Re: [PATCH] blk-iocost: fix lagging in-flight IOs detection

[Chengming Zhou <zhouchengming@bytedance.com>]

Hello, friendly ping...

Thanks.

On 2022/6/3 17:20, Chengming Zhou wrote:
> Since latency Qos detection doesn't account for IOs which
> are in-flight for longer than a period, we detect them by
> comparing vdone against period start. If lagging behind
> IOs from past periods, don't increase vrate, but don't let
> cmds which take a very long time pin lagging for too long.
> 
> The current code compares vdone against (vnow - period_vtime),
> which is thought to be the period start. But the earliest
> period start should be (vnow - period_vtime - margins.target).
> 
> And we will slide forward vtime and vdone on each period timer,
> so we can't depend on vtime to check if the iocg has pinned
> lagging for longer than MAX_LAGGING_PERIODS.
> 
> This patch adds lagging_periods in iocg to record how long
> the iocg has pinned lagging, will not be thought as lagging
> if longer than MAX_LAGGING_PERIODS.
> 
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> ---
> v2:
>  - add lagging_periods to check very long lagging iocg.
> ---
>  block/blk-iocost.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/block/blk-iocost.c b/block/blk-iocost.c
> index 33a11ba971ea..998bb38ffb37 100644
> --- a/block/blk-iocost.c
> +++ b/block/blk-iocost.c
> @@ -541,6 +541,8 @@ struct ioc_gq {
>  	u64				indebt_since;
>  	u64				indelay_since;
>  
> +	int				lagging_periods;
> +
>  	/* this iocg's depth in the hierarchy and ancestors including self */
>  	int				level;
>  	struct ioc_gq			*ancestors[];
> @@ -2257,10 +2259,13 @@ static void ioc_timer_fn(struct timer_list *timer)
>  		if ((ppm_rthr != MILLION || ppm_wthr != MILLION) &&
>  		    !atomic_read(&iocg_to_blkg(iocg)->use_delay) &&
>  		    time_after64(vtime, vdone) &&
> -		    time_after64(vtime, now.vnow -
> -				 MAX_LAGGING_PERIODS * period_vtime) &&
> -		    time_before64(vdone, now.vnow - period_vtime))
> -			nr_lagging++;
> +		    time_before64(vdone, ioc->period_at_vtime - ioc->margins.target)) {
> +			if (iocg->lagging_periods < MAX_LAGGING_PERIODS) {
> +				nr_lagging++;
> +				iocg->lagging_periods++;
> +			}
> +		} else if (iocg->lagging_periods)
> +			iocg->lagging_periods = 0;
>  
>  		/*
>  		 * Determine absolute usage factoring in in-flight IOs to avoid