linux-kernel - Re: [Patch 0/7] Per cpu thread hotplug infrastructure

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Date:	Fri, 20 Jul 2012 23:58:30 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [Patch 0/7] Per cpu thread hotplug infrastructure - V3

On 07/20/2012 11:23 PM, Paul E. McKenney wrote:
> On Fri, Jul 20, 2012 at 08:30:30PM +0530, Srivatsa S. Bhat wrote:
>> On 07/20/2012 08:05 PM, Paul E. McKenney wrote:
>>> On Fri, Jul 20, 2012 at 06:47:30PM +0530, Srivatsa S. Bhat wrote:
>>>> On 07/19/2012 05:24 AM, Paul E. McKenney wrote:
>>>>> On Wed, Jul 18, 2012 at 11:06:52PM +0530, Srivatsa S. Bhat wrote:
>>>>>> On 07/16/2012 08:52 PM, Paul E. McKenney wrote:
>>>>>>> On Mon, Jul 16, 2012 at 10:42:34AM -0000, Thomas Gleixner wrote:
>>>>>>>> The following series implements the infrastructure for parking and
>>>>>>>> unparking kernel threads to avoid the full teardown and fork on cpu
>>>>>>>> hotplug operations along with management infrastructure for hotplug
>>>>>>>> and users.
>>>>>>>>
>>>>>>>> Changes vs. V2:
>>>>>>>>
>>>>>>>>  Use callbacks for all functionality. Thanks to Rusty for pointing
>>>>>>>>  that out. It makes the use sites nice and simple and keeps all the
>>>>>>>>  code which would be duplicated otherwise on the core.
>>>>>>>
>>>>>>> Hello, Thomas,
>>>>>>>
>>>>>>> What version should I apply this patchset to?  I tried v3.5-rc7, but
>>>>>>> got lots of warnings (one shown below) and the watchdog patch did not
>>>>>>> apply.
>>>>>>>
>>>>>>
>>>>>> Hi Paul,
>>>>>>
>>>>>> This patchset applies cleanly on Thomas' smp/hotplug branch in the -tip
>>>>>> tree.
>>>>>
>>>>> Thank you, Srivatsa, works much better.  Still get "scheduling while
>>>>> atomic", looking into that.
>>>>>
>>>>
>>>> Got a chance to run this patchset now.. Even I am getting "scheduling while
>>>> atomic" messages like shown below..  Hmmm...
>>>
>>> Here is what little I have done so far (lots of completing demands on time
>>> this week, but I should have a goodly block of time to focus on this today):
>>>
>>> 1.	The failure is from the softirq modifications.  Reverting that
>>> 	commit gets rid of the failures.
>>>
>>> 2.	As one would expect, CONFIG_PREEMPT=n kernels do not have the
>>> 	problem, which of course indicates a preempt_disable() imbalance.
>>
>> Right..
> 
> Except that the imbalance is not in softirq like I was thinking, but
> rather in smpboot.  See patch below, which clears this up for me.
> 
> 							Thanx, Paul
> 
>>> 3.	I was unable to spot the problem by inspection, but this is not
>>> 	too surprising given the high level of distraction this week.
>>>
>>> 4.	Instrumentation shows that preempt_count() grows slowly with
>>> 	time, but with the upper bits zero.  This confirms the
>>> 	preempt_disable imbalance.
>>>
>>> 5.	I am currently placing WARN_ONCE() calls in the code to track
>>> 	this down.  When I do find it, I fully expect to feel very stupid
>>> 	about my efforts on #3 above.  ;-)
>>>
>>
>> Hehe :-) I'll also see if I can dig out the problem..
> 
>  smpboot.c |    4 ++--
>  softirq.c |    3 ++-
>  2 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/smpboot.c b/kernel/smpboot.c
> index 1c1458f..b2545c8 100644
> --- a/kernel/smpboot.c
> +++ b/kernel/smpboot.c
> @@ -148,12 +148,12 @@ static int smpboot_thread_fn(void *data)
>  		}
> 
>  		if (!ht->thread_should_run(td->cpu)) {
> -			schedule_preempt_disabled();
> +			preempt_enable();
> +			schedule();

Oh, of *course*! The trailing preempt_disable() would end up causing
trouble as we go into the next iteration of the loop..

>  		} else {
>  			set_current_state(TASK_RUNNING);
>  			preempt_enable();
>  			ht->thread_fn(td->cpu);
> -			preempt_disable();
>  		}
>  	}
>  }
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index 82ca065..090e1b9 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -744,9 +744,10 @@ static void run_ksoftirqd(unsigned int cpu)
>  	local_irq_disable();
>  	if (local_softirq_pending()) {
>  		__do_softirq();
> +		rcu_note_context_switch(cpu);
>  		local_irq_enable();
>  		cond_resched();
> -		rcu_note_context_switch(cpu);
> +		return;
>  	}
>  	local_irq_enable();
>  }
> 

Tested your fix, it works great!

Regards,
Srivatsa S. Bhat

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