linux-kernel - Re: [PATCH 5/7] blk-mq-sched: add framework for MQ capable IO schedulers

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Message-ID: <20161215192940.GA29747@vader.dhcp.TheFacebook.com>
Date:   Thu, 15 Dec 2016 11:29:40 -0800
From:   Omar Sandoval <osandov@...ndov.com>
To:     Jens Axboe <axboe@...com>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, paolo.valente@...aro.org,
        osandov@...com
Subject: Re: [PATCH 5/7] blk-mq-sched: add framework for MQ capable IO
 schedulers

Hey, Jens, a couple of minor nits below.

One bigger note: adding the blk_mq_sched_*() helpers and keeping the
blk_mq_*() helpers that they replaced seems risky. For example,
blk_mq_free_request() is superseded by blk_mq_sched_put_request(), but
we kept blk_mq_free_request(). There are definitely some codepaths that
are still using blk_mq_free_request() that are now wrong
(__nvme_submit_user_cmd() is the most obvious one I saw).

Can we get rid of the old, non-sched functions? Or maybe even make old
interface do the sched stuff instead of adding blk_mq_sched_*()?

On Wed, Dec 14, 2016 at 10:26:06PM -0700, Jens Axboe wrote:
> Signed-off-by: Jens Axboe <axboe@...com>
> ---
>  block/Makefile           |   2 +-
>  block/blk-core.c         |   7 +-
>  block/blk-exec.c         |   3 +-
>  block/blk-flush.c        |   7 +-
>  block/blk-mq-sched.c     | 375 +++++++++++++++++++++++++++++++++++++++++++++++
>  block/blk-mq-sched.h     | 190 ++++++++++++++++++++++++
>  block/blk-mq-tag.c       |   1 +
>  block/blk-mq.c           | 192 ++++++++++--------------
>  block/blk-mq.h           |   3 +
>  block/elevator.c         | 186 +++++++++++++++++------
>  include/linux/blk-mq.h   |   3 +-
>  include/linux/elevator.h |  29 ++++
>  12 files changed, 833 insertions(+), 165 deletions(-)
>  create mode 100644 block/blk-mq-sched.c
>  create mode 100644 block/blk-mq-sched.h
> 

[snip]

> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 8d1cec8e25d1..d10a246a3bc7 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2267,10 +2229,10 @@ static int blk_mq_queue_reinit_dead(unsigned int cpu)
>   * Now CPU1 is just onlined and a request is inserted into ctx1->rq_list
>   * and set bit0 in pending bitmap as ctx1->index_hw is still zero.
>   *
> - * And then while running hw queue, flush_busy_ctxs() finds bit0 is set in
> - * pending bitmap and tries to retrieve requests in hctx->ctxs[0]->rq_list.
> - * But htx->ctxs[0] is a pointer to ctx0, so the request in ctx1->rq_list
> - * is ignored.
> + * And then while running hw queue, blk_mq_flush_busy_ctxs() finds bit0 is set
> + * in pending bitmap and tries to retrieve requests in hctx->ctxs[0]->rq_list.
> + * But htx->ctxs[0] is a pointer to ctx0, so the request in ctx1->rq_list is
> + * ignored.
>   */

This belongs in patch 4 where flush_busy_ctxs() got renamed.

>  static int blk_mq_queue_reinit_prepare(unsigned int cpu)
>  {
> diff --git a/block/blk-mq.h b/block/blk-mq.h
> index e59f5ca520a2..a5ddc860b220 100644
> --- a/block/blk-mq.h
> +++ b/block/blk-mq.h
> @@ -47,6 +47,9 @@ struct blk_mq_tags *blk_mq_init_rq_map(struct blk_mq_tag_set *set,
>   */
>  void __blk_mq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
>  				bool at_head);
> +void blk_mq_insert_requests(struct blk_mq_hw_ctx *hctx, struct blk_mq_ctx *ctx,
> +				struct list_head *list);
> +void blk_mq_process_sw_list(struct blk_mq_hw_ctx *hctx);

Looks like the declaration of blk_mq_process_sw_list() survived a rebase
even though it doesn't exist anymore.

[snip]