[Devel] Re: [PATCH] io-controller: Preempt a non-rt queue if a rt ioq is present in ancestor or sibling groups

[Gui Jianfeng]

Vivek Goyal wrote:
> On Mon, Jun 22, 2009 at 03:44:08PM +0800, Gui Jianfeng wrote:
>> Preempt the ongoing non-rt ioq if there are rt ioqs waiting for dispatching
>> in ancestor or sibling groups. It will give other group's rt ioq an chance 
>> to dispatch ASAP.
>>
> 
> Hi Gui,
> 
> Will new preempton logic of traversing up the hiearchy so that both new
> queue and old queue are at same level to take a preemption decision not
> take care of above scenario?

Hi Vivek,

Would you explain a bit what do you mean about "both new queue and old queue 
are at same level to take a preemption decision". I don't understand it well.

> 
> Please have a look at bfq_find_matching_entity().
> 
> At the same time we probably don't want to preempt the non-rt queue
> with an RT queue in sibling group until and unless sibling group is an
> RT group.
> 
> 		root
> 		/  \
> 	   BEgrpA  BEgrpB
> 	      |     |	
> 	  BEioq1   RTioq2
> 
> Above we got two BE group A and B and assume ioq in group A is being
> served and then an RT request in group B comes. Because group B is an
> BE class group, we should not preempt the queue in group A.

  Yes, i also have this concern. So, it does not allow non-rt group preempts
  another group. I'll check whether there is a way to address this issue.

> 
> Thanks
> Vivek
> 
> 
>> Signed-off-by: Gui Jianfeng <guijianfeng at cn.fujitsu.com>
>> ---
>>  block/elevator-fq.c |   44 +++++++++++++++++++++++++++++++++++++++-----
>>  block/elevator-fq.h |    1 +
>>  2 files changed, 40 insertions(+), 5 deletions(-)
>>
>> diff --git a/block/elevator-fq.c b/block/elevator-fq.c
>> index 2ad40eb..80526fd 100644
>> --- a/block/elevator-fq.c
>> +++ b/block/elevator-fq.c
>> @@ -3245,8 +3245,16 @@ void elv_add_ioq_busy(struct elv_fq_data *efqd, struct io_queue *ioq)
>>  	elv_mark_ioq_busy(ioq);
>>  	efqd->busy_queues++;
>>  	if (elv_ioq_class_rt(ioq)) {
>> +		struct io_entity *entity;
>>  		struct io_group *iog = ioq_to_io_group(ioq);
>> +
>>  		iog->busy_rt_queues++;
>> +		entity = iog->entity.parent;
>> +
>> +		for_each_entity(entity) {
>> +			iog = io_entity_to_iog(entity);
>> +			iog->sub_busy_rt_queues++;
>> +		}
>>  	}
>>  
>>  #ifdef CONFIG_DEBUG_GROUP_IOSCHED
>> @@ -3290,9 +3298,18 @@ void elv_del_ioq_busy(struct elevator_queue *e, struct io_queue *ioq,
>>  	elv_clear_ioq_busy(ioq);
>>  	BUG_ON(efqd->busy_queues == 0);
>>  	efqd->busy_queues--;
>> +
>>  	if (elv_ioq_class_rt(ioq)) {
>> +		struct io_entity *entity;
>>  		struct io_group *iog = ioq_to_io_group(ioq);
>> +
>>  		iog->busy_rt_queues--;
>> +		entity = iog->entity.parent;
>> +
>> +		for_each_entity(entity) {
>> +			iog = io_entity_to_iog(entity);
>> +			iog->sub_busy_rt_queues--;
>> +		}
>>  	}
>>  
>>  	elv_deactivate_ioq(efqd, ioq, requeue);
>> @@ -3735,12 +3752,32 @@ int elv_iosched_expire_ioq(struct request_queue *q, int slice_expired,
>>  	return ret;
>>  }
>>  
>> +static int check_rt_queue(struct io_queue *ioq)
>> +{
>> +	struct io_group *iog;
>> +	struct io_entity *entity;
>> +
>> +	iog = ioq_to_io_group(ioq);
>> +
>> +	if (iog->busy_rt_queues)
>> +		return 1;
>> +
>> +	entity = iog->entity.parent;
>> +
>> +	for_each_entity(entity) {
>> +		iog = io_entity_to_iog(entity);
>> +		if (iog->sub_busy_rt_queues)
>> +			return 1;
>> +	}
>> +
>> +	return 0;
>> +}
>> +
>>  /* Common layer function to select the next queue to dispatch from */
>>  void *elv_fq_select_ioq(struct request_queue *q, int force)
>>  {
>>  	struct elv_fq_data *efqd = &q->elevator->efqd;
>>  	struct io_queue *new_ioq = NULL, *ioq = elv_active_ioq(q->elevator);
>> -	struct io_group *iog;
>>  	int slice_expired = 1;
>>  
>>  	if (!elv_nr_busy_ioq(q->elevator))
>> @@ -3811,12 +3848,9 @@ void *elv_fq_select_ioq(struct request_queue *q, int force)
>>  	/*
>>  	 * If we have a RT cfqq waiting, then we pre-empt the current non-rt
>>  	 * cfqq.
>> -	 *
>> -	 * TODO: This does not seem right across the io groups. Fix it.
>>  	 */
>> -	iog = ioq_to_io_group(ioq);
>>  
>> -	if (!elv_ioq_class_rt(ioq) && iog->busy_rt_queues) {
>> +	if (!elv_ioq_class_rt(ioq) && check_rt_queue(ioq)) {
>>  		/*
>>  		 * We simulate this as cfqq timed out so that it gets to bank
>>  		 * the remaining of its time slice.
>> diff --git a/block/elevator-fq.h b/block/elevator-fq.h
>> index b3193f8..be6c1af 100644
>> --- a/block/elevator-fq.h
>> +++ b/block/elevator-fq.h
>> @@ -248,6 +248,7 @@ struct io_group {
>>  	 * non-RT cfqq in service when this value is non-zero.
>>  	 */
>>  	unsigned int busy_rt_queues;
>> +	unsigned int sub_busy_rt_queues;
>>  
>>  	int deleting;
>>  	unsigned short iocg_id;
>> -- 
>> 1.5.4.rc3
>>
> 
> 
> 

-- 
Regards
Gui Jianfeng

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