On 11/25/20 3:19 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); >> + >> return bio_split(bio, sectors, GFP_NOIO, bs); >> } >> >> diff --git a/block/blk-mq.c b/block/blk-mq.c >> index 55bcee5dc032..ce1f3628e4c2 100644 >> --- a/block/blk-mq.c >> +++ b/block/blk-mq.c >> @@ -2265,6 +2265,9 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio) >> blk_mq_sched_insert_request(rq, false, true, true); >> } >> >> + if (bio_flagged(bio, BIO_SPLIT)) >> + return BLK_QC_T_NONE; >> + > > Not sure the new bio flag is really required for this case, just wondering > why not take the following simple way? BTW we are really going to run > out of bio flag. > Please consider the following case: One big bio got split into two split bios. At the first call of blk_mq_submit_bio(), the input @bio (actually the original big bio) indeed gets split. The split bio gets enqueued to hw queue and the returned cookie is BLK_QC_T_NONE, while the remained bio gets buffered in bio_list. So far so good. Then when calling blk_mq_submit_bio() the second time, the input @bio is indeed the remained bio. At this time, it will not get split and you will get a *valid* cookie. And since the cookie of last split bio will actually overrides the previous cookie, you will get a *valid* cookie as a result. > diff --git a/block/blk-mq.c b/block/blk-mq.c > index 55bcee5dc032..1139b1efd712 100644 > --- a/block/blk-mq.c > +++ b/block/blk-mq.c > @@ -2157,6 +2157,7 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio) > unsigned int nr_segs; > blk_qc_t cookie; > blk_status_t ret; > + struct bio *orig_bio = bio; > > blk_queue_bounce(q, &bio); > __blk_queue_split(&bio, &nr_segs); > @@ -2265,6 +2266,10 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio) > blk_mq_sched_insert_request(rq, false, true, true); > } > > + /* don't poll splitted bio */ > + if (orig_bio != bio) > + return BLK_QC_T_NONE; > + > return cookie; > queue_exit: > blk_queue_exit(q); > > Thanks, > Ming > -- Thanks, Jeffle
