On Sun, Sep 10, 2017 at 12:45:15PM +0800, Ming Lei wrote:
> On Fri, Sep 08, 2017 at 04:54:39PM -0700, Omar Sandoval wrote:
> > On Sat, Sep 02, 2017 at 11:17:20PM +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.
> > >
> > > Reviewed-by: Bart Van Assche <[email protected]>
> > > Signed-off-by: Ming Lei <[email protected]>
> > > ---
> > > block/blk-mq-sched.c | 61
> > > +++++++++++++++++++++++++++++++++++++++++++++-----
> > > include/linux/blk-mq.h | 2 ++
> > > 2 files changed, 57 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> > > index f69752961a34..735e432294ab 100644
> > > --- a/block/blk-mq-sched.c
> > > +++ b/block/blk-mq-sched.c
> > > @@ -89,9 +89,9 @@ static bool blk_mq_sched_restart_hctx(struct
> > > blk_mq_hw_ctx *hctx)
> > > return false;
> > > }
> > >
> > > -static void blk_mq_do_dispatch(struct request_queue *q,
> > > - struct elevator_queue *e,
> > > - struct blk_mq_hw_ctx *hctx)
> > > +static void blk_mq_do_dispatch_sched(struct request_queue *q,
> > > + struct elevator_queue *e,
> > > + struct blk_mq_hw_ctx *hctx)
> > > {
> > > LIST_HEAD(rq_list);
> > >
> > > @@ -105,6 +105,42 @@ static void blk_mq_do_dispatch(struct request_queue
> > > *q,
> > > } while (blk_mq_dispatch_rq_list(q, &rq_list));
> > > }
> > >
> > > +static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
> > > + struct blk_mq_ctx *ctx)
> > > +{
> > > + unsigned idx = ctx->index_hw;
> > > +
> > > + if (++idx == hctx->nr_ctx)
> > > + idx = 0;
> > > +
> > > + return hctx->ctxs[idx];
> > > +}
> > > +
> > > +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_dispatch_rq_from_ctx(hctx, ctx);
> > > + if (!rq)
> > > + break;
> > > + list_add(&rq->queuelist, &rq_list);
> > > +
> > > + /* round robin for fair dispatch */
> > > + ctx = blk_mq_next_ctx(hctx, rq->mq_ctx);
> >
> > Hm... this next ctx will get skipped if the dispatch on the previous ctx
> > fails, since we call blk_mq_next_ctx() again. Seems unfair. Maybe move
> > the blk_mq_next_ctx() from the if (!dispatched) below into the if (!rq)
> > above?
>
> In case of if (!rq), that means there isn't any request in all ctxs
> belonging to this hctx, so it is reasonable to start the dispatch from
> any one of these ctxs next time, include the next one.
Yep, that case is okay.
> If dispatch fails on previous ctx, the rq from that ctx will be
> put into ->dispatch, so it is fair to start dispatch from next ctx
> next time too.
I'm talking about this case
LIST_HEAD(rq_list);
struct blk_mq_ctx *ctx = READ_ONCE(hctx->dispatch_from);
bool dispatched;
/*
* Let's say that ctxs 0, 1, and 2 all have requests pending and
* hctx->dispatch_from was ctx0, so ctx is ctx0 when we start.
*/
do {
struct request *rq;
rq = blk_mq_dispatch_rq_from_ctx(hctx, ctx);
if (!rq)
break;
list_add(&rq->queuelist, &rq_list);
/* Now rq is a request from ctx0 */
/* round robin for fair dispatch */
ctx = blk_mq_next_ctx(hctx, rq->mq_ctx);
/* Now ctx is ctx1. */
dispatched = blk_mq_dispatch_rq_list(q, &rq_list);
/* If we couldn't dispatch, we break here. */
} while (dispatched);
if (!dispatched)
/*
* Now we set hctx->dispatch_from to ctx2, so we've
* skipped over ctx1.
*/
WRITE_ONCE(hctx->dispatch_from, blk_mq_next_ctx(hctx, ctx));
