[dm-devel] [PATCH V6 5/5] blk-mq-sched: don't dequeue request until all in ->dispatch are flushed

[Omar Sandoval]

On Mon, Oct 09, 2017 at 07:24:24PM +0800, Ming Lei wrote:
> During dispatching, we moved all requests from hctx->dispatch to
> one temporary list, then dispatch them one by one from this list.
> Unfortunately during this period, run queue from other contexts
> may think the queue is idle, then start to dequeue from sw/scheduler
> queue and still try to dispatch because ->dispatch is empty. This way
> hurts sequential I/O performance because requests are dequeued when
> lld queue is busy.
> 
> This patch introduces the state of BLK_MQ_S_DISPATCH_BUSY to
> make sure that request isn't dequeued until ->dispatch is
> flushed.
> 
> Reviewed-by: Bart Van Assche <bart.vanassche at wdc.com>
> Reviewed-by: Christoph Hellwig <hch at lst.de>

I think this will do for now.

Reviewed-by: Omar Sandoval <osandov at fb.com>

> Signed-off-by: Ming Lei <ming.lei at redhat.com>
> ---
>  block/blk-mq-debugfs.c |  1 +
>  block/blk-mq-sched.c   | 38 ++++++++++++++++++++++++++++++++------
>  block/blk-mq.c         |  5 +++++
>  include/linux/blk-mq.h |  1 +
>  4 files changed, 39 insertions(+), 6 deletions(-)