Re: [PATCH 1/1] blk-mq: fix blk_mq_hw_ctx active request accounting

[Ming Lei <ming.lei@redhat.com>]

Hello Tian,

On Sat, May 13, 2023 at 03:05:34PM -0400, 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:inboundIOReacto 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.

RQF_MQ_INFLIGHT won't be cleared when the request is freed normally
from blk_mq_free_request().

> 
>   320    320  kworker/29:1H __blk_mq_free_request rq_flags 0x220c0 in-flight 1

RQF_MQ_INFLIGHT/RQF_DONTPREP/RQF_IO_STAT/RQF_STATS is set, and it isn't
a FLUSH request.

>          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]'

If __blk_mq_free_request() is called from timeout, that means this
request has been freed by blk_mq_free_request() already, so __blk_mq_dec_active_requests
should have been run.

However, one case is that __blk_mq_dec_active_requests isn't called in
blk_mq_end_request_batch, so maybe your driver is nvme with multiple
NSs, so can you try the following patch?

diff --git a/block/blk-mq.c b/block/blk-mq.c
index f6dad0886a2f..9c5dd5aa289c 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1062,6 +1062,9 @@ void blk_mq_end_request_batch(struct io_comp_batch *iob)
 		if (iob->need_ts)
 			__blk_mq_end_request_acct(rq, now);
 
+		if (rq->rq_flags & RQF_MQ_INFLIGHT)
+			__blk_mq_dec_active_requests(rq->mq_hctx);
+
 		rq_qos_done(rq->q, rq);
 
 		/*

Thanks, 
Ming