Re: [PATCH] srcu: Delegate work to the first online cpu if using SRCU_SIZE_SMALL

[Michael Ellerman <mpe@ellerman.id.au>]

"Paul E. McKenney" <paulmck@kernel.org> writes:
> On Wed, Oct 26, 2022 at 11:27:16AM +0800, Pingfan Liu wrote:
>> commit 994f706872e6 ("srcu: Make Tree SRCU able to operate without
>> snp_node array") assumes that cpu 0 is always online, but that is not
>> the truth when using maxcpus=1 in the command line for the kdump kernel.
>> 
>> On a PowerPC, the following kdump kernel hanging is observed:
>
> Adding a few PowerPC folks on CC for their thoughts systems booting with
> some CPU other than CPU 0 as the boot CPU.  Is this intended/supported?

Yes, unfortunately.

It comes as part of kdump, where a random CPU crashes and kexec's into a
new kernel, so we can end up booting the 2nd kernel on any CPU.

I have objections to the distro practice of booting the kdump kernel
with maxcpus=1, but no one has ever listened to me :)

>> ...
>> [    1.740036] systemd[1]: Hostname set to <xyz.com>
>> [  243.686240] INFO: task systemd:1 blocked for more than 122 seconds.
>> [  243.686264]       Not tainted 6.1.0-rc1 #1
>> [  243.686272] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> [  243.686281] task:systemd         state:D stack:0     pid:1     ppid:0      flags:0x00042000
>> [  243.686296] Call Trace:
>> [  243.686301] [c000000016657640] [c000000016657670] 0xc000000016657670 (unreliable)
>> [  243.686317] [c000000016657830] [c00000001001dec0] __switch_to+0x130/0x220
>> [  243.686333] [c000000016657890] [c000000010f607b8] __schedule+0x1f8/0x580
>> [  243.686347] [c000000016657940] [c000000010f60bb4] schedule+0x74/0x140
>> [  243.686361] [c0000000166579b0] [c000000010f699b8] schedule_timeout+0x168/0x1c0
>> [  243.686374] [c000000016657a80] [c000000010f61de8] __wait_for_common+0x148/0x360
>> [  243.686387] [c000000016657b20] [c000000010176bb0] __flush_work.isra.0+0x1c0/0x3d0
>> [  243.686401] [c000000016657bb0] [c0000000105f2768] fsnotify_wait_marks_destroyed+0x28/0x40
>> [  243.686415] [c000000016657bd0] [c0000000105f21b8] fsnotify_destroy_group+0x68/0x160
>> [  243.686428] [c000000016657c40] [c0000000105f6500] inotify_release+0x30/0xa0
>> [  243.686440] [c000000016657cb0] [c0000000105751a8] __fput+0xc8/0x350
>> [  243.686452] [c000000016657d00] [c00000001017d524] task_work_run+0xe4/0x170
>> [  243.686464] [c000000016657d50] [c000000010020e94] do_notify_resume+0x134/0x140
>> [  243.686478] [c000000016657d80] [c00000001002eb18] interrupt_exit_user_prepare_main+0x198/0x270
>> [  243.686493] [c000000016657de0] [c00000001002ec60] syscall_exit_prepare+0x70/0x180
>> [  243.686505] [c000000016657e10] [c00000001000bf7c] system_call_vectored_common+0xfc/0x280
>> [  243.686520] --- interrupt: 3000 at 0x7fffa47d5ba4
>> [  243.686528] NIP:  00007fffa47d5ba4 LR: 0000000000000000 CTR: 0000000000000000
>> [  243.686538] REGS: c000000016657e80 TRAP: 3000   Not tainted  (6.1.0-rc1)
>> [  243.686548] MSR:  800000000000d033 <SF,EE,PR,ME,IR,DR,RI,LE>  CR: 42044440  XER: 00000000
>> [  243.686572] IRQMASK: 0
>> [  243.686572] GPR00: 0000000000000006 00007ffffa606710 00007fffa48e7200 0000000000000000
>> [  243.686572] GPR04: 0000000000000002 000000000000000a 0000000000000000 0000000000000001
>> [  243.686572] GPR08: 000001000c172dd0 0000000000000000 0000000000000000 0000000000000000
>> [  243.686572] GPR12: 0000000000000000 00007fffa4ff4bc0 0000000000000000 0000000000000000
