linux-kernel - Re: [PATCH] blk-mq: Put driver tag in blk_mq_dispatch_rq

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Date:   Mon, 20 Apr 2020 05:48:32 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     John Garry <john.garry@...wei.com>, axboe@...nel.dk
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        ming.lei@...hat.com
Subject: Re: [PATCH] blk-mq: Put driver tag in blk_mq_dispatch_rq_list() when
 no budget

On 4/20/20 1:22 AM, John Garry wrote:
> On 18/04/2020 03:43, Bart Van Assche wrote:
>> On 2020-04-16 04:18, John Garry wrote:
>>> If in blk_mq_dispatch_rq_list() we find no budget, then we break of the
>>> dispatch loop, but the request may keep the driver tag, evaulated
>>> in 'nxt' in the previous loop iteration.
>>>
>>> Fix by putting the driver tag for that request.
>>>
>>> Signed-off-by: John Garry <john.garry@...wei.com>
>>>
>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>> index 8e56884fd2e9..a7785df2c944 100644
>>> --- a/block/blk-mq.c
>>> +++ b/block/blk-mq.c
>>> @@ -1222,8 +1222,10 @@ bool blk_mq_dispatch_rq_list(struct 
>>> request_queue *q, struct list_head *list,
>>>           rq = list_first_entry(list, struct request, queuelist);
>>>           hctx = rq->mq_hctx;
>>> -        if (!got_budget && !blk_mq_get_dispatch_budget(hctx))
>>> +        if (!got_budget && !blk_mq_get_dispatch_budget(hctx)) {
>>> +            blk_mq_put_driver_tag(rq);
>>>               break;
>>> +        }
>>>           if (!blk_mq_get_driver_tag(rq)) {
>>>               /*
>>
>> Is this something that can only happen if q->mq_ops->queue_rq(hctx, &bd)
>> returns another value than BLK_STS_OK, BLK_STS_RESOURCE and
>> BLK_STS_DEV_RESOURCE? 
> 
> Right, as that case is handled in blk_mq_handle_dev_resource()
> 
> If so, please add a comment in the source code
>> that explains this.
> 
> So important that we should now do this in an extra patch?

Probably not, unless someone else feels strongly about this.

Thanks,

Bart.