Re: [PATCH] blk-mq: put the driver tag of nxt rq before first one is requeued

[Ming Lei]

On Tue, Sep 12, 2017 at 08:45:19PM -0600, Jens Axboe wrote:
> On 09/12/2017 08:42 PM, jianchao.wang wrote:
> > 
> > 
> > On 09/13/2017 10:23 AM, Jens Axboe wrote:
> >> On 09/12/2017 07:39 PM, jianchao.wang wrote:
> >>>
> >>>
> >>> On 09/13/2017 09:24 AM, Ming Lei wrote:
> >>>> On Wed, Sep 13, 2017 at 09:01:25AM +0800, jianchao.wang wrote:
> >>>>> Hi ming
> >>>>>
> >>>>> On 09/12/2017 06:23 PM, Ming Lei wrote:
> >>>>>>> @@ -1029,14 +1029,20 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
> >>>>>>>  		if (list_empty(list))
> >>>>>>>  			bd.last = true;
> >>>>>>>  		else {
> >>>>>>> -			struct request *nxt;
> >>>>>>> -
> >>>>>>>  			nxt = list_first_entry(list, struct request, queuelist);
> >>>>>>>  			bd.last = !blk_mq_get_driver_tag(nxt, NULL, false);
> >>>>>>>  		}
> >>>>>>>  
> >>>>>>>  		ret = q->mq_ops->queue_rq(hctx, &bd);
> >>>>>>>  		if (ret == BLK_STS_RESOURCE) {
> >>>>>>> +			/*
> >>>>>>> +			 * If an I/O scheduler has been configured and we got a
> >>>>>>> +			 * driver tag for the next request already, free it again.
> >>>>>>> +			 */
> >>>>>>> +			if (!list_empty(list)) {
> >>>>>>> +				nxt = list_first_entry(list, struct request, queuelist);
> >>>>>>> +				blk_mq_put_driver_tag(nxt);
> >>>>>>> +			}
> >>>>>> The following way might be more simple and clean:
> >>>>>>
> >>>>>> 			if (nxt)
> >>>>>> 				blk_mq_put_driver_tag(nxt);
> >>>>>>
> >>>>>> meantime 'nxt' need to be cleared inside the 'if (list_empty(list))'
> >>>>>> before .queue_rq().
> >>>>>
> >>>>> I had ever thought about that, but to avoid add extra command in the 
> >>>>> fast path, I made the patch above.
> >>>>
> >>>> Got it, so how about changing to the following way simply:
> >>>>
> >>>>  			if (nxt && !list_empty(list))
> >>>>  				blk_mq_put_driver_tag(nxt);
> >>>>
> >>> It seems that we even could change it as following:
> >>>                         if (!list_empty(list))
> >>>   				blk_mq_put_driver_tag(nxt);
> >>
> >> This is starting to get too clever for its own good, I generally don't
> >> like to sacrifice readability for performance. In reality, the compiler
> >> probably figures it out anyway...
> >>
> >> So either make it explicit, or add a nice comment as to why it is the
> >> way that it is.
> >>
> > yes, it indeed leads to compiler warning of "may be used uninitialized"
> > maybe the original one could be taken back.
> > 			if (!list_empty(list)) {
> > 				nxt = list_first_entry(list, struct request, queuelist);
> > 				blk_mq_put_driver_tag(nxt);
> > 			}
> > It is more readable and could avoid the warning.
> 
> Exactly, and especially the readability is the key element here. It's
> just not worth it to try and be too clever, especially not for something
> like this. When you read the above, you immediately know what the code
> does without needing a comment. That's not true for the other construct.
> You both have to read other parts of the function to figure out what it
> does, AND read the entire function to ensure it always does the right
> thing. Fragile.

OK, fair enough wrt. readability.

Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>

For the original post.

-- 
Ming