linux-kernel - Re: [PATCH 2/4] blk-flush: count inflight flush

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Date:   Wed, 28 Jun 2023 20:55:53 +0800
From:   Chengming Zhou <chengming.zhou@...ux.dev>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     axboe@...nel.dk, tj@...nel.org, hch@....de,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        zhouchengming@...edance.com
Subject: Re: [PATCH 2/4] blk-flush: count inflight flush_data requests

On 2023/6/28 15:22, Ming Lei wrote:
> On Wed, Jun 28, 2023 at 12:55:49PM +0800, Chengming Zhou wrote:
>> On 2023/6/28 12:13, Ming Lei wrote:
>>> On Tue, Jun 27, 2023 at 08:08:52PM +0800, chengming.zhou@...ux.dev wrote:
>>>> From: Chengming Zhou <zhouchengming@...edance.com>
>>>>
>>>> The flush state machine use a double list to link all inflight
>>>> flush_data requests, to avoid issuing separate post-flushes for
>>>> these flush_data requests which shared PREFLUSH.
>>>>
>>>> So we can't reuse rq->queuelist, this is why we need rq->flush.list
>>>>
>>>> In preparation of the next patch that reuse rq->queuelist for flush
>>>> state machine, we change the double linked list to a u64 counter,
>>>> which count all inflight flush_data requests.
>>>>
>>>> This is ok since we only need to know if there is any inflight
>>>> flush_data request, so a u64 counter is good. The only problem I can
>>>> think of is that u64 counter may overflow, which should be unlikely happen.
>>>
>>> It won't overflow, q->nr_requests is 'unsigned long', which should have
>>> been limited to one more reasonable value, such as 2 * BLK_MQ_MAX_DEPTH, so
>>> u16 should be big enough in theory.
>>
>> Ah, right. q->nr_requests is 'unsigned long' and q->queue_depth is 'unsigned int',
>> so 'unsigned long' counter here won't overflow.
> 
> Not like q->nr_requests, q->queue_depth usually means the whole queue's depth,
> which may cover all hw queue's depth. And it is only used by scsi, but it
> should be held in "unsigned short" too.
> 
>>
>> Should I change it to smaller 'unsigned short' or just leave it as 'unsigned long' ?
>> (Now the size of struct blk_flush_queue is exactly 64 bytes)
> 
> You have to limit q->nr_requests first, which may need a bit more work for avoiding
> compiling warning or sort of thing. And 64k is big enough for holding per-queue
> scheduler request.
> 
> Once it is done, it is fine to define this counter as 'unsigned short'.
> 

Ok, I looked around these related code, found it maybe subtle to me for now.
So I'd better just leave it 'unsigned long' here. :)

Thanks.