Re: [PATCH] blk-throttle: fix zero wait time for iops throttled group

[Konstantin Khlebnikov]

On 08.07.2019 18:29, Konstantin Khlebnikov wrote:
> After commit 991f61fe7e1d ("Blk-throttle: reduce tail io latency when iops
> limit is enforced") wait time could be zero even if group is throttled and
> cannot issue requests right now. As a result throtl_select_dispatch() turns
> into busy-loop under irq-safe queue spinlock.

To be clear: this almost instantly kills entire machine - other cpus stuck at sending ipi.

> Fix is simple: always round up target time to the next throttle slice.
>
> Fixes: 991f61fe7e1d ("Blk-throttle: reduce tail io latency when iops limit is enforced")
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # v4.19+
> ---
>   block/blk-throttle.c |    9 +++------
>   1 file changed, 3 insertions(+), 6 deletions(-)
>
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index 9ea7c0ecad10..8ab6c8153223 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -881,13 +881,10 @@ static bool tg_with_in_iops_limit(struct throtl_grp *tg, struct bio *bio,
>   	unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;
>   	u64 tmp;
>   
> -	jiffy_elapsed = jiffy_elapsed_rnd = jiffies - tg->slice_start[rw];
> -
> -	/* Slice has just started. Consider one slice interval */
> -	if (!jiffy_elapsed)
> -		jiffy_elapsed_rnd = tg->td->throtl_slice;
> +	jiffy_elapsed = jiffies - tg->slice_start[rw];
>   
> -	jiffy_elapsed_rnd = roundup(jiffy_elapsed_rnd, tg->td->throtl_slice);
> +	/* Round up to the next throttle slice, wait time must be nonzero */
> +	jiffy_elapsed_rnd = roundup(jiffy_elapsed + 1, tg->td->throtl_slice);
>   
>   	/*
>   	 * jiffy_elapsed_rnd should not be a big value as minimum iops can be