On 12/05/2016 10:00 AM, Ming Lei wrote: > On Sat, Dec 3, 2016 at 11:15 AM, Jens Axboe <[email protected]> wrote: >> No functional changes with this patch, it's just in preparation for >> supporting legacy schedulers on blk-mq. >> >> Signed-off-by: Jens Axboe <[email protected]> >> --- >> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
