Hi,

Did anything change recently with respect to RCU stall detection or
sock_def_readable()? My tests have been failing pretty much in the same
place every other time I run it. It's getting rather annoying. The test
runs:

  trace-cmd record -p function -e syscalls ./hackbench 50

(enables syscall trace events along with function tracing and records to a file)

On a VM with the attached config (it has LOCKDEP enabled), and every
other run the following triggers:

[  219.564147] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
[  219.566376] rcu:     Tasks blocked on level-0 rcu_node (CPUs 0-3): P3152/1:b..l
[  219.568975] rcu:     (detected by 0, t=6502 jiffies, g=30165, q=175 ncpus=4)
[  219.571215] task:hackbench_64    state:R  running task     stack:0     pid:3152  tgid:3152  ppid:3145   task_flags:0x400000 flags:0x00080000
[  219.575470] Call Trace:
[  219.576432]  <TASK>
[  219.577396]  __schedule+0x4ac/0x12f0
[  219.578731]  preempt_schedule_common+0x26/0xe0
[  219.580319]  ? preempt_schedule_thunk+0x16/0x30
[  219.581956]  preempt_schedule_thunk+0x16/0x30
[  219.583550]  ? _raw_spin_unlock_irqrestore+0x39/0x70
[  219.585328]  _raw_spin_unlock_irqrestore+0x5d/0x70
[  219.587039]  sock_def_readable+0x9c/0x2b0
[  219.588509]  unix_stream_sendmsg+0x2d7/0x710
[  219.590104]  sock_write_iter+0x185/0x190
[  219.591591]  vfs_write+0x457/0x5b0
[  219.592900]  ksys_write+0xc8/0xf0
[  219.594173]  do_syscall_64+0x117/0x1660
[  219.595586]  ? irqentry_exit+0xd9/0x690
[  219.596992]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[  219.598809] RIP: 0033:0x7f30e2eb0190
[  219.600239] RSP: 002b:00007ffd4e5f9368 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
[  219.602828] RAX: ffffffffffffffda RBX: 00007ffd4e5f94f8 RCX: 00007f30e2eb0190
[  219.605254] RDX: 0000000000000001 RSI: 00007ffd4e5f938f RDI: 0000000000000006
[  219.607598] RBP: 00007ffd4e5f93e0 R08: 00057bcf00000000 R09: 0000000000000000
[  219.609969] R10: 00007f30e2dd14d0 R11: 0000000000000202 R12: 0000000000000000
[  219.612453] R13: 00007ffd4e5f9510 R14: 000055c31b856dd8 R15: 00007f30e2fdb020
[  219.614933]  </TASK>

Always with the exact same stacktrace!

Note, the machine is fine. It's not locked up at all. It appears to be
something that might be blocking RCU for a little longer than RCU would
like.

I'm not sure if RCU changed its stall detection that makes it more
sensitive or if the sock_def_readable() did something different that
causes more contention or something, but this has started with the
7.0-rc1.

Any ideas?

-- Steve