Currently cfq does round robin among cfqq and allocates bigger slices to higher prio queue. But it also does additional logic of putting higher priority queues ahead of lower priority queues in the service tree. cfq_slice_offset() determines the postion of a queue in the service tree.
I think it was done so that higher prio queues can get even higher share of disk. Other advantage of it could be trying to provide service differentiation on SSD where we don't idle on queues. So instead of trying to provide bigger slice lenght for higher prio queue, one can try to schedule the queue more number of times. I don't think in practice it will work very well and reason being that there are not many queues on service tree. As we don't idle, we dispatch the request and expire the queue. So if queue depth is 32, ideally you need to have more than 32 cfqq doing IO (assuming each queue dipatches one read and waits for it to finish). And after that one can hope to see some service differentiaton and that too very unpredictable. So I would not count on it and rather keep it simple that on SSD we don't get ioprio differentiation. Even after we move to vdisktime logic, one can introduce above kind of appriximations where higher prio/weight queue is not put at the end but instead we give it some vdisktime boost. So this patch puts every new queue at the end of service tree by default. Existing queues get their position in the tree depending on how much slice did they use recently and what's their prio/weight. This patch only introduces the functionality of adding queues at the end of service tree. Later patches will introduce the functionality of determining vdisktime (hence position in service tree) based on slice used and weight. If a queue is being requeued, then it will already be on service tree and we can't determine the rb_key of last element using cfq_rb_last(). So we always remove the queue from service tree first. This is just an intermediate patch to show clearly how I am chaning existing functionality. Did not want to lump it together with bigger patches. Signed-off-by: Vivek Goyal <[email protected]> --- block/cfq-iosched.c | 49 ++++++++++++++++--------------------------------- 1 files changed, 16 insertions(+), 33 deletions(-) diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index 58f1bdc..7136ede 100644 --- a/block/cfq-iosched.c +++ b/block/cfq-iosched.c @@ -1139,16 +1139,6 @@ cfq_find_next_rq(struct cfq_data *cfqd, struct cfq_queue *cfqq, return cfq_choose_req(cfqd, next, prev, blk_rq_pos(last)); } -static unsigned long cfq_slice_offset(struct cfq_data *cfqd, - struct cfq_queue *cfqq) -{ - /* - * just an approximation, should be ok. - */ - return (cfqq->cfqg->nr_cfqq - 1) * (cfq_prio_slice(cfqd, 1, 0) - - cfq_prio_slice(cfqd, cfq_cfqq_sync(cfqq), cfqq->ioprio)); -} - static inline s64 cfqg_key(struct cfq_rb_root *st, struct cfq_group *cfqg) { @@ -1628,41 +1618,34 @@ static void cfq_service_tree_add(struct cfq_data *cfqd, struct cfq_queue *cfqq, bool new_cfqq = RB_EMPTY_NODE(&cfqq->rb_node); st = st_for(cfqq->cfqg, cfqq_class(cfqq), cfqq_type(cfqq)); - if (cfq_class_idle(cfqq)) { + if (!new_cfqq) { + cfq_rb_erase(&cfqq->rb_node, cfqq->service_tree); + cfqq->service_tree = NULL; + } + if (!add_front) { rb_key = CFQ_IDLE_DELAY; parent = rb_last(&st->rb); - if (parent && parent != &cfqq->rb_node) { + if (parent) { __cfqq = rb_entry(parent, struct cfq_queue, rb_node); rb_key += __cfqq->rb_key; } else rb_key += jiffies; - } else if (!add_front) { - /* - * Get our rb key offset. Subtract any residual slice - * value carried from last service. A negative resid - * count indicates slice overrun, and this should position - * the next service time further away in the tree. - */ - rb_key = cfq_slice_offset(cfqd, cfqq) + jiffies; - rb_key -= cfqq->slice_resid; - cfqq->slice_resid = 0; + if (!cfq_class_idle(cfqq)) { + /* + * Subtract any residual slice * value carried from + * last service. A negative resid count indicates + * slice overrun, and this should position + * the next service time further away in the tree. + */ + rb_key -= cfqq->slice_resid; + cfqq->slice_resid = 0; + } } else { rb_key = -HZ; __cfqq = cfq_rb_first(st); rb_key += __cfqq ? __cfqq->rb_key : jiffies; } - if (!new_cfqq) { - /* - * same position, nothing more to do - */ - if (rb_key == cfqq->rb_key && cfqq->service_tree == st) - return; - - cfq_rb_erase(&cfqq->rb_node, cfqq->service_tree); - cfqq->service_tree = NULL; - } - left = 1; parent = NULL; cfqq->service_tree = st; -- 1.7.7.6 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
