On 10/06/2014 06:27 AM, Bart Van Assche wrote:
> Ensure that bt_clear_tag() increments bs->wait_cnt if one or more
> threads are waiting for a tag. Remove a superfluous
> waitqueue_active() call from blk_mq_tag_wakeup_all(). This patch
> avoids that bt_get() hangs as follows if the number of hardware
> contexts is below the number of CPU threads:
>
> INFO: task fio:6739 blocked for more than 120 seconds.
> Not tainted 3.17.0-rc7+ #1
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> fio D ffff88085fcd2740 0 6739 6688 0x00000000
> ffff8807fb96f830 0000000000000086 ffff8808350eb000 ffff8807fb96ffd8
> 0000000000012740 0000000000012740 ffff88085b885000 ffff8808350eb000
> ffff88085fcd2fe0 ffff8807fb96f920 ffff88080061ddc8 ffffe8ffffcc4600
> Call Trace:
> [<ffffffff8142f52d>] io_schedule+0x9d/0x130
> [<ffffffff812016bf>] bt_get+0xef/0x180
> [<ffffffff8107f440>] ? prepare_to_wait_event+0x110/0x110
> [<ffffffff81201a0f>] blk_mq_get_tag+0x9f/0xd0
> [<ffffffff811fdedb>] __blk_mq_alloc_request+0x1b/0x200
> [<ffffffff811ff3bc>] blk_mq_map_request+0x15c/0x1b0
> [<ffffffff8120079e>] blk_mq_make_request+0x6e/0x270
> [<ffffffff8110a99f>] ? mempool_alloc+0x4f/0x130
> [<ffffffff811f3af0>] generic_make_request+0xc0/0x110
> [<ffffffff811f3bab>] submit_bio+0x6b/0x140
> [<ffffffff81110e4b>] ? set_page_dirty_lock+0x2b/0x40
> [<ffffffff811eea57>] ? bio_set_pages_dirty+0x47/0x60
> [<ffffffff811907d0>] do_blockdev_direct_IO+0x2080/0x3220
> [<ffffffff8118bac0>] ? I_BDEV+0x10/0x10
> [<ffffffff811919bc>] __blockdev_direct_IO+0x4c/0x50
> [<ffffffff8118bac0>] ? I_BDEV+0x10/0x10
> [<ffffffff8118c1de>] blkdev_direct_IO+0x4e/0x50
> [<ffffffff8118bac0>] ? I_BDEV+0x10/0x10
> [<ffffffff81109a13>] generic_file_read_iter+0x513/0x5e0
> [<ffffffff8119d8a7>] ? kiocb_free+0x37/0x40
> [<ffffffff8118c660>] ? ioctl_by_bdev+0x40/0x40
> [<ffffffff8118c697>] blkdev_read_iter+0x37/0x40
> [<ffffffff8119e6b4>] aio_run_iocb+0x1e4/0x3c0
> [<ffffffff8114dfa6>] ? kmem_cache_alloc+0xd6/0x3d0
> [<ffffffff8114e045>] ? kmem_cache_alloc+0x175/0x3d0
> [<ffffffff8119f5cc>] do_io_submit+0x11c/0x490
> [<ffffffff8119f950>] SyS_io_submit+0x10/0x20
> [<ffffffff81432fd2>] system_call_fastpath+0x16/0x1b
>
> Signed-off-by: Bart Van Assche <bvanass...@acm.org>
> Cc: Christoph Hellwig <h...@lst.de>
> Cc: Ming Lei <ming....@canonical.com>
> Cc: Robert Elliott <elli...@hp.com>
> Cc: <sta...@vger.kernel.org>
> ---
> block/blk-mq-tag.c | 31 +++++++++++++++----------------
> 1 file changed, 15 insertions(+), 16 deletions(-)
>
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index b5088f0..08d3b1c 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -80,8 +80,7 @@ static void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags)
> for (i = 0; i < BT_WAIT_QUEUES; i++) {
> struct bt_wait_state *bs = &bt->bs[wake_index];
>
> - if (waitqueue_active(&bs->wait))
> - wake_up(&bs->wait);
> + wake_up(&bs->wait);
>
> wake_index = bt_index_inc(wake_index);
> }
> @@ -346,20 +345,20 @@ static void bt_clear_tag(struct blk_mq_bitmap_tags *bt,
> unsigned int tag)
> */
> clear_bit_unlock(TAG_TO_BIT(bt, tag), &bt->map[index].word);
>
> - bs = bt_wake_ptr(bt);
> - if (!bs)
> - return;
> -
> - wait_cnt = atomic_dec_return(&bs->wait_cnt);
> - if (wait_cnt == 0) {
> -wake:
> - atomic_add(bt->wake_cnt, &bs->wait_cnt);
> - bt_index_atomic_inc(&bt->wake_index);
> - wake_up(&bs->wait);
> - } else if (wait_cnt < 0) {
> - wait_cnt = atomic_inc_return(&bs->wait_cnt);
> - if (!wait_cnt)
> - goto wake;
> + for (;;) {
> + bs = bt_wake_ptr(bt);
> + if (!bs)
> + return;
> +
> + wait_cnt = atomic_dec_return(&bs->wait_cnt);
> + if (unlikely(wait_cnt < 0))
> + wait_cnt = atomic_inc_return(&bs->wait_cnt);
> + if (wait_cnt == 0) {
> + atomic_add(bt->wake_cnt, &bs->wait_cnt);
> + bt_index_atomic_inc(&bt->wake_index);
> + wake_up(&bs->wait);
> + return;
> + }
> }
> }
I've been able to reproduce this this morning, and your patch does seem
to fix it. The inc/add logic is making my head spin a bit. And we now
end up banging a lot more on the waitqueue lock through
prepare_to_wait(), so there's a substantial performance regression to go
with the change.
I'll fiddle with this a bit and see if we can't retain existing
performance properties under tag contention, while still fixing the hang.
--
Jens Axboe
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