From bc07c10a3603a5ab3ef01ba42b3d41f9ac63d1b6 Mon Sep 17 00:00:00 2001 From: Ming Lei Date: Mon, 17 Aug 2015 10:31:51 +0800 Subject: [PATCH] block: loop: support DIO & AIO There are at least 3 advantages to use direct I/O and AIO on read/write loop's backing file: 1) double cache can be avoided, then memory usage gets decreased a lot 2) not like user space direct I/O, there isn't cost of pinning pages 3) avoid context switch for obtaining good throughput - in buffered file read, random I/O top throughput is often obtained only if they are submitted concurrently from lots of tasks; but for sequential I/O, most of times they can be hit from page cache, so concurrent submissions often introduce unnecessary context switch and can't improve throughput much. There was such discussion[1] to use non-blocking I/O to improve the problem for application. - with direct I/O and AIO, concurrent submissions can be avoided and random read throughput can't be affected meantime xfstests(-g auto, ext4) is basically passed when running with direct I/O(aio), one exception is generic/232, but it failed in loop buffered I/O(4.2-rc6-next-20150814) too. Follows the fio test result for performance purpose: 4 jobs fio test inside ext4 file system over loop block 1) How to run - KVM: 4 VCPUs, 2G RAM - linux kernel: 4.2-rc6-next-20150814(base) with the patchset - the loop block is over one image on SSD. - linux psync, 4 jobs, size 1500M, ext4 over loop block - test result: IOPS from fio output 2) Throughput(IOPS) becomes a bit better with direct I/O(aio) ------------------------------------------------------------- test cases |randread |read |randwrite |write | ------------------------------------------------------------- base |8015 |113811 |67442 |106978 ------------------------------------------------------------- base+loop aio |8136 |125040 |67811 |111376 ------------------------------------------------------------- - somehow, it should be caused by more page cache avaiable for application or one extra page copy is avoided in case of direct I/O 3) context switch - context switch decreased by ~50% with loop direct I/O(aio) compared with loop buffered I/O(4.2-rc6-next-20150814) 4) memory usage from /proc/meminfo ------------------------------------------------------------- | Buffers | Cached ------------------------------------------------------------- base | > 760MB | ~950MB ------------------------------------------------------------- base+loop direct I/O(aio) | < 5MB | ~1.6GB ------------------------------------------------------------- - so there are much more page caches available for application with direct I/O [1] https://lwn.net/Articles/612483/ Signed-off-by: Ming Lei Reviewed-by: Christoph Hellwig Signed-off-by: Jens Axboe --- drivers/block/loop.c | 98 ++++++++++++++++++++++++++++++++++++++++++-- drivers/block/loop.h | 2 + 2 files changed, 97 insertions(+), 3 deletions(-) diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 75db3b98ec2b..23376084a5cb 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -445,6 +445,90 @@ static int lo_req_flush(struct loop_device *lo, struct request *rq) return ret; } +static inline void handle_partial_read(struct loop_cmd *cmd, long bytes) +{ + if (bytes < 0 || (cmd->rq->cmd_flags & REQ_WRITE)) + return; + + if (unlikely(bytes < blk_rq_bytes(cmd->rq))) { + struct bio *bio = cmd->rq->bio; + + bio_advance(bio, bytes); + zero_fill_bio(bio); + } +} + +static void lo_rw_aio_complete(struct kiocb *iocb, long ret, long ret2) +{ + struct loop_cmd *cmd = container_of(iocb, struct loop_cmd, iocb); + struct request *rq = cmd->rq; + + handle_partial_read(cmd, ret); + + if (ret > 0) + ret = 0; + else if (ret < 0) + ret = -EIO; + + rq->errors = ret; + blk_mq_complete_request(rq); +} + +static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd, + loff_t pos, bool rw) +{ + struct iov_iter iter; + struct bio_vec *bvec; + struct bio *bio = cmd->rq->bio; + struct file *file = lo->lo_backing_file; + int ret; + + /* nomerge for loop request queue */ + WARN_ON(cmd->rq->bio != cmd->rq->biotail); + + bvec = __bvec_iter_bvec(bio->bi_io_vec, bio->bi_iter); + iov_iter_bvec(&iter, ITER_BVEC | rw, bvec, + bio_segments(bio), blk_rq_bytes(cmd->rq)); + + cmd->iocb.ki_pos = pos; + cmd->iocb.ki_filp = file; + cmd->iocb.ki_complete = lo_rw_aio_complete; + cmd->iocb.ki_flags = IOCB_DIRECT; + + if (rw == WRITE) + ret = file->f_op->write_iter(&cmd->iocb, &iter); + else + ret = file->f_op->read_iter(&cmd->iocb, &iter); + + if (ret != -EIOCBQUEUED) + cmd->iocb.ki_complete(&cmd->iocb, ret, 0); + return 0; +} + + +static inline int lo_rw_simple(struct loop_device *lo, + struct request *rq, loff_t pos, bool rw) +{ + struct loop_cmd *cmd = blk_mq_rq_to_pdu(rq); + + if (cmd->use_aio) + return lo_rw_aio(lo, cmd, pos, rw); + + /* + * lo_write_simple and lo_read_simple should have been covered + * by io submit style function like lo_rw_aio(), one blocker + * is that lo_read_simple() need to call flush_dcache_page after + * the page is written from kernel, and it isn't easy to handle + * this in io submit style function which submits all segments + * of the req at one time. And direct read IO doesn't need to + * run flush_dcache_page(). + */ + if (rw == WRITE) + return lo_write_simple(lo, rq, pos); + else + return lo_read_simple(lo, rq, pos); +} + static int do_req_filebacked(struct loop_device *lo, struct request *rq) { loff_t pos; @@ -460,13 +544,13 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq) else if (lo->transfer) ret = lo_write_transfer(lo, rq, pos); else - ret = lo_write_simple(lo, rq, pos); + ret = lo_rw_simple(lo, rq, pos, WRITE); } else { if (lo->transfer) ret = lo_read_transfer(lo, rq, pos); else - ret = lo_read_simple(lo, rq, pos); + ret = lo_rw_simple(lo, rq, pos, READ); } return ret; @@ -1570,6 +1654,12 @@ static int loop_queue_rq(struct blk_mq_hw_ctx *hctx, if (lo->lo_state != Lo_bound) return -EIO; + if (lo->use_dio && !(cmd->rq->cmd_flags & (REQ_FLUSH | + REQ_DISCARD))) + cmd->use_aio = true; + else + cmd->use_aio = false; + queue_kthread_work(&lo->worker, &cmd->work); return BLK_MQ_RQ_QUEUE_OK; @@ -1589,7 +1679,9 @@ static void loop_handle_cmd(struct loop_cmd *cmd) failed: if (ret) cmd->rq->errors = -EIO; - blk_mq_complete_request(cmd->rq); + /* complete non-aio request */ + if (!cmd->use_aio || ret) + blk_mq_complete_request(cmd->rq); } static void loop_queue_work(struct kthread_work *work) diff --git a/drivers/block/loop.h b/drivers/block/loop.h index d1de2217c09a..fb2237c73e61 100644 --- a/drivers/block/loop.h +++ b/drivers/block/loop.h @@ -69,6 +69,8 @@ struct loop_cmd { struct kthread_work work; struct request *rq; struct list_head list; + bool use_aio; /* use AIO interface to handle I/O */ + struct kiocb iocb; }; /* Support for loadable transfer modules */