linux-kernel - Re: [PATCH] blk-iocost: fix false positive lagging

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Date:   Wed, 1 Jun 2022 20:32:32 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     tj@...nel.org, axboe@...nel.dk
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blk-iocost: fix false positive lagging

On 2022/5/28 16:17, Chengming Zhou wrote:
> On 2022/5/26 21:35, Chengming Zhou wrote:
>> I found many false positive lagging during iocost test.
>>
>> Since iocg->vtime will be advanced to (vnow - margins.target)
>> in hweight_after_donation(), which called throw away excess,
>> the iocg->done_vtime will also be advanced that much.
>>
>>        period_at_vtime  <--period_vtime-->  vnow
>>               |                              |
>>   --------------------------------------------------->
>>         |<--->|
>>      margins.target
>>         |->
>>   vtime, done_vtime
>>
>> If that iocg has some inflight io when vnow, but its done_vtime
>> is before period_at_vtime, ioc_timer_fn() will think it has
>> lagging io, even these io maybe issued just before now.
>>
>> This patch change the condition to check if vdone is before
>> (period_at_vtime - margins.target) instead of period_at_vtime.
>>
>> But there is another problem that this patch doesn't fix.
>> Since vtime will be advanced, we can't check if vtime is
>> after (vnow - MAX_LAGGING_PERIODS * period_vtime) to tell
>> whether this iocg pin lagging for too long.
>>
>> Maybe we can add lagging_periods in iocg to record how many
>> periods this iocg pin lagging, but I don't know when to clean it.
> 
> Hello tejun, I add lagging_periods in iocg based on the original patch,
> to record how many periods this iocg pin lagging. So we can use it to
> avoid letting cmds which take a very long time pin lagging for too long.
> 
> Thanks.
> 
> 
> 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
> 

Hi, I tested with this version, previous false laggings are gone. So I wonder
if I should send v2 for review?

Thanks!

> 
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
>> ---
>>  block/blk-iocost.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/block/blk-iocost.c b/block/blk-iocost.c
>> index 33a11ba971ea..42e301b7527b 100644
>> --- a/block/blk-iocost.c
>> +++ b/block/blk-iocost.c
>> @@ -2259,7 +2259,7 @@ static void ioc_timer_fn(struct timer_list *timer)
>>  		    time_after64(vtime, vdone) &&
>>  		    time_after64(vtime, now.vnow -
>>  				 MAX_LAGGING_PERIODS * period_vtime) &&
>> -		    time_before64(vdone, now.vnow - period_vtime))
>> +		    time_before64(vdone, ioc->period_at_vtime - ioc->margins.target))
>>  			nr_lagging++;
>>  
>>  		/*