linux-kernel - Re: [PATCH 06/14] blkcg: associate blkg when associating a device

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Date:   Tue, 4 Dec 2018 14:48:39 -0500
From:   Josef Bacik <josef@...icpanda.com>
To:     Dennis Zhou <dennis@...nel.org>
Cc:     Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Josef Bacik <josef@...icpanda.com>, kernel-team@...com,
        linux-block@...r.kernel.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/14] blkcg: associate blkg when associating a device

On Tue, Dec 04, 2018 at 01:35:52PM -0500, Dennis Zhou wrote:
> Previously, blkg association was handled by controller specific code in
> blk-throttle and blk-iolatency. However, because a blkg represents a
> relationship between a blkcg and a request_queue, it makes sense to keep
> the blkg->q and bio->bi_disk->queue consistent.
> 
> This patch moves association into the bio_set_dev macro(). This should
> cover the majority of cases where the device is set/changed keeping the
> two pointers consistent. Fallback code is added to
> blkcg_bio_issue_check() to catch any missing paths.
> 
> Signed-off-by: Dennis Zhou <dennis@...nel.org>
> ---
>  block/bio.c                |  1 +
>  block/blk-iolatency.c      |  4 +---
>  block/blk-throttle.c       |  1 -
>  include/linux/bio.h        |  2 ++
>  include/linux/blk-cgroup.h | 11 +++--------
>  5 files changed, 7 insertions(+), 12 deletions(-)
> 
> diff --git a/block/bio.c b/block/bio.c
> index 41ebb3f8e2fc..1e852ab904aa 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -2074,6 +2074,7 @@ void bio_associate_blkg(struct bio *bio)
>  
>  	rcu_read_unlock();
>  }
> +EXPORT_SYMBOL_GPL(bio_associate_blkg);
>  
>  /**
>   * bio_disassociate_task - undo bio_associate_current()
> diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
> index cdbd10564e66..e6b47c255521 100644
> --- a/block/blk-iolatency.c
> +++ b/block/blk-iolatency.c
> @@ -472,14 +472,12 @@ static void check_scale_change(struct iolatency_grp *iolat)
>  static void blkcg_iolatency_throttle(struct rq_qos *rqos, struct bio *bio)
>  {
>  	struct blk_iolatency *blkiolat = BLKIOLATENCY(rqos);
> -	struct blkcg_gq *blkg;
> +	struct blkcg_gq *blkg = bio->bi_blkg;
>  	bool issue_as_root = bio_issue_as_root_blkg(bio);
>  
>  	if (!blk_iolatency_enabled(blkiolat))
>  		return;
>  
> -	bio_associate_blkg(bio);
> -	blkg = bio->bi_blkg;
>  	bio_issue_init(&bio->bi_issue, bio_sectors(bio));
>  
>  	while (blkg && blkg->parent) {
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index 228c3a007ebc..1c6529df2002 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -2118,7 +2118,6 @@ static inline void throtl_update_latency_buckets(struct throtl_data *td)
>  static void blk_throtl_assoc_bio(struct bio *bio)
>  {
>  #ifdef CONFIG_BLK_DEV_THROTTLING_LOW
> -	bio_associate_blkg(bio);
>  	bio_issue_init(&bio->bi_issue, bio_sectors(bio));
>  #endif
>  }
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 62715a5a4f32..6ee2ea8b378a 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -491,12 +491,14 @@ do {						\
>  		bio_clear_flag(bio, BIO_THROTTLED);\
>  	(bio)->bi_disk = (bdev)->bd_disk;	\
>  	(bio)->bi_partno = (bdev)->bd_partno;	\
> +	bio_associate_blkg(bio);		\
>  } while (0)
>  
>  #define bio_copy_dev(dst, src)			\
>  do {						\
>  	(dst)->bi_disk = (src)->bi_disk;	\
>  	(dst)->bi_partno = (src)->bi_partno;	\
> +	bio_clone_blkcg_association(dst, src);	\
>  } while (0)
>  
>  #define bio_dev(bio) \
> diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
> index c08e96e521ed..3c87ae71156f 100644
> --- a/include/linux/blk-cgroup.h
> +++ b/include/linux/blk-cgroup.h
> @@ -802,21 +802,17 @@ static inline bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg
>  static inline bool blkcg_bio_issue_check(struct request_queue *q,
>  					 struct bio *bio)
>  {
> -	struct blkcg *blkcg;
>  	struct blkcg_gq *blkg;
>  	bool throtl = false;
>  
> -	rcu_read_lock();
> +	if (!bio->bi_blkg)
> +		bio_associate_blkg(bio);
>  

Should we maybe WARN_ON_ONCE() here since this really shouldn't happen?
Otherwise you can add

Reviewed-by: Josef Bacik <josef@...icpanda.com>

Thanks,

Josef