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Subject: Re: [PATCH -next] blk-wbt: don't throttle swap writes in direct
 reclaim
To: libaokun@huaweicloud.com, linux-block@vger.kernel.org
Cc: axboe@kernel.dk, hch@infradead.org, linux-kernel@vger.kernel.org,
 yangerkun@huawei.com, houtao1@huawei.com, libaokun1@huawei.com,
 "yukuai (C)" <yukuai3@huawei.com>
References: <20240604030522.3686177-1-libaokun@huaweicloud.com>
From: Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <8b17831e-0132-17af-7ec0-6e05057c3530@huaweicloud.com>
Date: Tue, 4 Jun 2024 15:01:28 +0800
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?? 2024/06/04 11:05, libaokun@huaweicloud.com д??:
> From: Baokun Li <libaokun1@huawei.com>
> 
> Now we avoid throttling swap writes by determining whether the current
> process is kswapd (aka current_is_kswapd()), but swap writes can come
> from either kswapd or direct reclaim, so the swap writes from direct
> reclaim will still be throttled.
> 
> When a process holds a lock to allocate a free page, and enters direct
> reclaim because there is no free memory, then it might trigger a hung
> due to the wbt throttling that causes other processes to fail to get
> the lock.
> 
> Both kswapd and direct reclaim set the REQ_SWAP flag, so use REQ_SWAP
> instead of current_is_kswapd() to avoid throttling swap writes. Also
> renamed WBT_KSWAPD to WBT_SWAP and WBT_RWQ_KSWAPD to WBT_RWQ_SWAP.
> 
> Signed-off-by: Baokun Li <libaokun1@huawei.com>

LGTM
Reviewed-by: Yu Kuai <yukuai3@huawei.com>

> ---
>   block/blk-wbt.c | 18 +++++++++---------
>   1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/block/blk-wbt.c b/block/blk-wbt.c
> index 64472134dd26..aaacf2f5b223 100644
> --- a/block/blk-wbt.c
> +++ b/block/blk-wbt.c
> @@ -37,7 +37,7 @@
>   enum wbt_flags {
>   	WBT_TRACKED		= 1,	/* write, tracked for throttling */
>   	WBT_READ		= 2,	/* read */
> -	WBT_KSWAPD		= 4,	/* write, from kswapd */
> +	WBT_SWAP		= 4,	/* write, from swap_writepage() */
>   	WBT_DISCARD		= 8,	/* discard */
>   
>   	WBT_NR_BITS		= 4,	/* number of bits */
> @@ -45,7 +45,7 @@ enum wbt_flags {
>   
>   enum {
>   	WBT_RWQ_BG		= 0,
> -	WBT_RWQ_KSWAPD,
> +	WBT_RWQ_SWAP,
>   	WBT_RWQ_DISCARD,
>   	WBT_NUM_RWQ,
>   };
> @@ -172,8 +172,8 @@ static bool wb_recent_wait(struct rq_wb *rwb)
>   static inline struct rq_wait *get_rq_wait(struct rq_wb *rwb,
>   					  enum wbt_flags wb_acct)
>   {
> -	if (wb_acct & WBT_KSWAPD)
> -		return &rwb->rq_wait[WBT_RWQ_KSWAPD];
> +	if (wb_acct & WBT_SWAP)
> +		return &rwb->rq_wait[WBT_RWQ_SWAP];
>   	else if (wb_acct & WBT_DISCARD)
>   		return &rwb->rq_wait[WBT_RWQ_DISCARD];
>   
> @@ -528,7 +528,7 @@ static bool close_io(struct rq_wb *rwb)
>   		time_before(now, rwb->last_comp + HZ / 10);
>   }
>   
> -#define REQ_HIPRIO	(REQ_SYNC | REQ_META | REQ_PRIO)
> +#define REQ_HIPRIO	(REQ_SYNC | REQ_META | REQ_PRIO | REQ_SWAP)
>   
>   static inline unsigned int get_limit(struct rq_wb *rwb, blk_opf_t opf)
>   {
> @@ -539,13 +539,13 @@ static inline unsigned int get_limit(struct rq_wb *rwb, blk_opf_t opf)
>   
>   	/*
>   	 * At this point we know it's a buffered write. If this is
> -	 * kswapd trying to free memory, or REQ_SYNC is set, then
> +	 * swap trying to free memory, or REQ_SYNC is set, then
>   	 * it's WB_SYNC_ALL writeback, and we'll use the max limit for
>   	 * that. If the write is marked as a background write, then use
>   	 * the idle limit, or go to normal if we haven't had competing
>   	 * IO for a bit.
>   	 */
> -	if ((opf & REQ_HIPRIO) || wb_recent_wait(rwb) || current_is_kswapd())
> +	if ((opf & REQ_HIPRIO) || wb_recent_wait(rwb))
>   		limit = rwb->rq_depth.max_depth;
>   	else if ((opf & REQ_BACKGROUND) || close_io(rwb)) {
>   		/*
> @@ -622,8 +622,8 @@ static enum wbt_flags bio_to_wbt_flags(struct rq_wb *rwb, struct bio *bio)
>   	if (bio_op(bio) == REQ_OP_READ) {
>   		flags = WBT_READ;
>   	} else if (wbt_should_throttle(bio)) {
> -		if (current_is_kswapd())
> -			flags |= WBT_KSWAPD;
> +		if (bio->bi_opf & REQ_SWAP)
> +			flags |= WBT_SWAP;
>   		if (bio_op(bio) == REQ_OP_DISCARD)
>   			flags |= WBT_DISCARD;
>   		flags |= WBT_TRACKED;
>