Re: [PATCH 3/4] cfq-iosched: Remove prio_change logic for workload selection

[Gui Jianfeng <guijianfeng@cn.fujitsu.com>]

Vivek Goyal wrote:
> On Thu, Dec 17, 2009 at 05:20:30PM +0800, Gui Jianfeng wrote:
>> Vivek Goyal wrote:
>>> o CFQ now internally divides cfq queues in therr workload categories. sync-idle,
>>>   sync-noidle and async. Which workload to run depends primarily on rb_key
>>>   offset across three service trees. Which is a combination of mulitiple things
>>>   including what time queue got queued on the service tree.
>>>
>>>   There is one exception though. That is if we switched the prio class, say
>>>   we served some RT tasks and again started serving BE class, then with-in
>>>   BE class we always started with sync-noidle workload irrespective of rb_key
>>>   offset in service trees.
>>>
>>>   This can provide better latencies for sync-noidle workload in the presence
>>>   of RT tasks.
>>>
>>> o This patch gets rid of that exception and which workload to run with-in
>>>   class always depends on lowest rb_key across service trees. The reason
>>>   being that now we have multiple BE class groups and if we always switch
>>>   to sync-noidle workload with-in group, we can potentially starve a sync-idle
>>>   workload with-in group. Same is true for async workload which will be in
>>>   root group. Also the workload-switching with-in group will become very
>>>   unpredictable as it now depends whether some RT workload was running in
>>>   the system or not.
>>>
>>> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
>> Actually, in current CFQ, there still has bug in prio_changed logic.
>> Because prio_changed = (cfqd->serving_prio != previous_prio), and previous_prio
>> might come from another group, in this case, prio_changed becomes invalid, but
>> cfq_choose_wl() still relies on prio_changed to calculate serving_type, this
>> doesn't make sence IMHO. 
> 
> Ok, so you are referring to the case when we don't save workload state
> because it has expired at the time of group expiry.
> 
> How about saving the serving prio all the time at the time of group expiry,
> irrespective of the fact whether workload has expired or not at the time of
> group expiry and while we also need to always restore the serving prio back
> when group is rescheduled in.

Hi Vivek,

Yes, i think so also.

