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.
--
Jens Axboe
