linux-kernel - Re: [PATCH v2 2/2] CPU hotplug, stop-machine: Plug race-window that leads to "IPI-to-offline-CPU"

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Date:	Mon, 12 May 2014 01:37:58 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>, peterz@...radead.org,
	tglx@...utronix.de, mingo@...nel.org, rusty@...tcorp.com.au,
	fweisbec@...il.com, hch@...radead.org, mgorman@...e.de,
	riel@...hat.com, bp@...e.de, rostedt@...dmis.org,
	mgalbraith@...e.de, ego@...ux.vnet.ibm.com,
	paulmck@...ux.vnet.ibm.com, oleg@...hat.com, rjw@...ysocki.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] CPU hotplug, stop-machine: Plug race-window that
 leads to "IPI-to-offline-CPU"

On 05/10/2014 08:36 AM, Tejun Heo wrote:
> On Wed, May 07, 2014 at 03:31:51AM +0530, Srivatsa S. Bhat wrote:
>> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
>> index 01fbae5..7abb361 100644
>> --- a/kernel/stop_machine.c
>> +++ b/kernel/stop_machine.c
>> @@ -165,12 +165,13 @@ static void ack_state(struct multi_stop_data *msdata)
>>  		set_state(msdata, msdata->state + 1);
>>  }
>>  
>> +
> 
> Why add a new line here?

Argh, a stray newline.. will remove it.

> 
>>  /* This is the cpu_stop function which stops the CPU. */
>>  static int multi_cpu_stop(void *data)
>>  {
>>  	struct multi_stop_data *msdata = data;
>>  	enum multi_stop_state curstate = MULTI_STOP_NONE;
>> -	int cpu = smp_processor_id(), err = 0;
>> +	int cpu = smp_processor_id(), num_active_cpus, err = 0;
> 
> 	TYPE var0 = INIT0, var1, var2 = INIT2;
> 
> looks kinda weird.  Maybe collect initialized ones to one side or
> separate out uninitialized one to a separate declaration?
>

Yeah, now that you point out, it does look very odd. I don't
remember why I wrote it that way in the first place! :-(
I'll fix this in the next version. Thanks!

> Also, isn't nr_active_cpus more common way of naming it?
> 

Sure, will use this convention.

>>  	unsigned long flags;
>>  	bool is_active;
>>  
>> @@ -180,15 +181,38 @@ static int multi_cpu_stop(void *data)
>>  	 */
>>  	local_save_flags(flags);
>>  
>> -	if (!msdata->active_cpus)
>> +	if (!msdata->active_cpus) {
>>  		is_active = cpu == cpumask_first(cpu_online_mask);
>> -	else
>> +		num_active_cpus = 1;
>> +	} else {
>>  		is_active = cpumask_test_cpu(cpu, msdata->active_cpus);
>> +		num_active_cpus = cpumask_weight(msdata->active_cpus);
>> +	}
>>  
>>  	/* Simple state machine */
>>  	do {
>>  		/* Chill out and ensure we re-read multi_stop_state. */
>>  		cpu_relax();
>> +
>> +		/*
>> +		 * In the case of CPU offline, we don't want the other CPUs to
>> +		 * send IPIs to the active_cpu (the one going offline) after it
>> +		 * has entered the _DISABLE_IRQ state (because, then it will
>> +		 * notice the IPIs only after it goes offline). So ensure that
>> +		 * the active_cpu always follows the others while entering
>> +		 * each subsequent state in this state-machine.
>> +		 *
>> +		 * msdata->thread_ack tracks the number of CPUs that are yet to
>> +		 * move to the next state, during each transition. So make the
>> +		 * active_cpu(s) wait until ->thread_ack indicates that the
>> +		 * active_cpus are the only ones left to complete the transition.
>> +		 */
>> +		if (is_active) {
>> +			/* Wait until all the non-active threads ack the state */
>> +			while (atomic_read(&msdata->thread_ack) > num_active_cpus)
>> +				cpu_relax();
>> +		}
> 
> Wouldn't it be cleaner to separate this out to a separate stage so
> that there are two separate DISABLE_IRQ stages - sth like
> MULTI_STOP_DISABLE_IRQ_INACTIVE and MULTI_STOP_DISABLE_IRQ_ACTIVE?
> The above adds an ad-hoc mechanism on top of the existing mechanism
> which is built to sequence similar things anyway.
>

Indeed, that looks like a much more elegant method! Thanks a lot for the
suggestion Tejun, I'll use that in the next version of the patchset.

Thank you!

Regards,
Srivatsa S. Bhat

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