Re: [PATCH V7 5/6] block: support PREEMPT_ONLY

Ming Lei
2017-10-03T08:22:25Z

On Mon, Oct 02, 2017 at 06:50:24AM -0700, Christoph Hellwig wrote:
> > +void blk_set_preempt_only(struct request_queue *q, bool preempt_only)
> > +{
> > + blk_mq_freeze_queue(q);
> > + if (preempt_only)
> > + queue_flag_set_unlocked(QUEUE_FLAG_PREEMPT_ONLY, q);
> > + else
> > + queue_flag_clear_unlocked(QUEUE_FLAG_PREEMPT_ONLY, q);
> > + blk_mq_unfreeze_queue(q);
> > +}
> > +EXPORT_SYMBOL(blk_set_preempt_only);
>
> What do you need the queue freeze for?

The main reason is for draining requests in queue before setting PREEMP_ONLY.

>
> > + /*
> > + * preempt_only flag has to be set after queue is frozen,
> > + * so it can be checked here lockless and safely
> > + */
> > + if (blk_queue_preempt_only(q)) {
>
> We can always check a single bit flag safely, so I really don't
> understand that comment.
>
> > + if (!(flags & BLK_REQ_PREEMPT))
> > + goto slow_path;
> > + }
> > +
> > if (percpu_ref_tryget_live(&q->q_usage_counter))
> > return 0;
> > -
> > + slow_path:
>
> Also this looks a very spaghetti, why not:
>
>
> if (!blk_queue_preempt_only(q) || (flags & BLK_MQ_REQ_PREEMPT)) {
> if (percpu_ref_tryget_live(&q->q_usage_counter))
> return 0;
> }

Looks fine, will do it in next version.

--
Ming