linux-kernel - Re: [PATCH -next v3 2/2] blk-throttle: fix io hung due to configuration updates

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Date:   Fri, 20 May 2022 09:22:17 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     Michal Koutný <mkoutny@...e.com>
CC:     <tj@...nel.org>, <axboe@...nel.dk>, <ming.lei@...hat.com>,
        <geert@...ux-m68k.org>, <cgroups@...r.kernel.org>,
        <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <yi.zhang@...wei.com>
Subject: Re: [PATCH -next v3 2/2] blk-throttle: fix io hung due to
 configuration updates

在 2022/05/20 0:10, Michal Koutný 写道:
> On Thu, May 19, 2022 at 08:14:28PM +0800, "yukuai (C)" <yukuai3@...wei.com> wrote:
>> tg_with_in_bps_limit:
>>   jiffy_elapsed_rnd = jiffies - tg->slice_start[rw];
>>   tmp = bps_limit * jiffy_elapsed_rnd;
>>   do_div(tmp, HZ);
>>   bytes_allowed = tmp; -> how many bytes are allowed in this slice,
>> 		         incluing dispatched.
>>   if (tg->bytes_disp[rw] + bio_size <= bytes_allowed)
>>    *wait = 0 -> no need to wait if this bio is within limit
>>
>>   extra_bytes = tg->bytes_disp[rw] + bio_size - bytes_allowed;
>>   -> extra_bytes is based on 'bytes_disp'
>>
>> For example:
>>
>> 1) bps_limit is 2k, we issue two io, (1k and 9k)
>> 2) the first io(1k) will be dispatched, bytes_disp = 1k, slice_start = 0
>>     the second io(9k) is waiting for (9 - (2 - 1)) / 2 = 4 s
> 
> The 2nd io arrived at 1s, the wait time is 4s, i.e. it can be dispatched
> at 5s (i.e. 10k/*2kB/s = 5s).
No, the example is that the second io arrived together with first io.
> 
>> 3) after 3 s, we update bps_limit to 1k, then new waiting is caculated:
>>
>> without this patch:  bytes_disp = 0, slict_start =3:
>> bytes_allowed = 1k	                            <--- why 1k and not 0?
Because slice_start == jiffies, bytes_allowed is equal to bps_limit
>> extra_bytes = 9k - 1k = 8k
>> wait = 8s
> 
> This looks like it was calculated at time 4s (1s after new config was
> set).
No... it was caculated at time 3s:

jiffy_elapsed_rnd = roundup(jiffy_elapsed_rnd, tg->td->throtl_slice);

jiffies should be greater than 3s here, thus jiffy_elapsed_rnd is
3s + throtl_slice (I'm using throtl_slice = 1s here, it should not
affect result)
> 
>>
>> whth this patch: bytes_disp = 0.5k, slice_start =  0,
>> bytes_allowed = 1k * 3 + 1k = 4k
>> extra_bytes =  0.5k + 9k - 4k = 5.5k
>> wait = 5.5s
> 
> This looks like calculated at 4s, so the IO would be waiting till
> 4s+5.5s = 9.5s.
wait time is based on extra_bytes, this is really 5.5s, add 4s is
wrong here.

bytes_allowed = ((jiffies - slice_start) / Hz + 1) * bps_limit
extra_bytes = bio_size + bytes_disp - bytes_allowed
wait = extra_bytes / bps_limit
> 
> As I don't know why using time 4s, I'll shift this calculation to the
> time 3s (when the config changes):
> 
> bytes_disp = 0.5k, slice_start =  0,
> bytes_allowed = 1k * 3  = 3k
> extra_bytes =  0.5k + 9k - 3k = 7.5k
6.5k
> wait = 7.5s
> 
> In absolute time, the IO would wait till 3s+7.5s = 10.5s
Like I said above, wait time should not add (jiffies - slice_start)
> 
> OK, either your 9.5s or my 10.5s looks weird (although earlier than
> original 4s+8s=12s).
> However, the IO should ideally only wait till
> 
>      3s + (9k -   (6k    -    1k)     ) / 1k/s =
>           bio - (allowed - dispatched)  / new_limit
> 
>     =3s + 4k / 1k/s = 7s
> 
>     ('allowed' is based on old limit)
> 
> Or in another example, what if you change the config from 2k/s to ∞k/s
> (unlimited, let's neglect the arithmetic overflow that you handle
> explicitly, imagine a big number but not so big to be greater than
> division result).
> 
> In such a case, the wait time should be zero, i.e. IO should be
> dispatched right at the time of config change.

I thought about it, however, IMO, this is not a good idea. If user
updated config quite frequently, io throttle will be invalid.

Thanks,
Kuai
> (With your patch that still calculates >0 wait time (and the original
> behavior gives >0 wait too.)
> 
>> I hope I can expliain it clearly...
> 
> Yes, thanks for pointing me to relevant parts.
> I hope I grasped them correctly.
> 
> IOW, your patch and formula make the wait time shorter but still IO can
> be delayed indefinitely if you pass a sequence of new configs. (AFAIU)
> 
> Regards,
> Michal
> .
>