On Fri, Sep 02 2016 at 11:12am -0400, Mike Snitzer <snitzer@xxxxxxxxxx> wrote: > So in the case of blk-mq request-based DM: we cannot expect > blk_mq_freeze_queue(), during suspend, to complete if requests are > getting requeued to the blk-mq queue via BLK_MQ_RQ_QUEUE_BUSY. Looking closer at blk-mq. Currently __blk_mq_run_hw_queue() will move any requeued requests to the hctx->dispatch list and then performs async blk_mq_run_hw_queue(). To do what you hoped (have blk_mq_freeze_queue() discontinue all use of blk-mq hw queues during DM suspend) I think we'd need blk-mq to: 1) avoid processing requeued IO if blk_mq_freeze_queue() was used to freeze the queue. Meaning it'd have to hold requeued work longer than it currently does. 2) Then once blk_mq_unfreeze_queue() is called it'd allow requeues to proceed. This would be catering to a very specific requirement of DM (given it re-queues IO back to the request_queue during suspend). BUT all said, relative to request-based DM multipath, what we have is perfectly fine on a correctness level: the requests are re-queued because the blk-mq DM device is suspended. Unfortunately on an efficiency level DM suspend creates a lot of busy looping in blk-mq, with 100% cpu usage in a threads with names "kworker/3:1H", ideally we'd avoid that! -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
