Re: [PATCH] blk-throttle: fix lower bps rate by throtl_trim_slice()

[Ming Lei <ming.lei@redhat.com>]

On Wed, Feb 26, 2025 at 09:16:27AM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> The bio submission time may be a few jiffies more than the expected
> waiting time, due to 'extra_bytes' can't be divided in
> tg_within_bps_limit(), and also due to timer wakeup delay. In this
> case, adjust slice_start to jiffies will discard the extra wait time,
> causing lower rate than expected.
> 
> This problem will cause blktests throtl/001 failure in case of
> CONFIG_HZ_100=y, fix it by preserving one finished slice in
> throtl_trim_slice() and allowing deviation between [0, 2 slices).

I think it only can cover single default slice deviation, since
throtl_trim_slice() just keeps dispatch data in the previous single
default slice. Or can you add words on how to allow 2 default slices
deviation?

> 
> For example, assume bps_limit is 1000bytes, 1 jiffes is 10ms, and
> slice is 20ms(2 jiffies), expected rate is 1000 / 1000 * 20 = 20 bytes
> per slice.
> 
> If user issues two 21 bytes IO, then wait time will be 30ms for the
> first IO:
> 
> bytes_allowed = 20, extra_bytes = 1;
> jiffy_wait = 1 + 2 = 3 jiffies
> 
> and consider
> extra 1 jiffies by timer, throtl_trim_slice() will be called at:
> 
> jiffies = 40ms
> slice_start = 0ms, slice_end= 40ms
> bytes_disp = 21
> 
> In this case, before the patch, real rate in the first two slices is
> 10.5 bytes per slice, and slice will be updated to:
> 
> jiffies = 40ms
> slice_start = 40ms, slice_end = 60ms,
> bytes_disp = 0;
> 
> Hence the second IO will have to wait another 30ms;
> 
> With the patch, the real rate in the first slice is 20 bytes per slice,
> which is the same as expected, and slice will be updated:
> 
> jiffies=40ms,
> slice_start = 20ms, slice_end = 60ms,
> bytes_disp = 1;
> 
> And now, there is still 19 bytes allowed in the second slice, and the
> second IO will only have to wait 10ms;
> 
> Fixes: e43473b7f223 ("blkio: Core implementation of throttle policy")
> Reported-by: Ming Lei <ming.lei@redhat.com>
> Closes: https://lore.kernel.org/linux-block/20250222092823.210318-3-yukuai1@huaweicloud.com/
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>  block/blk-throttle.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index 8d149aff9fd0..cb472cf7b6b6 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -599,14 +599,23 @@ static inline void throtl_trim_slice(struct throtl_grp *tg, bool rw)
>  	 * sooner, then we need to reduce slice_end. A high bogus slice_end
>  	 * is bad because it does not allow new slice to start.
>  	 */
> -
>  	throtl_set_slice_end(tg, rw, jiffies + tg->td->throtl_slice);
>  
>  	time_elapsed = rounddown(jiffies - tg->slice_start[rw],
>  				 tg->td->throtl_slice);
> -	if (!time_elapsed)
> +	/* Don't trim slice until at least 2 slices are used */
> +	if (time_elapsed < tg->td->throtl_slice * 2)
>  		return;
>  
> +	/*
> +	 * The bio submission time may be a few jiffies more than the expected
> +	 * waiting time, due to 'extra_bytes' can't be divided in
> +	 * tg_within_bps_limit(), and also due to timer wakeup delay. In this
> +	 * case, adjust slice_start to jiffies will discard the extra wait time,
> +	 * causing lower rate than expected. Therefore, one slice is preserved,
> +	 * allowing deviation that is less than two slices.
> +	 */
> +	time_elapsed -= tg->td->throtl_slice;

Please document that default slice window size is doubled actually in
this way.


Thanks,
Ming