On Fri, Oct 28, 2022 at 10:28:27PM +1100, Michael Ellerman wrote:
> "Paul E. McKenney" <paulmck@xxxxxxxxxx> 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 :)
I know that feeling... :-/
And thank you for your answers!
And one more that I should have asked to begin with...
Is it possible for the system booting on CPU N to not have CPU 0 in the
cpu_possible_mask? As in is it possible for CPU 0's per-CPU variables
to be unmapped?
Thanx, Paul
> >> ...
> >> [ 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@xxxxxxxxx>
> >
> > 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
