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    SubjectRe: [PATCH V5 6/7] blk-mq-sched: improve dispatching from sw queue
    On Sat, Sep 30, 2017 at 06:27:19PM +0800, Ming Lei wrote:
    > SCSI devices use host-wide tagset, and the shared
    > driver tag space is often quite big. Meantime
    > there is also queue depth for each lun(.cmd_per_lun),
    > which is often small.
    >
    > So lots of requests may stay in sw queue, and we
    > always flush all belonging to same hw queue and
    > dispatch them all to driver, unfortunately it is
    > easy to cause queue busy because of the small
    > per-lun queue depth. Once these requests are flushed
    > out, they have to stay in hctx->dispatch, and no bio
    > merge can participate into these requests, and
    > sequential IO performance is hurted.
    >
    > This patch improves dispatching from sw queue when
    > there is per-request-queue queue depth by taking
    > request one by one from sw queue, just like the way
    > of IO scheduler.

    Once again, use your line length real estate for your change logs..

    > +static void blk_mq_do_dispatch_ctx(struct request_queue *q,
    > + struct blk_mq_hw_ctx *hctx)
    > +{
    > + LIST_HEAD(rq_list);
    > + struct blk_mq_ctx *ctx = READ_ONCE(hctx->dispatch_from);
    > + bool dispatched;
    > +
    > + do {
    > + struct request *rq;
    > +
    > + rq = blk_mq_dequeue_from_ctx(hctx, ctx);

    This probably should be merged wit hthe patch that introduceѕ
    blk_mq_dequeue_from_ctx.

    > + if (!rq)
    > + break;
    > + list_add(&rq->queuelist, &rq_list);

    Btw, do we really need to return a request from blk_mq_dequeue_from_ctx,
    or would it be easier to just add it directly to the list passed as
    an argument. I did wonder that about the existing code already.

    > +
    > + /* round robin for fair dispatch */
    > + ctx = blk_mq_next_ctx(hctx, rq->mq_ctx);
    > +
    > + dispatched = blk_mq_dispatch_rq_list(q, &rq_list);
    > + } while (dispatched);
    > +
    > + if (!dispatched)
    > + WRITE_ONCE(hctx->dispatch_from, ctx);

    No need for the dispatched argument, just write this as:

    } while (blk_mq_dispatch_rq_list(q, &rq_list));

    WRITE_ONCE(hctx->dispatch_from, ctx);

    and do an early return instead of a break from inside the loop.

    > + } else if (!has_sched_dispatch && !q->queue_depth) {
    > + /*
    > + * If there is no per-request_queue depth, we
    > + * flush all requests in this hw queue, otherwise
    > + * pick up request one by one from sw queue for
    > + * avoiding to mess up I/O merge when dispatch
    > + * run out of resource, which can be triggered
    > + * easily by per-request_queue queue depth
    > + */

    Use your 80 line real estate for comments..

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