On Sat, 2017-09-23 at 01:44 +0800, Ming Lei wrote: > On Fri, Sep 22, 2017 at 03:06:16PM +0000, Bart Van Assche wrote: > > On Fri, 2017-09-22 at 09:35 +0800, Ming Lei wrote: > > > + /* > > > + * blk-mq's SCHED_RESTART can cover this requeue, so > > > + * we needn't to deal with it by DELAY_REQUEUE. More > > > + * importantly, we have to return DM_MAPIO_REQUEUE > > > + * so that blk-mq can get the queue busy feedback, > > > + * otherwise I/O merge can be hurt. > > > + */ > > > + if (q->mq_ops) > > > + return DM_MAPIO_REQUEUE; > > > + else > > > + return DM_MAPIO_DELAY_REQUEUE; > > > } > > > > This patch is inferior to what I posted because this patch does not avoid > > the delay if multiple LUNs are associated with the same SCSI host. Consider > > e.g. the following configuration: > > * A single SCSI host with two SCSI LUNs associated to that host, e.g. /dev/sda > > and /dev/sdb. > > * A dm-mpath instance has been created on top of /dev/sda. > > If all tags are in use by requests queued to /dev/sdb, no dm requests are in > > progress and a request is submitted against the dm-mpath device then the > > blk_get_request(q, GFP_ATOMIC) call will fail. The request will be requeued > > and the queue will be rerun after a delay. > > > > My patch does not introduce a delay in this case. > > That delay may not matter because SCHED_RESTART will run queue just > after one request is completed. Did you understand what I wrote? SCHED_RESTART will be set for /dev/sdb but not for the dm queue. That's what I was trying to explain to you in my previous e-mail. > There is at least one issue with get_request(GFP_NOIO): AIO > performance regression may not be caused, or even AIO may not > be possible. For example, user runs fio(libaio, randread, single > job, queue depth: 64, device: dm-mpath disk), if get_request(GFP_NOIO) > often blocks because of shared tags or out of tag, the actual queue > depth won't reach 64 at all, and may be just 1 in the worst case. > Once the actual queue depth is decreased much, random I/O performance > should be hurt a lot. That's why we need to modify scsi_lld_busy(). If scsi_lld_busy() will be modified as I proposed in a previous e-mail then it will become very unlikely that no tag is available when blk_get_request() is called. With that scsi_lld_busy() modification it is even possible that we don't need to modify the dm-mpath driver. Bart.
