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

From: Gui Jianfeng
Date: Thu Dec 17 2009 - 04:25:20 EST

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@xxxxxxxxxx>

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.
But with this change, this bug is gone.

Reviewed-by: Gui Jianfeng <guijianfeng@xxxxxxxxxxxxxx>

> ---
> 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;
>
> /*

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