On 11/27/18 4:49 PM, Jens Axboe wrote:
> On 11/27/18 4:31 PM, Omar Sandoval wrote:
>> On Mon, Nov 26, 2018 at 09:35:50AM -0700, Jens Axboe wrote:
>>> Do it for the nr_hw_queues == 1 case, but only do it for the multi queue
>>> case if we have requests for multiple devices in the plug.
>>>
>>> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
>>> ---
>>> block/blk-core.c | 1 +
>>> block/blk-mq.c | 7 +++++--
>>> include/linux/blkdev.h | 1 +
>>> 3 files changed, 7 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/block/blk-core.c b/block/blk-core.c
>>> index be9233400314..c9758d185357 100644
>>> --- a/block/blk-core.c
>>> +++ b/block/blk-core.c
>>> @@ -1780,6 +1780,7 @@ void blk_start_plug(struct blk_plug *plug)
>>> INIT_LIST_HEAD(&plug->mq_list);
>>> INIT_LIST_HEAD(&plug->cb_list);
>>> plug->rq_count = 0;
>>> + plug->do_sort = false;
>>>
>>> /*
>>> * Store ordering should not be needed here, since a potential
>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>> index 99c66823d52f..6a249bf6ed00 100644
>>> --- a/block/blk-mq.c
>>> +++ b/block/blk-mq.c
>>> @@ -1678,7 +1678,8 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
>>> list_splice_init(&plug->mq_list, &list);
>>> plug->rq_count = 0;
>>>
>>> - list_sort(NULL, &list, plug_rq_cmp);
>>> + if (plug->do_sort)
>>> + list_sort(NULL, &list, plug_rq_cmp);
>>>
>>> this_q = NULL;
>>> this_hctx = NULL;
>>> @@ -1935,6 +1936,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
>>>
>>> list_add_tail(&rq->queuelist, &plug->mq_list);
>>> plug->rq_count++;
>>> + plug->do_sort = true;
>>> } else if (plug && !blk_queue_nomerges(q)) {
>>> blk_mq_bio_to_request(rq, bio);
>>>
>>> @@ -1958,7 +1960,8 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
>>> data.hctx = same_queue_rq->mq_hctx;
>>> blk_mq_try_issue_directly(data.hctx, same_queue_rq,
>>> &cookie);
>>> - }
>>> + } else if (plug->rq_count > 1)
>>> + plug->do_sort = true;
>>
>> If plug->rq_count == 2, there's no benefit to sorting, either. The
>> nr_hw_queues == 1 case could also avoid sorting in that case. So maybe
>> this whole patch could just be replaced with:
>
> Heh yes, good point, it should be 3 at least. But if you look at the
> later mq plug patch, we only sort for that one if we have multiple
> queues. So the logic should be something ala:
>
> if (plug->rq_count > 2 && plug->has_multiple_queues)
>
> since that's the only case we want to sort for.
How about something like this?
diff --git a/block/blk-core.c b/block/blk-core.c
index be9233400314..d107d016b92b 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1780,6 +1780,7 @@ void blk_start_plug(struct blk_plug *plug)
INIT_LIST_HEAD(&plug->mq_list);
INIT_LIST_HEAD(&plug->cb_list);
plug->rq_count = 0;
+ plug->multiple_queues = false;
/*
* Store ordering should not be needed here, since a potential
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 7b7dff85cf6c..02daa32c5d77 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1677,7 +1677,8 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
list_splice_init(&plug->mq_list, &list);
plug->rq_count = 0;
- list_sort(NULL, &list, plug_rq_cmp);
+ if (plug->rq_count > 2 && plug->multiple_queues)
+ list_sort(NULL, &list, plug_rq_cmp);
this_q = NULL;
this_hctx = NULL;
@@ -1866,6 +1867,20 @@ void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx,
}
}
+static void blk_add_rq_to_plug(struct blk_plug *plug, struct request *rq)
+{
+ list_add_tail(&rq->queuelist, &plug->mq_list);
+ plug->rq_count++;
+ if (!plug->multiple_queues && !list_is_singular(&plug->mq_list)) {
+ struct request *tmp;
+
+ tmp = list_first_entry(&plug->mq_list, struct request,
+ queuelist);
+ if (tmp->q != rq->q)
+ plug->multiple_queues = true;
+ }
+}
+
static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
{
const int is_sync = op_is_sync(bio->bi_opf);
@@ -1932,8 +1947,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
trace_block_plug(q);
}
- list_add_tail(&rq->queuelist, &plug->mq_list);
- plug->rq_count++;
+ blk_add_rq_to_plug(plug, rq);
} else if (plug && !blk_queue_nomerges(q)) {
blk_mq_bio_to_request(rq, bio);
@@ -1948,8 +1962,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
same_queue_rq = NULL;
if (same_queue_rq)
list_del_init(&same_queue_rq->queuelist);
- list_add_tail(&rq->queuelist, &plug->mq_list);
- plug->rq_count++;
+ blk_add_rq_to_plug(plug, rq);
blk_mq_put_ctx(data.ctx);
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 02732cae6080..08d940f85fa0 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1131,6 +1131,7 @@ struct blk_plug {
struct list_head mq_list; /* blk-mq requests */
struct list_head cb_list; /* md requires an unplug callback */
unsigned short rq_count;
+ bool multiple_queues;
};
#define BLK_MAX_REQUEST_COUNT 16
#define BLK_PLUG_FLUSH_SIZE (128 * 1024)
--
Jens Axboe
