On Fri, Sep 08, 2017 at 01:56:29PM -0700, Omar Sandoval wrote:
> On Fri, Sep 08, 2017 at 01:54:48PM -0700, Omar Sandoval wrote:
> > On Fri, Sep 08, 2017 at 01:48:32PM -0700, Omar Sandoval wrote:
> > > On Sat, Sep 02, 2017 at 11:17:16PM +0800, Ming Lei wrote:
> > > > When hw queue is busy, we shouldn't take requests from
> > > > scheduler queue any more, otherwise it is difficult to do
> > > > IO merge.
> > > >
> > > > This patch fixes the awful IO performance on some
> > > > SCSI devices(lpfc, qla2xxx, ...) when mq-deadline/kyber
> > > > is used by not taking requests if hw queue is busy.
> > > >
> > > > Reviewed-by: Bart Van Assche <bart.vanass...@wdc.com>
> > >
> > > Jens and I did some testing with xfs/297 and couldn't find any issues,
> > > so
> > >
> > > Reviewed-by: Omar Sandoval <osan...@fb.com>
> >
> > I take it back, the comment below needs to be updated
> >
> > > > Signed-off-by: Ming Lei <ming....@redhat.com>
> > > > ---
> > > > block/blk-mq-sched.c | 6 +++---
> > > > 1 file changed, 3 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> > > > index 4ab69435708c..845e5baf8af1 100644
> > > > --- a/block/blk-mq-sched.c
> > > > +++ b/block/blk-mq-sched.c
> > > > @@ -94,7 +94,7 @@ void blk_mq_sched_dispatch_requests(struct
> > > > blk_mq_hw_ctx *hctx)
> > > > struct request_queue *q = hctx->queue;
> > > > struct elevator_queue *e = q->elevator;
> > > > const bool has_sched_dispatch = e &&
> > > > e->type->ops.mq.dispatch_request;
> > > > - bool did_work = false;
> > > > + bool do_sched_dispatch = true;
> > > > LIST_HEAD(rq_list);
> > > >
> > > > /* RCU or SRCU read lock is needed before checking quiesced
> > > > flag */
> > > > @@ -125,7 +125,7 @@ void blk_mq_sched_dispatch_requests(struct
> > > > blk_mq_hw_ctx *hctx)
> > > > */
> > > > if (!list_empty(&rq_list)) {
> > > > blk_mq_sched_mark_restart_hctx(hctx);
> > > > - did_work = blk_mq_dispatch_rq_list(q, &rq_list);
> > > > + do_sched_dispatch = blk_mq_dispatch_rq_list(q,
> > > > &rq_list);
> > > > } else if (!has_sched_dispatch) {
> > > > blk_mq_flush_busy_ctxs(hctx, &rq_list);
> > > > blk_mq_dispatch_rq_list(q, &rq_list);
> > > > @@ -136,7 +136,7 @@ void blk_mq_sched_dispatch_requests(struct
> > > > blk_mq_hw_ctx *hctx)
> > > > * on the dispatch list, OR if we did have work but weren't able
> > > > * to make progress.
> > > > */
> >
> > This should be updated to say:
> >
> > We want to dispatch from the scheduler if there was nothing on the
> > dispatch list or we were able to dispatch from the dispatch list.
>
> By the way, did you experiment with changing blk_mq_dispatch_rq_list()
> to return true IFF _all_ requests were dispatched, instead of _any_
> requests were dispatched?
I understand that changes nothing, because dispatch may happen
concurrently, and queue busy may be triggered in any one of the contexts
and cause rq to be added to ->dispatch.
So even all requests are dispatched successfully from
blk_mq_dispatch_rq_list(), rq still may be added to ->dispatch
from other contexts, and you will see the check in patch
"blk-mq-sched: don't dequeue request until all in ->dispatch are flushed".
--
Ming
