Re: [PATCH] block: iocost: ensure hweight_inuse is at least 1

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi,

在 2024/11/22 15:26, Kunhai Dai 写道:
> The hweight_inuse calculation in transfer_surpluses() could potentially
> result in a value of 0, which would lead to division by zero errors in
> subsequent calculations that use this value as a divisor.
> 
> Signed-off-by: Kunhai Dai <daikunhai@didiglobal.com>
> ---
>   block/blk-iocost.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/block/blk-iocost.c b/block/blk-iocost.c
> index 384aa15e8260..65cdb55d30cc 100644
> --- a/block/blk-iocost.c
> +++ b/block/blk-iocost.c
> @@ -1999,9 +1999,10 @@ static void transfer_surpluses(struct list_head *surpluses, struct ioc_now *now)
>   		parent = iocg->ancestors[iocg->level - 1];
>   
>   		/* b' = gamma * b_f + b_t' */
> -		iocg->hweight_inuse = DIV64_U64_ROUND_UP(
> -			(u64)gamma * (iocg->hweight_active - iocg->hweight_donating),
> -			WEIGHT_ONE) + iocg->hweight_after_donation;
> +		iocg->hweight_inuse = max_t(u64, 1,
> +			DIV64_U64_ROUND_UP(
> +				(u64)gamma * (iocg->hweight_active - iocg->hweight_donating),
> +				WEIGHT_ONE) + iocg->hweight_after_donation);

I'm confused, how could DIV64_U64_Round_UP() end up less than 1?

#define DIV64_U64_ROUND_UP(ll, d)       \
         ({ u64 _tmp = (d); div64_u64((ll) + _tmp - 1, _tmp); })

AFAIK, the only case that could happen is that
iocg->hweight_active - iocg->hweight_donating is 0, then I don't
get it now how cound active iocg donate all the hweight, if this
really happend perhaps the better solution is to avoid such case.

Thanks,
Kuai

>   
>   		/* w' = s' * b' / b'_p */
>   		inuse = DIV64_U64_ROUND_UP(
>