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Message-ID: <bbd8744f-d938-c4a5-cb02-145c9875ea53@bytedance.com>
Date:   Mon, 16 May 2022 20:18:54 +0800
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Subject: Re: [PATCH] blk-iocos: fix inuse clamp when iocg deactivate or free
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To:     tj@kernel.org, axboe@kernel.dk
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        duanxiongchun@bytedance.com, songmuchun@bytedance.com
References: <20220516101909.99768-1-zhouchengming@bytedance.com>
From:   Chengming Zhou <zhouchengming@bytedance.com>
In-Reply-To: <20220516101909.99768-1-zhouchengming@bytedance.com>
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On 2022/5/16 18:19, Chengming Zhou wrote:
> For an active leaf node, its inuse shouldn't be zero or exceed
> its active, but it's not true when deactivate idle iocg or delete
> iocg in ioc_pd_free().
> 
> Although inuse of 1 is very small, it could cause noticeable hwi
> decrease in the long running server. So we'd better fix it.
> 
> And check iocg->child_active_sum is enough for inner iocg, remove
> the needless list_empty check by the way.
> 
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> ---
>  block/blk-iocost.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-iocost.c b/block/blk-iocost.c
> index 2570732b92d1..84374ebcc402 100644
> --- a/block/blk-iocost.c
> +++ b/block/blk-iocost.c
> @@ -1073,11 +1073,11 @@ static void __propagate_weights(struct ioc_gq *iocg, u32 active, u32 inuse,
>  	 * @active. An active internal node's inuse is solely determined by the
>  	 * inuse to active ratio of its children regardless of @inuse.
>  	 */
> -	if (list_empty(&iocg->active_list) && iocg->child_active_sum) {
> +	if (iocg->child_active_sum) {
>  		inuse = DIV64_U64_ROUND_UP(active * iocg->child_inuse_sum,
>  					   iocg->child_active_sum);
>  	} else {
> -		inuse = clamp_t(u32, inuse, 1, active);
> +		inuse = clamp_t(u32, inuse, 0, active);

I found the commit message is wrong after a second look at the test data,
inuse value will be zero when active is zero, since:

#define clamp_t(type, val, lo, hi) min_t(type, max_t(type, val, lo), hi)

So clamp_t(u32, 0, 1, 0) will return 0, deactivate idle iocg or delete iocg
will set its inuse to zero correctly.

The inuse -> 1 happened in the test data turn out to be iocg_incur_debt(),
which call __propagate_weights() with active = weight, inuse = 0, so
clamp_t(u32, 0, 1, active) return 1.

Then this effect is very small, unlikely to have an impact in practice. Should
I modify the commit message to send v2 or just drop it?

Thanks.

>  	}
>  
>  	iocg->last_inuse = iocg->inuse;