Hi Jens Thanks for your kindly response. On 11/13/18 9:44 PM, Jens Axboe wrote: > On 11/13/18 2:56 AM, Jianchao Wang wrote: >> When issue request directly and the task is migrated out of the >> original cpu where it allocates request, hctx could be ran on >> the cpu where it is not mapped. >> To fix this, >> - insert the request forcibly if BLK_MQ_F_BLOCKING is set. >> - check whether the current is mapped to the hctx, if not, insert >> forcibly. >> - invoke __blk_mq_issue_directly under preemption disabled. > > I'm not too crazy about this one, adding a get/put_cpu() in the hot > path, and a cpumask test. The fact is that most/no drivers care > about strict placement. We always try to do so, if convenient, > since it's faster, but this seems to be doing the opposite. > > I'd be more inclined to have a driver flag if it needs guaranteed > placement, using one an ops BLK_MQ_F_STRICT_CPU flag or similar. > > What do you think? > I'd inclined blk-mq should comply with a unified rule, no matter the issuing directly path or inserting one. Then blk-mq would have a simpler model. And also this guarantee could be a little good for drivers, especially the case where cpu and hw queue mapping is 1:1. Regarding with hot path, do you concern about the nvme device ? If so, how about split a standalone path for it ? Thanks Jianchao
