Re: [PATCH 4/4] blk-wbt: throttle discards like background writes

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, May 07, 2018 at 10:13:35AM -0600, Jens Axboe wrote:
> Throttle discards like we would any background write. Discards should
> be background activity, so if they are impacting foreground IO, then
> we will throttle them down.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>

Looks ok,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
>  block/blk-stat.h |  6 +++---
>  block/blk-wbt.c  | 43 ++++++++++++++++++++++++++-----------------
>  block/blk-wbt.h  |  4 +++-
>  3 files changed, 32 insertions(+), 21 deletions(-)
> 
> diff --git a/block/blk-stat.h b/block/blk-stat.h
> index 2dd36347252a..c22049a8125e 100644
> --- a/block/blk-stat.h
> +++ b/block/blk-stat.h
> @@ -10,11 +10,11 @@
>  
>  /*
>   * from upper:
> - * 3 bits: reserved for other usage
> + * 4 bits: reserved for other usage
>   * 12 bits: size
> - * 49 bits: time
> + * 48 bits: time
>   */
> -#define BLK_STAT_RES_BITS	3
> +#define BLK_STAT_RES_BITS	4
>  #define BLK_STAT_SIZE_BITS	12
>  #define BLK_STAT_RES_SHIFT	(64 - BLK_STAT_RES_BITS)
>  #define BLK_STAT_SIZE_SHIFT	(BLK_STAT_RES_SHIFT - BLK_STAT_SIZE_BITS)
> diff --git a/block/blk-wbt.c b/block/blk-wbt.c
> index 25d202345965..a7a724580033 100644
> --- a/block/blk-wbt.c
> +++ b/block/blk-wbt.c
> @@ -106,6 +106,8 @@ static inline struct rq_wait *get_rq_wait(struct rq_wb *rwb,
>  {
>  	if (wb_acct & WBT_KSWAPD)
>  		return &rwb->rq_wait[WBT_RWQ_KSWAPD];
> +	else if (wb_acct & WBT_DISCARD)
> +		return &rwb->rq_wait[WBT_RWQ_DISCARD];
>  
>  	return &rwb->rq_wait[WBT_RWQ_BG];
>  }
> @@ -143,10 +145,13 @@ void __wbt_done(struct rq_wb *rwb, enum wbt_flags wb_acct)
>  	}
>  
>  	/*
> -	 * If the device does write back caching, drop further down
> -	 * before we wake people up.
> +	 * For discards, our limit is always the background. For writes, if
> +	 * the device does write back caching, drop further down before we
> +	 * wake people up.
>  	 */
> -	if (rwb->wc && !wb_recent_wait(rwb))
> +	if (wb_acct & WBT_DISCARD)
> +		limit = rwb->wb_background;
> +	else if (rwb->wc && !wb_recent_wait(rwb))
>  		limit = 0;
>  	else
>  		limit = rwb->wb_normal;
> @@ -483,6 +488,9 @@ static inline unsigned int get_limit(struct rq_wb *rwb, unsigned long rw)
>  {
>  	unsigned int limit;
>  
> +	if ((rw & REQ_OP_MASK) == REQ_OP_DISCARD)
> +		return rwb->wb_background;
> +
>  	/*
>  	 * At this point we know it's a buffered write. If this is
>  	 * kswapd trying to free memory, or REQ_SYNC is set, then
> @@ -564,21 +572,20 @@ static void __wbt_wait(struct rq_wb *rwb, enum wbt_flags wb_acct,
>  
>  static inline bool wbt_should_throttle(struct rq_wb *rwb, struct bio *bio)
>  {
> -	const int op = bio_op(bio);
> -
> -	/*
> -	 * If not a WRITE, do nothing
> -	 */
> -	if (op != REQ_OP_WRITE)
> -		return false;
> -
> -	/*
> -	 * Don't throttle WRITE_ODIRECT
> -	 */
> -	if ((bio->bi_opf & (REQ_SYNC | REQ_IDLE)) == (REQ_SYNC | REQ_IDLE))
> +	switch (bio_op(bio)) {
> +	case REQ_OP_WRITE:
> +		/*
> +		 * Don't throttle WRITE_ODIRECT
> +		 */
> +		if ((bio->bi_opf & (REQ_SYNC | REQ_IDLE)) ==
> +		    (REQ_SYNC | REQ_IDLE))
> +			return false;
> +		/* fallthrough */
> +	case REQ_OP_DISCARD:
> +		return true;
> +	default:
>  		return false;
> -
> -	return true;
> +	}
>  }
>  
>  /*
> @@ -605,6 +612,8 @@ enum wbt_flags wbt_wait(struct rq_wb *rwb, struct bio *bio, spinlock_t *lock)
>  
>  	if (current_is_kswapd())
>  		ret |= WBT_KSWAPD;
> +	if (bio_op(bio) == REQ_OP_DISCARD)
> +		ret |= WBT_DISCARD;
>  
>  	__wbt_wait(rwb, ret, bio->bi_opf, lock);
>  
> diff --git a/block/blk-wbt.h b/block/blk-wbt.h
> index 8038b4a0d4ef..d6a125e49db5 100644
> --- a/block/blk-wbt.h
> +++ b/block/blk-wbt.h
> @@ -14,13 +14,15 @@ enum wbt_flags {
>  	WBT_TRACKED		= 1,	/* write, tracked for throttling */
>  	WBT_READ		= 2,	/* read */
>  	WBT_KSWAPD		= 4,	/* write, from kswapd */
> +	WBT_DISCARD		= 8,	/* discard */
>  
> -	WBT_NR_BITS		= 3,	/* number of bits */
> +	WBT_NR_BITS		= 4,	/* number of bits */
>  };
>  
>  enum {
>  	WBT_RWQ_BG		= 0,
>  	WBT_RWQ_KSWAPD,
> +	WBT_RWQ_DISCARD,
>  	WBT_NUM_RWQ,
>  };
>  
> -- 
> 2.7.4
> 
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