>> [  243.686572] GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>> [  243.686572] GPR20: 0000000132dfdc50 000000000000000e 0000000000189375 0000000000000000
>> [  243.686572] GPR24: 00007ffffa606ae0 0000000000000005 000001000c185490 000001000c172570
>> [  243.686572] GPR28: 000001000c172990 000001000c184850 000001000c172e00 00007fffa4fedd98
>> [  243.686683] NIP [00007fffa47d5ba4] 0x7fffa47d5ba4
>> [  243.686691] LR [0000000000000000] 0x0
>> [  243.686698] --- interrupt: 3000
>> [  243.686708] INFO: task kworker/u16:1:24 blocked for more than 122 seconds.
>> [  243.686717]       Not tainted 6.1.0-rc1 #1
>> [  243.686724] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> [  243.686733] task:kworker/u16:1   state:D stack:0     pid:24    ppid:2      flags:0x00000800
>> [  243.686747] Workqueue: events_unbound fsnotify_mark_destroy_workfn
>> [  243.686758] Call Trace:
>> [  243.686762] [c0000000166736e0] [c00000004fd91000] 0xc00000004fd91000 (unreliable)
>> [  243.686775] [c0000000166738d0] [c00000001001dec0] __switch_to+0x130/0x220
>> [  243.686788] [c000000016673930] [c000000010f607b8] __schedule+0x1f8/0x580
>> [  243.686801] [c0000000166739e0] [c000000010f60bb4] schedule+0x74/0x140
>> [  243.686814] [c000000016673a50] [c000000010f699b8] schedule_timeout+0x168/0x1c0
>> [  243.686827] [c000000016673b20] [c000000010f61de8] __wait_for_common+0x148/0x360
>> [  243.686840] [c000000016673bc0] [c000000010210840] __synchronize_srcu.part.0+0xa0/0xe0
>> [  243.686855] [c000000016673c30] [c0000000105f2c64] fsnotify_mark_destroy_workfn+0xc4/0x1a0
>> [  243.686868] [c000000016673ca0] [c000000010174ea8] process_one_work+0x2a8/0x570
>> [  243.686882] [c000000016673d40] [c000000010175208] worker_thread+0x98/0x5e0
>> [  243.686895] [c000000016673dc0] [c0000000101828d4] kthread+0x124/0x130
>> [  243.686908] [c000000016673e10] [c00000001000cd40] ret_from_kernel_thread+0x5c/0x64
>> [  366.566274] INFO: task systemd:1 blocked for more than 245 seconds.
>> [  366.566298]       Not tainted 6.1.0-rc1 #1
>> [  366.566305] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> [  366.566314] task:systemd         state:D stack:0     pid:1     ppid:0      flags:0x00042000
>> [  366.566329] Call Trace:
>> ...
>> 
>> In that case, note that maxcpus=1 instead of nr_cpus=1 is used in the
>> kernel command line on the PowerPC platform. Consequently, the crash cpu
>> is the only onlined cpu in the kdump kernel, but with its logical id not
>> necessary 0. While SRCU queues a sdp->work on cpu 0, on which no worker
>> thread is created, so sdp->work will be never executed and
>> __synchronize_srcu() can not be completed.
>> 
>> Tackle this issue by queueing sdp->work on the first onlined cpu.
>> 
>> Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
>
> Good catch!!!  New one on me!  ;-)
>
> But a few questions...
>
> 1.	As noted above, is booting without CPU 0 an intentional and
> 	supported feature of PowerPC?  If not, perhaps a better approach
> 	would be to rule out this configuration.
>
> 2.	When booting without CPU 0, is it guaranteed that CPU 0 will
> 	never come online?  If not, then isn't the patch below subject
> 	to failure modes when that happens?

In the general case no it's not guaranteed.

You could boot on CPU N and then later CPU0 could be brought online via
hotplug.

> 3.	More generally, when CPU N is the boot CPU, is it guaranteed
> 	that CPU M, M < N, will never come online?  Same as above on
> 	failure modes.

No.

cheers