again, after which it gets back to the runqueue and gets
a chance to run. But the only one who can do wake_up_process() for this
watchdog task in this case, is the kthread_unpark() code, which needs the
task to be in TASK_PARKED state, not TASK_INTERRUPTIBLE. So the call to
wake_up_state() returns without doing anything, leaving the watchdog task
non-runnable. This is what Frederic referred to, when he said that
enabling nmi watchdog via sysctl/procfs won't do anything.

> 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());
> ht->park(td->cpu);
> td->status = HP_THREAD_PARKED;
> }
> kthread_parkme();
> /* We might have been woken for stop */
> continue;
> }
> 
> As we have already called kthread_park this above if() condition gets true and
> it will park the thread wouldn't it?But this will happen after the schedule
> call which is not right as mentioned by fredrick.
> 

Regards,
Srivatsa S. Bhat