On Tue, Dec 6, 2016 at 1:09 AM, Jens Axboe <axboe@xxxxxx> wrote:
> 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.
OK, got it, thanks for your explanation.
So could we fall back to mempool_alloc() for allocating rq with mq
size if MQ rq allocator fails? Then in this way the extra rq allocation
in blk_mq_alloc_request() may be killed.
Thanks,
Ming
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