On 11/25/20 4:29 PM, Ming Lei wrote: > On Wed, Nov 25, 2020 at 02:41:47PM +0800, Jeffle Xu wrote: >> iopoll is initially for small size, latency sensitive IO. It doesn't >> work well for big IO, especially when it needs to be split to multiple >> bios. In this case, the returned cookie of __submit_bio_noacct_mq() is >> indeed the cookie of the last split bio. The completion of *this* last >> split bio done by iopoll doesn't mean the whole original bio has >> completed. Callers of iopoll still need to wait for completion of other >> split bios. >> >> Besides bio splitting may cause more trouble for iopoll which isn't >> supposed to be used in case of big IO. >> >> iopoll for split bio may cause potential race if CPU migration happens >> during bio submission. Since the returned cookie is that of the last >> split bio, polling on the corresponding hardware queue doesn't help >> complete other split bios, if these split bios are enqueued into >> different hardware queues. Since interrupts are disabled for polling >> queues, the completion of these other split bios depends on timeout >> mechanism, thus causing a potential hang. >> >> iopoll for split bio may also cause hang for sync polling. Currently >> both the blkdev and iomap-based fs (ext4/xfs, etc) support sync polling >> in direct IO routine. These routines will submit bio without REQ_NOWAIT >> flag set, and then start sync polling in current process context. The >> process may hang in blk_mq_get_tag() if the submitted bio has to be >> split into multiple bios and can rapidly exhaust the queue depth. The >> process are waiting for the completion of the previously allocated >> requests, which should be reaped by the following polling, and thus >> causing a deadlock. >> >> To avoid these subtle trouble described above, just disable iopoll for >> split bio. >> >> Suggested-by: Ming Lei <ming.lei@xxxxxxxxxx> >> Signed-off-by: Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx> >> Reviewed-by: Christoph Hellwig <hch@xxxxxx> >> --- >> block/bio.c | 2 ++ >> block/blk-merge.c | 12 ++++++++++++ >> block/blk-mq.c | 3 +++ >> include/linux/blk_types.h | 1 + >> 4 files changed, 18 insertions(+) >> >> diff --git a/block/bio.c b/block/bio.c >> index fa01bef35bb1..7f7ddc22a30d 100644 >> --- a/block/bio.c >> +++ b/block/bio.c >> @@ -684,6 +684,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src) >> bio_set_flag(bio, BIO_CLONED); >> if (bio_flagged(bio_src, BIO_THROTTLED)) >> bio_set_flag(bio, BIO_THROTTLED); >> + if (bio_flagged(bio_src, BIO_SPLIT)) >> + bio_set_flag(bio, BIO_SPLIT); >> bio->bi_opf = bio_src->bi_opf; >> bio->bi_ioprio = bio_src->bi_ioprio; >> bio->bi_write_hint = bio_src->bi_write_hint; >> diff --git a/block/blk-merge.c b/block/blk-merge.c >> index bcf5e4580603..a2890cebf99f 100644 >> --- a/block/blk-merge.c >> +++ b/block/blk-merge.c >> @@ -279,6 +279,18 @@ static struct bio *blk_bio_segment_split(struct request_queue *q, >> return NULL; >> split: >> *segs = nsegs; >> + >> + /* >> + * Bio splitting may cause subtle trouble such as hang when doing sync >> + * iopoll in direct IO routine. Given performance gain of iopoll for >> + * big IO can be trival, disable iopoll when split needed. We need >> + * BIO_SPLIT to identify bios need this workaround. Since currently >> + * only normal IO under mq routine may suffer this issue, BIO_SPLIT is >> + * only marked here. >> + */ >> + bio->bi_opf &= ~REQ_HIPRI; >> + bio_set_flag(bio, BIO_SPLIT); > > You may need to put the above into one helper, and call the helper for > other splitted cases(discard, write zero and write same) too. It could be, though currently only normal IO could be marked with REQ_HIPRI. -- Thanks, Jeffle
