Re: [PATCH 1/4] block: add mq_ops->queue_rqs hook

[Jens Axboe <axboe@kernel.dk>]

On 12/4/21 3:43 AM, Hannes Reinecke wrote:
> On 12/3/21 10:45 PM, Jens Axboe wrote:
>> If we have a list of requests in our plug list, send it to the driver in
>> one go, if possible. The driver must set mq_ops->queue_rqs() to support
>> this, if not the usual one-by-one path is used.
>>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>> ---
>>   block/blk-mq.c         | 24 +++++++++++++++++++++---
>>   include/linux/blk-mq.h |  8 ++++++++
>>   2 files changed, 29 insertions(+), 3 deletions(-)
>>
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index 22ec21aa0c22..9ac9174a2ba4 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -2513,6 +2513,7 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
>>   {
>>   	struct blk_mq_hw_ctx *this_hctx;
>>   	struct blk_mq_ctx *this_ctx;
>> +	struct request *rq;
>>   	unsigned int depth;
>>   	LIST_HEAD(list);
>>   
>> @@ -2521,7 +2522,26 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
>>   	plug->rq_count = 0;
>>   
>>   	if (!plug->multiple_queues && !plug->has_elevator && !from_schedule) {
>> -		blk_mq_run_dispatch_ops(plug->mq_list->q,
>> +		struct request_queue *q;
>> +
>> +		rq = plug->mq_list;
>> +		q = rq->q;
>> +
>> +		/*
>> +		 * Peek first request and see if we have a ->queue_rqs() hook.
>> +		 * If we do, we can dispatch the whole plug list in one go. We
>> +		 * already know at this point that all requests belong to the
>> +		 * same queue, caller must ensure that's the case.
>> +		 */
>> +		if (q->mq_ops->queue_rqs &&
>> +		    !(rq->mq_hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED)) {
> 
> What is the dependency on shared tags here?
>  From what I've seen it's just about submitting requests; the only 
> difference to shared tags is the way the tags are allocated.
> Care to explain?

For shared tags, we need to actively increment the use count per
request. This path doesn't do that, so it's disabled for now. It could
be done, but then it'd have to be in the caller, so I'd rather leave it
for a future optimization if anyone cares enough about this for shared
tags.

I can add a comment about it if that helps.

-- 
Jens Axboe