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

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Message-ID: <519B26FD.9080404@linux.vnet.ibm.com>
Date:	Tue, 21 May 2013 13:19:17 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	anish singh <anish198519851985@...il.com>
CC:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Don Zickus <dzickus@...hat.com>
Subject: Re: [RFC PATCH 6/8] kthread: Enable parking requests from setup()
 and unpark() callbacks

On 05/21/2013 11:04 AM, anish singh wrote:
> On Mon, May 20, 2013 at 9:31 PM, Frederic Weisbecker <fweisbec@...il.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.

Regards,
Srivatsa S. Bhat

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