Re: [PATCH v6] block: disable iopoll for split bio

From: JeffleXu <jefflexu@linux.alibaba.com>
To: Ming Lei <ming.lei@redhat.com>
Cc: axboe@kernel.dk, linux-block@vger.kernel.org,
	joseph.qi@linux.alibaba.com, hch@infradead.org
Subject: Re: [PATCH v6] block: disable iopoll for split bio
Date: Wed, 25 Nov 2020 17:17:07 +0800	[thread overview]
Message-ID: <cf5df115-5e27-6d43-e4f9-0ac680bf0849@linux.alibaba.com> (raw)
In-Reply-To: <20201125082937.GB28463@T590>


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@redhat.com>
>> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
>> Reviewed-by: Christoph Hellwig <hch@lst.de>
>> ---
>>  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