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

[Can Guo <cang@codeaurora.org>]

On 2021-05-14 12:04, 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?
> 

Are you saying that if we queue a new request on one of the UFS request
queues, since the active_queues is always above 0, we can never use
the full queue depth? If so, then I agree.

Reviewed-by: Can Guo <cang@codeaurora.org>

> 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;
> }
> 
> Thanks,
> Can Guo.
> 
>> 
>> The shared tag set code in the block layer keeps track of the number 
>> of
>> active request queues. blk_mq_tag_busy() is called before a request is
>> queued onto a hwq and blk_mq_tag_idle() is called some time after the 
>> hwq
>> became idle. blk_mq_tag_idle() is called from inside 
>> blk_mq_timeout_work().
>> Hence, blk_mq_tag_idle() is only called if a timer is associated with 
>> each
>> request that is submitted to a request queue that shares a tag set 
>> with
>> another request queue. Hence this patch that adds a 
>> blk_mq_start_request()
>> call in ufshcd_exec_dev_cmd(). This patch doubles the queue depth on 
>> my
>> test setup from 16 to 32.
>> 
>> In addition to increasing the usable queue depth, also fix the
>> documentation of the 'timeout' parameter in the header above
>> ufshcd_exec_dev_cmd().
>> 
>> Cc: Can Guo <cang@codeaurora.org>
>> Cc: Alim Akhtar <alim.akhtar@samsung.com>
>> Cc: Avri Altman <avri.altman@wdc.com>
>> Cc: Stanley Chu <stanley.chu@mediatek.com>
>> Cc: Bean Huo <beanhuo@micron.com>
>> Cc: Adrian Hunter <adrian.hunter@intel.com>
>> Fixes: 7252a3603015 ("scsi: ufs: Avoid busy-waiting by eliminating tag
>> conflicts")
>> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
>> ---
>>  drivers/scsi/ufs/ufshcd.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
>> index c96e36aab989..e669243354da 100644
>> --- a/drivers/scsi/ufs/ufshcd.c
>> +++ b/drivers/scsi/ufs/ufshcd.c
>> @@ -2838,7 +2838,7 @@ static int ufshcd_wait_for_dev_cmd(struct 
>> ufs_hba *hba,
>>   * ufshcd_exec_dev_cmd - API for sending device management requests
>>   * @hba: UFS hba
>>   * @cmd_type: specifies the type (NOP, Query...)
>> - * @timeout: time in seconds
>> + * @timeout: timeout in milliseconds
>>   *
>>   * NOTE: Since there is only one available tag for device management 
>> commands,
>>   * it is expected you hold the hba->dev_cmd.lock mutex.
>> @@ -2868,6 +2868,9 @@ static int ufshcd_exec_dev_cmd(struct ufs_hba 
>> *hba,
>>  	}
>>  	tag = req->tag;
>>  	WARN_ON_ONCE(!ufshcd_valid_tag(hba, tag));
>> +	/* Set the timeout such that the SCSI error handler is not 
>> activated. */
>> +	req->timeout = msecs_to_jiffies(2 * timeout);
>> +	blk_mq_start_request(req);
>> 
>>  	init_completion(&wait);
>>  	lrbp = &hba->lrb[tag];