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 <ming.lei@canonical.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@fb.com>
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@ -445,6 +445,90 @@ static int lo_req_flush(struct loop_device *lo, struct request *rq)
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return ret;
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}
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static inline void handle_partial_read(struct loop_cmd *cmd, long bytes)
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{
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if (bytes < 0 || (cmd->rq->cmd_flags & REQ_WRITE))
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return;
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if (unlikely(bytes < blk_rq_bytes(cmd->rq))) {
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struct bio *bio = cmd->rq->bio;
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bio_advance(bio, bytes);
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zero_fill_bio(bio);
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}
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}
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static void lo_rw_aio_complete(struct kiocb *iocb, long ret, long ret2)
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{
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struct loop_cmd *cmd = container_of(iocb, struct loop_cmd, iocb);
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struct request *rq = cmd->rq;
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handle_partial_read(cmd, ret);
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if (ret > 0)
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ret = 0;
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else if (ret < 0)
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ret = -EIO;
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rq->errors = ret;
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blk_mq_complete_request(rq);
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}
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static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
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loff_t pos, bool rw)
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{
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struct iov_iter iter;
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struct bio_vec *bvec;
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struct bio *bio = cmd->rq->bio;
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struct file *file = lo->lo_backing_file;
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int ret;
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/* nomerge for loop request queue */
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WARN_ON(cmd->rq->bio != cmd->rq->biotail);
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bvec = __bvec_iter_bvec(bio->bi_io_vec, bio->bi_iter);
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iov_iter_bvec(&iter, ITER_BVEC | rw, bvec,
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bio_segments(bio), blk_rq_bytes(cmd->rq));
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cmd->iocb.ki_pos = pos;
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cmd->iocb.ki_filp = file;
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cmd->iocb.ki_complete = lo_rw_aio_complete;
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cmd->iocb.ki_flags = IOCB_DIRECT;
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if (rw == WRITE)
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ret = file->f_op->write_iter(&cmd->iocb, &iter);
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else
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ret = file->f_op->read_iter(&cmd->iocb, &iter);
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if (ret != -EIOCBQUEUED)
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cmd->iocb.ki_complete(&cmd->iocb, ret, 0);
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return 0;
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}
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static inline int lo_rw_simple(struct loop_device *lo,
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struct request *rq, loff_t pos, bool rw)
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{
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struct loop_cmd *cmd = blk_mq_rq_to_pdu(rq);
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if (cmd->use_aio)
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return lo_rw_aio(lo, cmd, pos, rw);
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/*
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* lo_write_simple and lo_read_simple should have been covered
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* by io submit style function like lo_rw_aio(), one blocker
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* is that lo_read_simple() need to call flush_dcache_page after
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* the page is written from kernel, and it isn't easy to handle
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* this in io submit style function which submits all segments
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* of the req at one time. And direct read IO doesn't need to
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* run flush_dcache_page().
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*/
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if (rw == WRITE)
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return lo_write_simple(lo, rq, pos);
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else
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return lo_read_simple(lo, rq, pos);
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}
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static int do_req_filebacked(struct loop_device *lo, struct request *rq)
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{
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loff_t pos;
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@ -460,13 +544,13 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq)
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else if (lo->transfer)
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ret = lo_write_transfer(lo, rq, pos);
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else
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ret = lo_write_simple(lo, rq, pos);
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ret = lo_rw_simple(lo, rq, pos, WRITE);
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} else {
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if (lo->transfer)
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ret = lo_read_transfer(lo, rq, pos);
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else
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ret = lo_read_simple(lo, rq, pos);
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ret = lo_rw_simple(lo, rq, pos, READ);
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}
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return ret;
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@ -1570,6 +1654,12 @@ static int loop_queue_rq(struct blk_mq_hw_ctx *hctx,
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if (lo->lo_state != Lo_bound)
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return -EIO;
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if (lo->use_dio && !(cmd->rq->cmd_flags & (REQ_FLUSH |
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REQ_DISCARD)))
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cmd->use_aio = true;
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else
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cmd->use_aio = false;
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queue_kthread_work(&lo->worker, &cmd->work);
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return BLK_MQ_RQ_QUEUE_OK;
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@ -1589,7 +1679,9 @@ static void loop_handle_cmd(struct loop_cmd *cmd)
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failed:
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if (ret)
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cmd->rq->errors = -EIO;
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blk_mq_complete_request(cmd->rq);
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/* complete non-aio request */
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if (!cmd->use_aio || ret)
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blk_mq_complete_request(cmd->rq);
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}
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static void loop_queue_work(struct kthread_work *work)
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@ -69,6 +69,8 @@ struct loop_cmd {
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struct kthread_work work;
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struct request *rq;
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struct list_head list;
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bool use_aio; /* use AIO interface to handle I/O */
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struct kiocb iocb;
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};
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/* Support for loadable transfer modules */
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