On 11/14/18 11:02 AM, Ming Lei wrote: > On Wed, Nov 14, 2018 at 10:15 AM jianchao.wang > <jianchao.w.wang@xxxxxxxxxx> wrote: >> >> 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. > > I guess it is quite hard to respect this rule 100%, such as in case of > CPU hotplug. > Yes, it is indeed the case. Looks like this patch is contentious. I will drop this one and post later as a standalone one if necessary. Thanks Jianchao
