Re: [PATCH -next v2 2/4] blk-throttle: fix wrong comparation while 'carryover_ios/bytes' is negative

[Tejun Heo]

On Wed, Aug 16, 2023 at 09:27:06AM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
> 
> carryover_ios/bytes[] can be negative in the case that ios are
> dispatched in the slice in advance, and then configuration is updated.
> For example:
> 
> 1) set iops limit to 1000, and slice start is 0, slice end is 100ms;
> 2) current time is 0, and 100 ios are dispatched, those ios will not be
>    throttled, hence io_disp is 100;
> 3) still at current time 0, update iops limit to 100, then carryover_ios
>    is (0 - 100) = -100;
> 4) then, dispatch a new io at time 0, the expected result is that this
>    io will wait for 1s. The calculation in tg_within_iops_limit:
> 
>    io_disp = 0;
>    io_allowed = calculate_io_allowed + carryover_ios
> 	      = 10 + (-100) = -90;
>    io won't be throttled if (io_disp + 1 < io_allowed) passed.
> 
> Before this patch, in step 4) (io_disp + 1 < io_allowed) is passed,
> because -90 for unsigned value is very huge, and such io won't be
> throttled.
> 
> Fix this problem by checking if 'io/bytes_allowed' is negative first.
> 
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>

Acked-by: Tejun Heo <tj@xxxxxxxxxx>

Thanks.

-- 
tejun