On 12/05/2016 10:00 AM, Ming Lei wrote:
> On Sat, Dec 3, 2016 at 11:15 AM, Jens Axboe <axboe@xxxxxx> wrote:
>> No functional changes with this patch, it's just in preparation for
>> supporting legacy schedulers on blk-mq.
>>
>> Signed-off-by: Jens Axboe <axboe@xxxxxx>
>> ---
>> block/blk-core.c | 2 +-
>> block/blk-exec.c | 2 +-
>> block/blk-flush.c | 26 ++++++++++++++------------
>> block/blk.h | 12 +++++++++++-
>> 4 files changed, 27 insertions(+), 15 deletions(-)
>>
>> diff --git a/block/blk-core.c b/block/blk-core.c
>> index 3f2eb8d80189..0e23589ab3bf 100644
>> --- a/block/blk-core.c
>> +++ b/block/blk-core.c
>> @@ -1310,7 +1310,7 @@ static struct request *blk_old_get_request(struct request_queue *q, int rw,
>>
>> struct request *blk_get_request(struct request_queue *q, int rw, gfp_t gfp_mask)
>> {
>> - if (q->mq_ops)
>> + if (blk_use_mq_path(q))
>> return blk_mq_alloc_request(q, rw,
>> (gfp_mask & __GFP_DIRECT_RECLAIM) ?
>> 0 : BLK_MQ_REQ_NOWAIT);
>
> Another way might be to use mq allocator to allocate rq in case of mq_sched,
> such as: just replace mempool_alloc in __get_request() with
> blk_mq_alloc_request(), in this way, it should be possible to
> avoid one extra rq allocation in blk_mq_sched_dispatch(), and keep mq's benefit
> of rq preallocation, which can avoid to hold queue_lock during the
> allocation too.
One problem with the MQ rq allocation is that it's tied to the device
queue depth. This is a problem for scheduling, since we want to have a
larger pool of requests that the IO scheduler can use, so that we
actually have something that we can schedule with. This is a non-starter
on QD=1 devices, but it's also a problem for SATA with 31 effectively
usable tags.
That's why I split it in two, so we have the "old" requests that we hand
to the scheduler. I know the 'rq' field copy isn't super pretty, though.
--
Jens Axboe
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