Re: [PATCH V2 11/20] blk-mq: introduce helpers for operating ->dispatch list

Ming Lei
2017-08-24T07:11:06Z

On Thu, Aug 24, 2017 at 09:59:13AM +0900, Damien Le Moal wrote:
>
> On 8/23/17 05:43, Bart Van Assche wrote:
> > On Sat, 2017-08-05 at 14:56 +0800, Ming Lei wrote:
> >> +static inline bool blk_mq_has_dispatch_rqs(struct blk_mq_hw_ctx *hctx)
> >> +{
> >> + return !list_empty_careful(&hctx->dispatch);
> >> +}
> >> +
> >> +static inline void blk_mq_add_rq_to_dispatch(struct blk_mq_hw_ctx *hctx,
> >> + struct request *rq)
> >> +{
> >> + spin_lock(&hctx->lock);
> >> + list_add(&rq->queuelist, &hctx->dispatch);
> >> + blk_mq_hctx_set_dispatch_busy(hctx);
> >> + spin_unlock(&hctx->lock);
> >> +}
> >> +
> >> +static inline void blk_mq_add_list_to_dispatch(struct blk_mq_hw_ctx *hctx,
> >> + struct list_head *list)
> >> +{
> >> + spin_lock(&hctx->lock);
> >> + list_splice_init(list, &hctx->dispatch);
> >> + blk_mq_hctx_set_dispatch_busy(hctx);
> >> + spin_unlock(&hctx->lock);
> >> +}
> >> +
> >> +static inline void blk_mq_add_list_to_dispatch_tail(struct blk_mq_hw_ctx *hctx,
> >> + struct list_head *list)
> >> +{
> >> + spin_lock(&hctx->lock);
> >> + list_splice_tail_init(list, &hctx->dispatch);
> >> + blk_mq_hctx_set_dispatch_busy(hctx);
> >> + spin_unlock(&hctx->lock);
> >> +}
> >> +
> >> +static inline void blk_mq_take_list_from_dispatch(struct blk_mq_hw_ctx *hctx,
> >> + struct list_head *list)
> >> +{
> >> + spin_lock(&hctx->lock);
> >> + list_splice_init(&hctx->dispatch, list);
> >> + spin_unlock(&hctx->lock);
> >> +}
> >
> > Same comment for this patch: these helper functions are so short that I'm not
> > sure it is useful to introduce these helper functions.
> >
> > Bart.
>
> Personally, I like those very much as they give a place to hook up
> different dispatch_list handling without having to change blk-mq.c and
> blk-mq-sched.c all over the place.
>
> I am thinking of SMR (zoned block device) support here since we need to
> to sort insert write requests in blk_mq_add_rq_to_dispatch() and
> blk_mq_add_list_to_dispatch_tail(). For this last one, the name would

Could you explain a bit why you sort insert write rq in these two
helpers? which are only triggered in case of dispatch busy.

> become a little awkward though. This sort insert would be to avoid
> breaking a sequential write request sequence sent by the disk user. This
> is needed since this reordering breakage cannot be solved only from the
> SCSI layer.

Basically this patchset tries to flush hctx->dispatch first, then
fetch requests from scheduler queue after htct->dispatch is flushed.

But it isn't strictly in this way because the implementation is lockless.

So looks the request from hctx->dispatch won't break one coming
requests from scheduler.

--
Ming