Re: [PATCH 0/2] rcu: Fix series of spurious RCU softirqs

[Lai Jiangshan]

On 11/25/2010 03:38 PM, Frederic Weisbecker wrote:
> On Thu, Nov 25, 2010 at 11:42:34AM +0800, Lai Jiangshan wrote:
>> On 11/24/2010 08:31 AM, Frederic Weisbecker wrote:
>>> Hi,
>>>
>>> I've observed some not so unfrequent series of spurious rcu
>>> softirqs, sometimes happening at each ticks for a random
>>> while.
>>>
>>> These patches aims at fixing them.
>>>
>>> Thanks.
>>>
>>> Frederic Weisbecker (2):
>>>   rcu: Don't chase unnecessary quiescent states after extended grace periods
>>>   rcu: Stop checking quiescent states after grace period completion from remote
>>>
>>
>> If we ensure rdp->gpnum >= rdp->completed is always true, the problems as
>> you described will not be existed. Or maybe I misunderstand you.
>>
>> rdp->gpnum >= rdp->completed is a very important guarantee I think.
>> (In my RCURING, it is guaranteed.) I'm afraid there are some other
>> problems still hidden if it is not guaranteed.
>>
>> so I recommend: (code is better than words)
>>
>> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
>> index d5bc439..af4e87a 100644
>> --- a/kernel/rcutree.c
>> +++ b/kernel/rcutree.c
>> @@ -648,6 +648,13 @@ __rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
>>  
>>  		/* Remember that we saw this grace-period completion. */
>>  		rdp->completed = rnp->completed;
>> +
>> +		/* Ensure ->gpnum >= ->completed after NO_HZ */
>> +		if (unlikely(rnp->completed - rdp->gpnum > 0
>> +				|| rdp->gpnum - rnp->gpnum > 0)) {
>> +			rdp->gpnum = rnp->completed;
>> +			rdp->qs_pending = 0;
> 
> 
> That's an alternative to my first patch yeah.

Since rdp->gpnum >= rdp->completed is guaranteed.
your second patch is not needed, the problem is also fixed.

if rnp->gpnum == rnp->completed, rcu_report_qs_rdp() will not be called.
it is because rdp->qs_pending == 0 when rnp->gpnum == rnp->completed.


And if rdp->gpnum >= rdp->completed
> must be a guarantee outside the rnp lock, then it's certainly better because
> the lock is relaxed between rcu_process_gp_end() and note_new_gpnum(), and
> both values are async in this lockless frame.
> 
> But perhaps this shouldn't touch rdp->qs_pending:

if rdp->gpnum == rnp->completed, it means we don't need a qs for rdp->gpnum,
it is completed. so we must set rdp->qs_pending = 0;

when we really need a qs, rdp->qs_pending will be fixed in note_new_gp_new().

> 
> "if (rnp->completed > rdp->gpnum || rdp->gpnum > rnp->gpnum)" is not
> a guarantee that we don't need to find quiescent states.
> 
> but rnp->completed == rnp->gpnum would provide that guarantee.
> That said, note_new_gp_new() would fix the value of rdp->qs_pending.
> 
> Thanks.
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