From: Ming Lei <ming.lei@redhat.com>
To: Jeffle Xu <jefflexu@linux.alibaba.com>
Cc: axboe@kernel.dk, hch@infradead.org, viro@zeniv.linux.org.uk,
linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
joseph.qi@linux.alibaba.com, xiaoguang.wang@linux.alibaba.com
Subject: Re: [PATCH v3 1/2] block: disable iopoll for split bio
Date: Fri, 16 Oct 2020 20:51:51 +0800 [thread overview]
Message-ID: <20201016125151.GC1218835@T590> (raw)
In-Reply-To: <20201016091851.93728-2-jefflexu@linux.alibaba.com>
On Fri, Oct 16, 2020 at 05:18:50PM +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>
> ---
> block/blk-merge.c | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index bcf5e4580603..924db7c428b4 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -279,6 +279,20 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> return NULL;
> split:
> *segs = nsegs;
> +
> + /*
> + * bio splitting may cause more trouble for iopoll which isn't supposed
> + * to be used in case of big IO.
> + * 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.
> + */
> + bio->bi_opf &= ~REQ_HIPRI;
> +
> return bio_split(bio, sectors, GFP_NOIO, bs);
> }
The above change may not be enough, since caller of submit_bio() still
can call into blk_poll() even though REQ_HIPRI is cleared for splitted
bio, for avoiding this issue:
- Either we may add check in blk_poll() to only allow hctx with HCTX_TYPE_POLL
to poll,
- or return BLK_QC_T_NONE from blk_mq_submit_bio() if REQ_HIPRI is cleared.
thanks,
Ming
next prev parent reply other threads:[~2020-10-16 12:52 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-16 9:18 [PATCH v3 0/2] block, iomap: disable iopoll for split bio Jeffle Xu
2020-10-16 9:18 ` [PATCH v3 1/2] block: " Jeffle Xu
2020-10-16 12:51 ` Ming Lei [this message]
2020-10-16 9:18 ` [PATCH v3 2/2] block,iomap: disable iopoll when split needed Jeffle Xu
2020-10-16 10:26 ` Ming Lei
2020-10-16 11:02 ` JeffleXu
2020-10-16 12:39 ` Ming Lei
2020-10-16 13:30 ` JeffleXu
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