dd hangs when reading large partitions

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Hello,

I'm running into an issue which I don't know how to debug.
So I'm open to ideas and suggestions :-)

On my arm64 board, I have enabled Universal Flash Storage support.

I wanted to benchmark read performance, and noticed that the system
locks up when I read partitions larger than 3.5 GB, unless I tell
dd to use direct IO:

*** WITH O_DIRECT ***
# dd if=/dev/sda of=/dev/null bs=1M iflag=direct status=progress
57892929536 bytes (58 GB, 54 GiB) copied, 697.006 s, 83.1 MB/s 
55256+0 records in
55256+0 records out
57940115456 bytes (58 GB, 54 GiB) copied, 697.575 s, 83.1 MB/s

*** WITHOUT O_DIRECT ***
# dd if=/dev/sda of=/dev/null bs=1M status=progress
3853516800 bytes (3.9 GB, 3.6 GiB) copied, 49.0002 s, 78.6 MB/s


rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
rcu:    1-...0: (8242 ticks this GP) idle=106/1/0x4000000000000000 softirq=168/171 fqs=2626 
rcu:    6-...0: (99 GPs behind) idle=ec2/1/0x4000000000000000 softirq=71/71 fqs=2626 
rcu:    (detected by 7, t=5254 jiffies, g=-275, q=2)
Task dump for CPU 1:
kworker/1:1H    R  running task        0   675      2 0x0000002a
Workqueue: kblockd blk_mq_run_work_fn
Call trace:
 __switch_to+0x168/0x1d0
 0xffffffc0f6efbbc8
 blk_mq_run_work_fn+0x28/0x40
 process_one_work+0x208/0x470
 worker_thread+0x48/0x460
 kthread+0x128/0x130
 ret_from_fork+0x10/0x1c
Task dump for CPU 6:
kthreadd        R  running task        0     2      0 0x0000002a
Call trace:
 __switch_to+0x168/0x1d0
 0x5b36396f4e7d4000


rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
rcu:    1-...0: (8242 ticks this GP) idle=106/1/0x4000000000000000 softirq=168/171 fqs=10500 
rcu:    6-...0: (99 GPs behind) idle=ec2/1/0x4000000000000000 softirq=71/71 fqs=10500 
rcu:    (detected by 7, t=21009 jiffies, g=-275, q=2)
Task dump for CPU 1:
kworker/1:1H    R  running task        0   675      2 0x0000002a
Workqueue: kblockd blk_mq_run_work_fn
Call trace:
 __switch_to+0x168/0x1d0
 0xffffffc0f6efbbc8
 blk_mq_run_work_fn+0x28/0x40
 process_one_work+0x208/0x470
 worker_thread+0x48/0x460
 kthread+0x128/0x130
 ret_from_fork+0x10/0x1c
Task dump for CPU 6:
kthreadd        R  running task        0     2      0 0x0000002a
Call trace:
 __switch_to+0x168/0x1d0
 0x5b36396f4e7d4000


The system always hangs around the 3.6 GiB mark, wherever I start from.
How can I debug this issue?

Regards.



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