On Wed, Aug 30, 2017 at 09:22:42AM -0600, Jens Axboe wrote: > On 08/30/2017 09:19 AM, Ming Lei wrote: > > It is more reasonable to add requests to ->dispatch in way > > of FIFO style, instead of LIFO style. > > > > Also in this way, we can allow to insert request at the front > > of hw queue, which function is needed to fix one bug > > in blk-mq's implementation of blk_execute_rq() > > > > Reported-by: Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx> > > Tested-by: Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx> > > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx> > > --- > > block/blk-mq-sched.c | 2 +- > > block/blk-mq.c | 2 +- > > 2 files changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c > > index 4ab69435708c..8d97df40fc28 100644 > > --- a/block/blk-mq-sched.c > > +++ b/block/blk-mq-sched.c > > @@ -272,7 +272,7 @@ static bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx *hctx, > > * the dispatch list. > > */ > > spin_lock(&hctx->lock); > > - list_add(&rq->queuelist, &hctx->dispatch); > > + list_add_tail(&rq->queuelist, &hctx->dispatch); > > spin_unlock(&hctx->lock); > > return true; > > } > > diff --git a/block/blk-mq.c b/block/blk-mq.c > > index 4603b115e234..fed3d0c16266 100644 > > --- a/block/blk-mq.c > > +++ b/block/blk-mq.c > > @@ -1067,7 +1067,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list) > > blk_mq_put_driver_tag(rq); > > > > spin_lock(&hctx->lock); > > - list_splice_init(list, &hctx->dispatch); > > + list_splice_tail_init(list, &hctx->dispatch); > > spin_unlock(&hctx->lock); > > I'm not convinced this is safe, there's actually a reason why the > request is added to the front and not the back. We do have > reorder_tags_to_front() as a safe guard, but I'd much rather get rid of reorder_tags_to_front() is for reordering the requests in current list, this patch is for splicing list into hctx->dispatch, so I can't see it isn't safe, or could you explain it a bit? > that than make this change. > > What's your reasoning here? Your changelog doesn't really explain why Firstly the 2nd patch need to add one rq(such as RQF_PM) to the front of the hw queue, the simple way is to add it to the front of hctx->dispatch. Without this change, the 2nd patch can't work at all. Secondly this way is still reasonable: - one rq is added to hctx->dispatch because queue is busy - another rq is added to hctx->dispatch too because of same reason so it is reasonable to to add list into hctx->dispatch in FIFO style. Finally my patchset for 'improving SCSI-MQ perf' will change to not dequeue rq from sw/scheduler if ->dispatch isn't flushed. I believe it is reasonable and correct thing to do, with that change, there won't be difference between the two styles. -- Ming
