On 8/29/21 16:02, Damien Le Moal wrote:
On 2021/08/27 23:34, Bart Van Assche wrote:
On 8/26/21 9:49 PM, Damien Le Moal wrote:
So the mq-deadline priority patch reduces performance by nearly half at high QD.
(*) Note: in all cases using the mq-deadline scheduler, for the first run at
QD=1, I get this splat 100% of the time.
[ 95.173889] watchdog: BUG: soft lockup - CPU#0 stuck for 26s! [kworker/0:1H:757]
[ 95.292994] CPU: 0 PID: 757 Comm: kworker/0:1H Not tainted 5.14.0-rc7+ #1334
[ 95.307504] Workqueue: kblockd blk_mq_run_work_fn
[ 95.312243] RIP: 0010:_raw_spin_unlock_irqrestore+0x35/0x40
[ 95.415904] Call Trace:
[ 95.418373] try_to_wake_up+0x268/0x7c0
[ 95.422238] blk_update_request+0x25b/0x420
[ 95.426452] blk_mq_end_request+0x1c/0x120
[ 95.430576] null_handle_cmd+0x12d/0x270 [null_blk]
[ 95.435485] blk_mq_dispatch_rq_list+0x13c/0x7f0
[ 95.443826] __blk_mq_do_dispatch_sched+0xb5/0x2f0
[ 95.448653] __blk_mq_sched_dispatch_requests+0xf4/0x140
[ 95.453998] blk_mq_sched_dispatch_requests+0x30/0x60
[ 95.459083] __blk_mq_run_hw_queue+0x49/0x90
[ 95.463377] process_one_work+0x26c/0x570
[ 95.467421] worker_thread+0x55/0x3c0
[ 95.475313] kthread+0x140/0x160
[ 95.482774] ret_from_fork+0x1f/0x30
I don't see any function names in the above call stack that refer to the
mq-deadline scheduler? Did I perhaps overlook something? Anyway, if you can
tell me how to reproduce this (kernel commit + kernel config) I will take a
look.
Indeed, the stack trace does not show any mq-deadline function. But the
workqueue is stuck on _raw_spin_unlock_irqrestore() in the blk_mq_run_work_fn()
function. I suspect that the spinlock is dd->lock, so the CPU may be stuck on
entry to mq-deadline dispatch or finish request methods. Not entirely sure.
I got this splat with 5.4.0-rc7 (Linus tag patch) with the attached config.
Hi Damien,
Thank you for having shared the kernel configuration used in your test.
So far I have not yet been able to reproduce the above call trace in a
VM. Could the above call trace have been triggered by the mpt3sas driver
instead of the mq-deadline I/O scheduler?
Thanks,
Bart.
