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Date: Sun, 20 Dec 2009 12:19:56 +0800
From: Gui Jianfeng <guijianfeng@...fujitsu.com>
To: Vivek Goyal <vgoyal@...hat.com>
CC: linux-kernel@...r.kernel.org, jens.axboe@...cle.com,
nauman@...gle.com, lizf@...fujitsu.com, ryov@...inux.co.jp,
fernando@....ntt.co.jp, taka@...inux.co.jp, jmoyer@...hat.com,
m-ikeda@...jp.nec.com, czoccolo@...il.com, Alan.Brunelle@...com
Subject: Re: [PATCH 3/4] cfq-iosched: Remove prio_change logic for workload
selection
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@...hat.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@...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
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