linux-kernel - Re: [RFC PATCH 5/6] implement per-cpu&per-domain state machine call

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Date:	Wed, 07 Mar 2012 17:21:22 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Gilad Ben-Yossef <gilad@...yossef.com>
CC:	Lai Jiangshan <eag0628@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	patches@...aro.org
Subject: Re: [RFC PATCH 5/6] implement per-cpu&per-domain state machine call_srcu()

On 03/07/2012 04:10 PM, Gilad Ben-Yossef wrote:
> On Tue, Mar 6, 2012 at 4:44 PM, Lai Jiangshan <eag0628@...il.com> wrote:
>> On Tue, Mar 6, 2012 at 7:52 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>>> On Tue, 2012-03-06 at 17:57 +0800, Lai Jiangshan wrote:
>>>> +void srcu_barrier(struct srcu_struct *sp)
>>>> +{
>>>> +       struct srcu_sync sync;
>>>> +       struct srcu_head *head = &sync.head;
>>>> +       unsigned long chck_seq; /* snap */
>>>> +
>>>> +       int idle_loop = 0;
>>>> +       int cpu;
>>>> +       struct srcu_cpu_struct *scp;
>>>> +
>>>> +       spin_lock_irq(&sp->gp_lock);
>>>> +       chck_seq = sp->chck_seq;
>>>> +       for_each_possible_cpu(cpu) {
>>>
>>> ARGH!! this is really not ok.. so we spend all this time killing
>>> srcu_sync_expidited and co because they prod at all cpus for no good
>>> reason, and what do you do?
>>
>> it is srcu_barrier(), it have to wait all callbacks complete for all
>> cpus since it is per-cpu
>> implementation.
> 
> I would say it only needs to wait for callbacks to complete for all
> CPUs that has a callback pending.

Right.
The code above flush_workqueue() wait until all of them are delivered.
flush_workqueue() wait until all of them are completely invoked.

> 
> Unless I misunderstood something, that is what your code does already
> - it does not wait for completion,
> or schedules a work on a CPU that does not has a callback pending, right?
> 
>>
>>>
>>> Also, what happens if your cpu isn't actually online?
>>
>> The workqueue handles it, not here, if a cpu state machine has callbacks, the
>> state machine is started, if it has no callback,  srcu_barrier() does
>> nothing for
>> this cpu
> 
> I understand the point is that offline cpus wont have callbacks, so
> nothing would be
> done for them, but still, is that a reason to even check? why not use
> for_each_online_cpu

It is possible that the offline cpus have callbacks during hot-plugging.

> 
> I think that if a cpu that was offline went online after your check
> and managed to get an
> SRCU callback pending it is by definition not a callback srcu_barrier
> needs to wait for
> since it went pending at a later time then srcu_barrier was called. Or
> have I missed something?
> 
> Thanks,
> Gilad
> 

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