On Mon, May 01, 2017 at 03:06:16PM +0000, Bart Van Assche wrote: > On Sat, 2017-04-29 at 18:35 +0800, Ming Lei wrote: > > On Fri, Apr 28, 2017 at 06:09:40PM +0000, Bart Van Assche wrote: > > > On Fri, 2017-04-28 at 23:15 +0800, Ming Lei wrote: > > > > +static inline bool blk_mq_sched_may_use_hw_tag(struct request_queue *q) > > > > +{ > > > > + if (q->tag_set->flags & BLK_MQ_F_TAG_SHARED) > > > > + return false; > > > > + > > > > + if (blk_mq_get_queue_depth(q) < q->nr_requests) > > > > + return false; > > > > + > > > > + return true; > > > > +} > > > > > > The only user of shared tag sets I know of is scsi-mq. I think it's really > > > unfortunate that this patch systematically disables BLK_MQ_F_SCHED_USE_HW_TAG > > > for scsi-mq. > > > > In previous patch, I actually allow driver to pass this flag, but this > > feature is dropped in this post, just for making it simple & clean. > > If you think we need it for shared tag set, I can add it in v1. > > > > For shared tag sets, I suggest to not enable it at default, because > > scheduler is per request queue now, and generaly more requests available, > > better it performs. When tags are shared among several request > > queues, one of them may use tags up for its own scheduling, then > > starve others. But it should be possible and not difficult to allocate > > requests fairly for scheduling in this case if we switch to per-hctx > > scheduling. > > Hello Ming, > > Have you noticed that there is already a mechanism in the block layer to > avoid starvation if a tag set is shared? The hctx_may_queue() function > guarantees that each user that shares a tag set gets at least some tags. > The .active_queues counter keeps track of the number of hardware queues > that share a tag set. > > Bart. The scheduler tags are there to abstract away the hardware, and USE_HW_TAG should just be an optimization for when that abstraction is a noop. That's not the case when there are shared tags, and I doubt that the overhead of the scheduler tags is significant for scsi-mq. Let's stick with the behavior Ming had here.