Re: [RFC PATCH v2 1/7] block: Extand commit_rqs() to do batch processing

[Baolin Wang <baolin.wang7@gmail.com>]

On Mon, Apr 27, 2020 at 11:46 PM Christoph Hellwig <hch@infradead.org> wrote:
>
> extand in the subject really shpuld be 'extend'

Sorry for typo, and will fix in next version.

>
> On Sun, Apr 26, 2020 at 05:38:54PM +0800, Baolin Wang wrote:
> > From: Ming Lei <ming.lei@redhat.com>
> >
> > Now some SD/MMC host controllers can support packed command or packed request,
> > that means we can package several requests to host controller at one time
> > to improve performence.
> >
> > But the blk-mq always takes one request from the scheduler and dispatch it to
> > the device, regardless of the driver or the scheduler, so there should only
> > ever be one request in the local list in blk_mq_dispatch_rq_list(), that means
> > the bd.last is always true and the driver can not use bd.last to decide if
> > there are requests are pending now in hardware queue to help to package
> > requests.
> >
> > Thus this patch introduces a new 'BLK_MQ_F_FORCE_COMMIT_RQS' flag to call
> > .commit_rqs() to do batch processing if necessary.
> >
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > Tested-by: Baolin Wang <baolin.wang7@gmail.com>
> > Signed-off-by: Baolin Wang <baolin.wang7@gmail.com>
> > ---
> >  block/blk-mq-sched.c   | 29 ++++++++++++++++++++---------
> >  block/blk-mq.c         | 15 ++++++++++-----
> >  include/linux/blk-mq.h |  1 +
> >  3 files changed, 31 insertions(+), 14 deletions(-)
> >
> > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> > index 74cedea56034..3429a71a7364 100644
> > --- a/block/blk-mq-sched.c
> > +++ b/block/blk-mq-sched.c
> > @@ -85,11 +85,12 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
> >   * its queue by itself in its completion handler, so we don't need to
> >   * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
> >   */
> > -static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
> > +static bool blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
>
> This function already returns an int in the current for-5.8/block tree.

Thanks for pointing this out, and seems I should re-modify the return
values of the functions.

>
> > +             if (!(hctx->flags & BLK_MQ_F_FORCE_COMMIT_RQS)) {
> > +                     if (list_empty(list)) {
> > +                             bd.last = true;
> > +                     } else {
> > +                             nxt = list_first_entry(list, struct request,
> > +                                                    queuelist);
> > +                             bd.last = !blk_mq_get_driver_tag(nxt);
> > +                     }
> > +             } else {
> > +                     bd.last = false;
> >               }
>
> This seems a little odd in terms of code flow.  Why not:
>
>                 if (hctx->flags & BLK_MQ_F_FORCE_COMMIT_RQS) {
>                         bd.last = false;
>                 } else if (list_empty(list)) {
>                         bd.last = true;
>                 } else {
>                         nxt = list_first_entry(list, struct request, queuelist);
>                         bd.last = !blk_mq_get_driver_tag(nxt);
>                 }

Yes, looks better.

> > diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> > index f389d7c724bd..6a20f8e8eb85 100644
> > --- a/include/linux/blk-mq.h
> > +++ b/include/linux/blk-mq.h
> > @@ -391,6 +391,7 @@ struct blk_mq_ops {
> >  enum {
> >       BLK_MQ_F_SHOULD_MERGE   = 1 << 0,
> >       BLK_MQ_F_TAG_SHARED     = 1 << 1,
> > +     BLK_MQ_F_FORCE_COMMIT_RQS = 1 << 3,
>
> Maybe BLK_MQ_F_ALWAYS_COMMIT might be a better name?  Also this

Looks reasonable to me, and will do.

> flag (just like the existing ones..) could really use a comment
> explaining it.

OK, will add some comments. Thanks for your comments.

-- 
Baolin Wang