Re: [PATCH] ufs: Increase the usable queue depth

[Can Guo <cang@codeaurora.org>]

On 2021-05-14 12:19, Bart Van Assche wrote:
> On 5/13/21 9:04 PM, Can Guo wrote:
>> Hi Bart,
>> 
>> On 2021-05-14 00:49, Bart Van Assche wrote:
>>> With the current implementation of the UFS driver active_queues is 1
>>> instead of 0 if all UFS request queues are idle. That causes
>>> hctx_may_queue() to divide the queue depth by 2 when queueing a 
>>> request
>>> and hence reduces the usable queue depth.
>> 
>> This is interesting. When all UFS queues are idle, in 
>> hctx_may_queue(),
>> active_queues reads 1 (users == 1, depth == 32), where is it divided 
>> by 2?
>> 
>> static inline bool hctx_may_queue(struct blk_mq_hw_ctx *hctx,
>>                                  struct sbitmap_queue *bt)
>> {
>>        unsigned int depth, users;
>> 
>> ....
>>                users = atomic_read(&hctx->tags->active_queues);
>>        }
>> 
>>        if (!users)
>>                return true;
>> 
>>        /*
>>         * Allow at least some tags
>>         */
>>        depth = max((bt->sb.depth + users - 1) / users, 4U);
>>        return __blk_mq_active_requests(hctx) < depth;
>> }
> 
> Hi Can,
> 
> If no I/O scheduler has been configured then the active_queues counter
> is increased from inside blk_get_request() by blk_mq_tag_busy() before
> hctx_may_queue() is called. So if active_queues == 1 when the UFS 
> device
> is idle, the active_queues counter will be increased to 2 if a request
> is submitted to another request queue than hba->cmd_queue. This will
> cause the hctx_may_queue() calls from inside __blk_mq_alloc_request()
> and __blk_mq_get_driver_tag() to limit the queue depth to 32 / 2 = 16.
> 
> If an I/O scheduler has been configured then __blk_mq_get_driver_tag()
> will be the first function to call blk_mq_tag_busy() while processing a
> request. The hctx_may_queue() call in __blk_mq_get_driver_tag() will
> limit the queue depth to 32 / 2 = 16 if an I/O scheduler has been
> configured.
> 
> Bart.

Yes, I just figured out what you are saying from the commit message and
gave my reviewed-by tag. Thanks for the explanation and the fix.

Regards,
Can Guo.