| Message ID | 06a05d0b16a6504461502140c3d1e9c8cd91b862.1634589262.git.asml.silence@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | blk_mq_rq_ctx_init() optimisations | expand |
On Mon, Oct 18, 2021 at 09:37:28PM +0100, Pavel Begunkov wrote: > We should have enough of registers in blk_mq_rq_ctx_init(), store them > in local vars, so we don't keep reloading them. > > note: keeping q->elevator may look unnecessary, but it's also used > inside inlined blk_mq_tags_from_data(). Is this really making a difference? I'd expect todays hyper-optimizing compilers to not be tricked into specific register allocations just by adding a local variable.
On 10/19/21 07:00, Christoph Hellwig wrote: > On Mon, Oct 18, 2021 at 09:37:28PM +0100, Pavel Begunkov wrote: >> We should have enough of registers in blk_mq_rq_ctx_init(), store them >> in local vars, so we don't keep reloading them. >> >> note: keeping q->elevator may look unnecessary, but it's also used >> inside inlined blk_mq_tags_from_data(). > > Is this really making a difference? I'd expect todays hyper-optimizing > compilers to not be tricked into specific register allocations just by > adding a local variable. Looking again, there are only reads before the use site, so indeed shouldn't matter. Was left from first versions of the patch where it wasn't the case.
diff --git a/block/blk-mq.c b/block/blk-mq.c index 1d2e2fd4043e..fa4de25c3bcb 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -312,10 +312,14 @@ static inline bool blk_mq_need_time_stamp(struct request *rq) static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data, unsigned int tag, u64 alloc_time_ns) { + struct blk_mq_ctx *ctx = data->ctx; + struct blk_mq_hw_ctx *hctx = data->hctx; + struct request_queue *q = data->q; + struct elevator_queue *e = q->elevator; struct blk_mq_tags *tags = blk_mq_tags_from_data(data); struct request *rq = tags->static_rqs[tag]; - if (data->q->elevator) { + if (e) { rq->rq_flags = RQF_ELV; rq->tag = BLK_MQ_NO_TAG; rq->internal_tag = tag; @@ -330,13 +334,13 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data, else rq->start_time_ns = 0; /* csd/requeue_work/fifo_time is initialized before use */ - rq->q = data->q; - rq->mq_ctx = data->ctx; - rq->mq_hctx = data->hctx; + rq->q = q; + rq->mq_ctx = ctx; + rq->mq_hctx = hctx; rq->cmd_flags = data->cmd_flags; if (data->flags & BLK_MQ_REQ_PM) rq->rq_flags |= RQF_PM; - if (blk_queue_io_stat(data->q)) + if (blk_queue_io_stat(q)) rq->rq_flags |= RQF_IO_STAT; rq->rq_disk = NULL; rq->part = NULL;
We should have enough of registers in blk_mq_rq_ctx_init(), store them in local vars, so we don't keep reloading them. note: keeping q->elevator may look unnecessary, but it's also used inside inlined blk_mq_tags_from_data(). Signed-off-by: Pavel Begunkov <asml.silence@gmail.com> --- block/blk-mq.c | 14 +++++++++----- 1 file changed, 9 insertions(+), 5 deletions(-)