Re: [PATCH RFC 3/5] blk-mq: Facilitate a shared tags per tagset

From: John Garry <john.garry@huawei.com>
To: Bart Van Assche <bvanassche@acm.org>,
	Hannes Reinecke <hare@suse.de>,
	"axboe@kernel.dk" <axboe@kernel.dk>,
	"jejb@linux.ibm.com" <jejb@linux.ibm.com>,
	"martin.petersen@oracle.com" <martin.petersen@oracle.com>
Cc: "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
	"ming.lei@redhat.com" <ming.lei@redhat.com>,
	"hare@suse.com" <hare@suse.com>,
	"chenxiang (M)" <chenxiang66@hisilicon.com>
Subject: Re: [PATCH RFC 3/5] blk-mq: Facilitate a shared tags per tagset
Date: Tue, 19 Nov 2019 09:26:55 +0000	[thread overview]
Message-ID: <0b75b219-6145-a58a-77fa-74024cdec493@huawei.com> (raw)
In-Reply-To: <1ba51afd-cce5-f7b2-704c-06e00db027bc@huawei.com>

>>
>>>> @@ -341,8 +341,11 @@ void blk_mq_tagset_busy_iter(struct 
>>>> blk_mq_tag_set *tagset,
>>>>       int i;
>>>>
>>>>       for (i = 0; i < tagset->nr_hw_queues; i++) {
>>>> -        if (tagset->tags && tagset->tags[i])
>>>> +        if (tagset->tags && tagset->tags[i]) {
>>>>               blk_mq_all_tag_busy_iter(tagset->tags[i], fn, priv);
>>>
>>> As I mentioned earlier, wouldn't this iterate over all tags for all 
>>> hctx's, when we just want the tags for hctx[i]?
>>>
>>> Thanks,
>>> John
>>>
>>> [Not trimming reply for future reference]
>>>
>>>> +            if (tagset->share_tags)
>>>> +                break;
>>>> +        }
>>>>       }
>>>>   }
>>>>   EXPORT_SYMBOL(blk_mq_tagset_busy_iter);
>>
>> Since blk_mq_tagset_busy_iter() loops over all hardware queues all 
>> what is changed is the order in which requests are examined. I am not 
>> aware of any block driver that calls blk_mq_tagset_busy_iter() and 
>> that depends on the order of the requests passed to the callback 
>> function.
>>
> 
> OK, fine.
> 
> So, to me, this approach also seems viable then.
> 
> I am however not so happy with how we use blk_mq_tag_set.tags[0] for the 
> shared tags; I would like to use blk_mq_tag_set.shared_tags and make 
> blk_mq_tag_set.tags[] point at blk_mq_tag_set.shared_tags or maybe not 
> blk_mq_tag_set.tags[] at all. However maybe that change may be more 
> intrusive.
> 
> And another more real concern is that we miss a check somewhere for 
> rq->mq_hctx == hctx when examining the bits on the shared tags.

Another issue I notice is that using tags from just hctx0 may cause a 
breakage when associated with a different hw queue in the driver.

Specifically we may have blk_mq_alloc_rqs(hctx_idx = 
0)->blk_mq_init_request()->nvme_init_request(), and we would set all 
iod->nvmeq = nvmeq0; since we may actually use this iod on another hw 
queue, we're breaking that interface.

Thanks,
John