Re: [PATCH V5 7/7] blk-mq-sched: don't dequeue request until all in ->dispatch are flushed

[Christoph Hellwig]

This looks good in general:

Reviewed-by: Christoph Hellwig <h...@lst.de>

Minor nitpicks below:

>       const bool has_sched_dispatch = e && e->type->ops.mq.dispatch_request;

This is now only tested once, so you can remove the local variable
for it.

> +             /*
> +              * We may clear DISPATCH_BUSY just after it
> +              * is set from another context, the only cost
> +              * is that one request is dequeued a bit early,
> +              * we can survive that. Given the window is
> +              * small enough, no need to worry about performance
> +              * effect.
> +              */

Use your 80 line real estate for comments please.

>       if (!has_sched_dispatch)
> +             if (!q->queue_depth) {
> +                     blk_mq_flush_busy_ctxs(hctx, &rq_list);
> +                     blk_mq_dispatch_rq_list(q, &rq_list);
> +             } else {
> +                     blk_mq_do_dispatch_ctx(q, hctx);
> +             }
> +     } else {
>               blk_mq_do_dispatch_sched(q, e, hctx);
> +     }

Maybe flatten this out to:

        if (e && e->type->ops.mq.dispatch_request) {
                blk_mq_do_dispatch_sched(q, e, hctx);
        } else if (q->queue_depth) {
                blk_mq_do_dispatch_ctx(q, hctx);
        } else {
                blk_mq_flush_busy_ctxs(hctx, &rq_list);
                blk_mq_dispatch_rq_list(q, &rq_list);
        }