Thanks,
Gui

> 
> Thanks
> Vivek
> 
>> But with this change, this bug is gone.
>>
>> Reviewed-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
>>
>>> ---
>>>  block/cfq-iosched.c |   48 ++++++++++++------------------------------------
>>>  1 files changed, 12 insertions(+), 36 deletions(-)
>>>
>>> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
>>> index d9bfa09..8df4fe5 100644
>>> --- a/block/cfq-iosched.c
>>> +++ b/block/cfq-iosched.c
>>> @@ -292,8 +292,7 @@ static struct cfq_group *cfq_get_next_cfqg(struct cfq_data *cfqd);
>>>  
>>>  static struct cfq_rb_root *service_tree_for(struct cfq_group *cfqg,
>>>  					    enum wl_prio_t prio,
>>> -					    enum wl_type_t type,
>>> -					    struct cfq_data *cfqd)
>>> +					    enum wl_type_t type)
>>>  {
>>>  	if (!cfqg)
>>>  		return NULL;
>>> @@ -1146,7 +1145,7 @@ static void cfq_service_tree_add(struct cfq_data *cfqd, struct cfq_queue *cfqq,
>>>  #endif
>>>  
>>>  	service_tree = service_tree_for(cfqq->cfqg, cfqq_prio(cfqq),
>>> -						cfqq_type(cfqq), cfqd);
>>> +						cfqq_type(cfqq));
>>>  	if (cfq_class_idle(cfqq)) {
>>>  		rb_key = CFQ_IDLE_DELAY;
>>>  		parent = rb_last(&service_tree->rb);
>>> @@ -1609,7 +1608,7 @@ static struct cfq_queue *cfq_get_next_queue(struct cfq_data *cfqd)
>>>  {
>>>  	struct cfq_rb_root *service_tree =
>>>  		service_tree_for(cfqd->serving_group, cfqd->serving_prio,
>>> -					cfqd->serving_type, cfqd);
>>> +					cfqd->serving_type);
>>>  
>>>  	if (!cfqd->rq_queued)
>>>  		return NULL;
>>> @@ -1956,8 +1955,7 @@ static void cfq_setup_merge(struct cfq_queue *cfqq, struct cfq_queue *new_cfqq)
>>>  }
>>>  
>>>  static enum wl_type_t cfq_choose_wl(struct cfq_data *cfqd,
>>> -				struct cfq_group *cfqg, enum wl_prio_t prio,
>>> -				bool prio_changed)
>>> +				struct cfq_group *cfqg, enum wl_prio_t prio)
>>>  {
>>>  	struct cfq_queue *queue;
>>>  	int i;
>>> @@ -1965,24 +1963,9 @@ static enum wl_type_t cfq_choose_wl(struct cfq_data *cfqd,
>>>  	unsigned long lowest_key = 0;
>>>  	enum wl_type_t cur_best = SYNC_NOIDLE_WORKLOAD;
>>>  
>>> -	if (prio_changed) {
>>> -		/*
>>> -		 * When priorities switched, we prefer starting
>>> -		 * from SYNC_NOIDLE (first choice), or just SYNC
>>> -		 * over ASYNC
>>> -		 */
>>> -		if (service_tree_for(cfqg, prio, cur_best, cfqd)->count)
>>> -			return cur_best;
>>> -		cur_best = SYNC_WORKLOAD;
>>> -		if (service_tree_for(cfqg, prio, cur_best, cfqd)->count)
>>> -			return cur_best;
>>> -
>>> -		return ASYNC_WORKLOAD;
>>> -	}
>>> -
>>> -	for (i = 0; i < 3; ++i) {
>>> -		/* otherwise, select the one with lowest rb_key */
>>> -		queue = cfq_rb_first(service_tree_for(cfqg, prio, i, cfqd));
>>> +	for (i = 0; i <= SYNC_WORKLOAD; ++i) {
>>> +		/* select the one with lowest rb_key */
>>> +		queue = cfq_rb_first(service_tree_for(cfqg, prio, i));
>>>  		if (queue &&
>>>  		    (!key_valid || time_before(queue->rb_key, lowest_key))) {
>>>  			lowest_key = queue->rb_key;
>>> @@ -1996,8 +1979,6 @@ static enum wl_type_t cfq_choose_wl(struct cfq_data *cfqd,
>>>  
>>>  static void choose_service_tree(struct cfq_data *cfqd, struct cfq_group *cfqg)
>>>  {
>>> -	enum wl_prio_t previous_prio = cfqd->serving_prio;
>>> -	bool prio_changed;
>>>  	unsigned slice;
>>>  	unsigned count;
>>>  	struct cfq_rb_root *st;
>>> @@ -2025,24 +2006,19 @@ static void choose_service_tree(struct cfq_data *cfqd, struct cfq_group *cfqg)
>>>  	 * (SYNC, SYNC_NOIDLE, ASYNC), and to compute a workload
>>>  	 * expiration time
>>>  	 */
>>> -	prio_changed = (cfqd->serving_prio != previous_prio);
>>> -	st = service_tree_for(cfqg, cfqd->serving_prio, cfqd->serving_type,
>>> -				cfqd);
>>> +	st = service_tree_for(cfqg, cfqd->serving_prio, cfqd->serving_type);
>>>  	count = st->count;
>>>  
>>>  	/*
>>> -	 * If priority didn't change, check workload expiration,
>>> -	 * and that we still have other queues ready
>>> +	 * check workload expiration, and that we still have other queues ready
>>>  	 */
>>> -	if (!prio_changed && count &&
>>> -	    !time_after(jiffies, cfqd->workload_expires))
>>> +	if (count && !time_after(jiffies, cfqd->workload_expires))
>>>  		return;
>>>  
>>>  	/* otherwise select new workload type */
>>>  	cfqd->serving_type =
>>> -		cfq_choose_wl(cfqd, cfqg, cfqd->serving_prio, prio_changed);
>>> -	st = service_tree_for(cfqg, cfqd->serving_prio, cfqd->serving_type,
>>> -				cfqd);
>>> +		cfq_choose_wl(cfqd, cfqg, cfqd->serving_prio);
>>> +	st = service_tree_for(cfqg, cfqd->serving_prio, cfqd->serving_type);
>>>  	count = st->count;
>>>  
>>>  	/*
> 
> 
> 

-- 
Regards
Gui Jianfeng