On 3/12/20 3:15 AM, Ming Lei wrote:
> commit 01e99aeca397 ("blk-mq: insert passthrough request into
> hctx->dispatch directly") may change to add flush request to the tail
> of dispatch by applying the 'add_head' parameter of
> blk_mq_sched_insert_request.
>
> Turns out this way causes performance regression on NCQ controller because
> flush is non-NCQ command, which can't be queued when there is any in-flight
> NCQ command. When adding flush rq to the front of hctx->dispatch, it is
> easier to introduce extra time to flush rq's latency compared with adding
> to the tail of dispatch queue because of S_SCHED_RESTART, then chance of
> flush merge is increased, and less flush requests may be issued to
> controller.
>
> So always insert flush request to the front of dispatch queue just like
> before applying commit 01e99aeca397 ("blk-mq: insert passthrough request
> into hctx->dispatch directly").
Applied, thanks.
--
Jens Axboe
