linux-kernel - Re: [PATCH v9 10/12] block: don't check blk_rq_is_passthrough() in blk_do_io

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Date:   Thu, 10 Oct 2019 11:56:48 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Sagi Grimberg <sagi@...mberg.me>,
        Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [PATCH v9 10/12] block: don't check blk_rq_is_passthrough() in
 blk_do_io_stat()

Hey,

Thanks for the thorough review, lots here to go through. I'll address it
all as I have time and try to get the prep work done first, as soon as I
can.

On 2019-10-10 4:05 a.m., Christoph Hellwig wrote:
>> @@ -319,7 +319,7 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
>>  	rq->cmd_flags = op;
>>  	if (data->flags & BLK_MQ_REQ_PREEMPT)
>>  		rq->rq_flags |= RQF_PREEMPT;
>> -	if (blk_queue_io_stat(data->q))
>> +	if (blk_queue_io_stat(data->q) && !blk_rq_is_passthrough(rq))
>>  		rq->rq_flags |= RQF_IO_STAT;
> 
> This needs a comment why we don't account passthrough requests by
> default.  And I'm really curious about the answer, because I don't
> know it myself.

Yes, sadly, I don't know this answer either but the comment made it
appear that someone did it deliberately. Digging into git blame suggests
that it just evolved that way. The check was originally added in 2005
here with blk_fs_request():

commit d72d904a5367 ("[BLOCK] Update read/write block io statistics at
completion time")

blk_fs_request() evolved to become blk_rq_is_passthrough() but I suspect
no one ever considered whether we want to account the passthru requests.

So, I'll leave this restriction out and see if anyone complains.

Logan

>>   *	a) it's attached to a gendisk, and
>>   *	b) the queue had IO stats enabled when this request was started, and
>> - *	c) it's a file system request
>> + *	c) it's a file system request (RQF_IO_STAT will not be set otherwise)
> 
> c) should just go away now based on your changes.
> 
>>  static inline bool blk_do_io_stat(struct request *rq)
>>  {
>>  	return rq->rq_disk &&
>> -	       (rq->rq_flags & RQF_IO_STAT) &&
>> -		!blk_rq_is_passthrough(rq);
>> +	       (rq->rq_flags & RQF_IO_STAT);
> 
> The check can be collapsed onto a single line now.
>