On Mon, 3 Apr 2017, 9:47am, Jens Axboe wrote: > On 04/03/2017 10:41 AM, Arun Easi wrote: > > On Mon, 3 Apr 2017, 8:20am, Bart Van Assche wrote: > > > >> On Mon, 2017-04-03 at 09:29 +0200, Hannes Reinecke wrote: > >>> On 04/03/2017 08:37 AM, Arun Easi wrote: > >>>> If the above is true, then for a LLD to get tag# within it's max-tasks > >>>> range, it has to report max-tasks / number-of-hw-queues in can_queue, and > >>>> in the I/O path, use the tag and hwq# to arrive at a index# to use. This, > >>>> though, leads to a poor use of tag resources -- queue reaching it's > >>>> capacity while LLD can still take it. > >>> > >>> Shared tag sets continue to dog the block-mq on 'legacy' (ie non-NVMe) > >>> HBAs. ATM the only 'real' solution to this problem is indeed have a > >>> static split of the entire tag space by the number of hardware queues. > >>> With the mentioned tag-starvation problem. > >> > >> Hello Arun and Hannes, > >> > >> Apparently the current blk_mq_alloc_tag_set() implementation is well suited > >> for drivers like NVMe and ib_srp but not for traditional SCSI HBA drivers. > >> How about adding a BLK_MQ_F_* flag that tells __blk_mq_alloc_rq_maps() to > >> allocate a single set of tags for all hardware queues and also to add a flag > >> to struct scsi_host_template such that SCSI LLDs can enable this behavior? > >> > > > > Hi Bart, > > > > This would certainly be beneficial in my case. Moreover, it certainly > > makes sense to move the logic up where multiple drivers can leverage. > > > > Perhaps, use percpu_ida* interfaces to do that, but I think I read > > somewhere that, it is not efficient (enough?) and is the reason to go the > > current way for block tags. > > You don't have to change the underlying tag generation to solve this > problem, Bart already pretty much outlined a fix that would work. > percpu_ida works fine if you never use more than roughly half the > available space, it's a poor fit for request tags where we want to > retain good behavior and scaling at or near tag exhaustion. That's why > blk-mq ended up rolling its own, which is now generically available as > lib/sbitmap.c. > Sounds good. Thanks for the education, Jens. Regards, -Arun
