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. Regards, -Arun
