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

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

On 08.07.2019 22:08, Liu Bo wrote:
> On Mon, Jul 08, 2019 at 06:29:57PM +0300, 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.
>>
>> 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@yandex-team.ru>
>> Cc: stable@vger.kernel.org # 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
> 
> Did you use a tiny iops limit to run into this?

Yep. 25 iops

also kernel built with HZ=250, this might be related

> 
> thanks,
> -liubo
>