Re: [PATCH] blk-mq: fix blk_mq_hw_ctx active request accounting

[Hannes Reinecke <hare@suse.de>]

On 5/14/23 16:53, Tian Lan wrote:
> From: Tian Lan <tian.lan@twosigma.com>
> 
> The nr_active counter continues to increase over time which causes the
> blk_mq_get_tag to hang until the thread is rescheduled to a different
> core despite there are still tags available.
> 
> kernel-stack
> 
>    INFO: task inboundIOReacto:3014879 blocked for more than 2 seconds
>    Not tainted 6.1.15-amd64 #1 Debian 6.1.15~debian11
>    "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>    task:inboundIORe state:D stack:0  pid:3014879 ppid:4557 flags:0x00000000
>      Call Trace:
>      <TASK>
>      __schedule+0x351/0xa20
>      scheduler+0x5d/0xe0
>      io_schedule+0x42/0x70
>      blk_mq_get_tag+0x11a/0x2a0
>      ? dequeue_task_stop+0x70/0x70
>      __blk_mq_alloc_requests+0x191/0x2e0
> 
> kprobe output showing RQF_MQ_INFLIGHT bit is not cleared before
> __blk_mq_free_request being called.
> 
>    320    320  kworker/29:1H __blk_mq_free_request rq_flags 0x220c0
>           b'__blk_mq_free_request+0x1 [kernel]'
>           b'bt_iter+0x50 [kernel]'
>           b'blk_mq_queue_tag_busy_iter+0x318 [kernel]'
>           b'blk_mq_timeout_work+0x7c [kernel]'
>           b'process_one_work+0x1c4 [kernel]'
>           b'worker_thread+0x4d [kernel]'
>           b'kthread+0xe6 [kernel]'
>           b'ret_from_fork+0x1f [kernel]'
> 
> The issue is caused by the difference between blk_mq_free_request() and
> blk_mq_end_request_batch() wrt. when to call __blk_mq_dec_active_requests().
> The former does it before calling req_ref_put_and_test(), and the latter
> decreases the active request after req_ref_put_and_test().
> 
> - Fixes: f794f3351f26 ("block: add support for blk_mq_end_request_batch()")
> 
> Signed-off-by: Tian Lan <tian.lan@twosigma.com>
> Reviewed-by: Ming Lei <ming.lei@redhat.com>
> ---
>   block/blk-mq.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Ivo Totev, Andrew
Myers, Andrew McDonald, Martje Boudien Moerman