linux-kernel - Re: [PATCH v2] blk-throtl: optimize IOPS throttle for large IO scenarios

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Date:   Mon, 2 Aug 2021 09:30:29 +0800
From:   brookxu <brookxu.cn@...il.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH v2] blk-throtl: optimize IOPS throttle for large IO
 scenarios



Tejun Heo wrote on 2021/7/31 12:07 上午:
> On Fri, Jul 30, 2021 at 10:09:34AM +0800, brookxu wrote:
>>>> @@ -877,10 +900,19 @@ static inline void throtl_trim_slice(struct throtl_grp *tg, bool rw)
>>>>  	else
>>>>  		tg->bytes_disp[rw] = 0;
>>>>  
>>>> -	if (tg->io_disp[rw] >= io_trim)
>>>> +	if (tg_io_disp(tg, rw) >= io_trim) {
>>>
>>> Instead of checking this in multiple places, would it be simpler to transfer
>>> the atomic counters to the existing counters whenever we enter blk-throtl
>>> and leave the rest of the code as-is?
>>
>> If we do this, we need to do similar processing on the bio submission path and the bio
>> resubmission path in pending_timer. It seems that the code is more complicated?
> 
> Yeah, basically whenever we enter blk-throtl. Factored to a function,
> calling it on entry should be fairly clean, right? I wonder whether it'd be
> better to consolidate all atomic counter handling in a single location and
> all it does is transferring whatever's accumulated to the usual counters.
> Also, when you're reading & resetting the atomic counters, can you use a
> pattern like the following?
> 
>   main_counter += atomic_xchg(counter, 0);
> 
> Right now, there's a race window between reading and resetting.

Yeah, thanks for your suggestion, I will submit the next version later.

> Thanks.
>