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

From: Frederic Weisbecker
Date: Wed May 22 2013 - 11:18:44 EST

2013/5/21, anish singh <anish198519851985@xxxxxxxxx>:
> On Tue, May 21, 2013 at 1:19 PM, Srivatsa S. Bhat
> <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> wrote:
>> On 05/21/2013 11:04 AM, anish singh wrote:
>>> On Mon, May 20, 2013 at 9:31 PM, Frederic Weisbecker <fweisbec@xxxxxxxxx>
>>> 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.
> 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.

But we are scheduling in TASK_INTERRUPTIBLE mode so we are going to
sleep until some other thread wake us. But we are
unparked instead of being awoken. This just have no effect because
kthread_unpark() is a no-op on non-parked kthreads.
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