Re: [PATCH -next v2 4/4] blk-throttle: consider 'carryover_ios/bytes' in throtl_trim_slice()

[Tejun Heo <tj@kernel.org>]

On Wed, Aug 16, 2023 at 09:27:08AM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> Currently, 'carryover_ios/bytes' is not handled in throtl_trim_slice(),
> for consequence, 'carryover_ios/bytes' will be used to throttle bio
> multiple times, for example:
> 
> 1) set iops limit to 100, and slice start is 0, slice end is 100ms;
> 2) current time is 0, and 10 ios are dispatched, those io won't be
>    throttled and io_disp is 10;
> 3) still at current time 0, update iops limit to 1000, carryover_ios is
>    updated to (0 - 10) = -10;
> 4) in this slice(0 - 100ms), io_allowed = 100 + (-10) = 90, which means
>    only 90 ios can be dispatched without waiting;
> 5) assume that io is throttled in slice(0 - 100ms), and
>    throtl_trim_slice() update silce to (100ms - 200ms). In this case,
>    'carryover_ios/bytes' is not cleared and still only 90 ios can be
>    dispatched between 100ms - 200ms.
> 
> Fix this problem by updating 'carryover_ios/bytes' in
> throtl_trim_slice().
> 
> Fixes: a880ae93e5b5 ("blk-throttle: fix io hung due to configuration updates")
> Reported-by: zhuxiaohui <zhuxiaohui.400@bytedance.com>
> Link: https://lore.kernel.org/all/20230812072116.42321-1-zhuxiaohui.400@bytedance.com/
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun