Re: [PATCH 4/6] block: avoid ordered task state change for polled IO
From: jianchao.wang <hidden>
Date: 2018-11-12 09:35:07
Hi Jens On 11/10/18 11:13 PM, Jens Axboe wrote:
quoted hunk ↗ jump to hunk
We only really need the barrier if we're going to be sleeping, if we're just polling we're fine with the __set_current_state() variant. Signed-off-by: Jens Axboe <axboe@kernel.dk> --- fs/block_dev.c | 25 +++++++++++++++++++++---- 1 file changed, 21 insertions(+), 4 deletions(-)diff --git a/fs/block_dev.c b/fs/block_dev.c index c039abfb2052..e7550b1f9670 100644 --- a/fs/block_dev.c +++ b/fs/block_dev.c@@ -237,12 +237,23 @@ __blkdev_direct_IO_simple(struct kiocb *iocb, struct iov_iter *iter, qc = submit_bio(&bio); for (;;) { - set_current_state(TASK_UNINTERRUPTIBLE); + /* + * Using non-atomic task state for polling + */ + __set_current_state(TASK_UNINTERRUPTIBLE); + if (!READ_ONCE(bio.bi_private)) break;
When we don't use polling, the blkdev_bio_end_io_simple may come on a different cpu before
submit_bio returns
For example,
__blkdev_direct_IO_simple
qc = submit_bio(&bio)
blkdev_bio_end_io_simple
WRITE_ONCE(bio.bi_private, NULL)
wake_up_process(waiter)
__set_current_state(TASK_UNINTERRUPTIBLE)
READ_ONCE(bio.bi_private)
At the moment, a smp_rmb at least maybe needed before READ_ONCE(bio.bi_private)
to ensure the WRITE_ONCE(bio.bi_private, NULL) is seen if we miss the wakeup.
quoted hunk ↗ jump to hunk
+ if (!(iocb->ki_flags & IOCB_HIPRI) || - !blk_poll(bdev_get_queue(bdev), qc)) + !blk_poll(bdev_get_queue(bdev), qc)) { + /* + * If we're scheduling, ensure that task state + * change is ordered and seen. + */ + smp_mb(); io_schedule(); + } } __set_current_state(TASK_RUNNING);@@ -403,13 +414,19 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages) return -EIOCBQUEUED; for (;;) { - set_current_state(TASK_UNINTERRUPTIBLE); + /* + * See __blkdev_direct_IO_simple() loop + */ + __set_current_state(TASK_UNINTERRUPTIBLE); +
Similar with above.
if (!READ_ONCE(dio->waiter))
break;
if (!(iocb->ki_flags & IOCB_HIPRI) ||
- !blk_poll(bdev_get_queue(bdev), qc))
+ !blk_poll(bdev_get_queue(bdev), qc)) {
+ smp_mb();
io_schedule();
+ }
}
__set_current_state(TASK_RUNNING);
Thanks Jianchao