Re: [PATCH v6 1/4] rcu: Make call_rcu() lazy to save power

[Joel Fernandes]

On Tue, Sep 27, 2022 at 04:08:18PM +0200, Uladzislau Rezki wrote:
> On Mon, Sep 26, 2022 at 08:54:27PM +0000, Joel Fernandes wrote:
> > Hi Vlad,
> > 
> > On Mon, Sep 26, 2022 at 09:39:23PM +0200, Uladzislau Rezki wrote:
> > [...]
> > > > > On my KVM machine the boot time is affected:
> > > > > 
> > > > > <snip>
> > > > > [    2.273406] e1000 0000:00:03.0 eth0: Intel(R) PRO/1000 Network Connection
> > > > > [   11.945283] e1000 0000:00:03.0 ens3: renamed from eth0
> > > > > [   22.165198] sr 1:0:0:0: [sr0] scsi3-mmc drive: 4x/4x cd/rw xa/form2 tray
> > > > > [   22.165206] cdrom: Uniform CD-ROM driver Revision: 3.20
> > > > > [   32.406981] sr 1:0:0:0: Attached scsi CD-ROM sr0
> > > > > [  104.115418] process '/usr/bin/fstype' started with executable stack
> > > > > [  104.170142] EXT4-fs (sda1): mounted filesystem with ordered data mode. Quota mode: none.
> > > > > [  104.340125] systemd[1]: systemd 241 running in system mode. (+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD -IDN2 +IDN -PCRE2 default-hierarchy=hybrid)
> > > > > [  104.340193] systemd[1]: Detected virtualization kvm.
> > > > > [  104.340196] systemd[1]: Detected architecture x86-64.
> > > > > [  104.359032] systemd[1]: Set hostname to <pc638>.
> > > > > [  105.740109] random: crng init done
> > > > > [  105.741267] systemd[1]: Reached target Remote File Systems.
> > > > > <snip>
> > > > > 
> > > > > 2 - 11 and second delay is between 32 - 104. So there are still users which must
> > > > > be waiting for "RCU" in a sync way.
> > > > 
> > > > I was wondering if you can compare boot logs and see which timestamp does the
> > > > slow down start from. That way, we can narrow down the callback. Also another
> > > > idea is, add "trace_event=rcu:rcu_callback,rcu:rcu_invoke_callback
> > > > ftrace_dump_on_oops" to the boot params, and then manually call
> > > > "tracing_off(); panic();" from the code at the first printk that seems off in
> > > > your comparison of good vs bad. For example, if "crng init done" timestamp is
> > > > off, put the "tracing_off(); panic();" there. Then grab the serial console
> > > > output to see what were the last callbacks that was queued/invoked.
> > 
> > Would you be willing to try these steps? Meanwhile I will try on my side as
> > well with the .config you sent me in another email.
> >
> Not exactly those steps. But see below:
> 
> <snip>
> [    2.291319] e1000 0000:00:03.0 eth0: Intel(R) PRO/1000 Network Connection
> [   17.302946] e1000 0000:00:03.0 ens3: renamed from eth0
> <snip>
> 
> 15 seconds delay between two prints. I have logged all call_rcu() users
> between those two prints:
> 
> <snip>
> # tracer: nop
> #
> # entries-in-buffer/entries-written: 166/166   #P:64
> #
> #                                _-----=> irqs-off/BH-disabled
> #                               / _----=> need-resched
> #                              | / _---=> hardirq/softirq
> #                              || / _--=> preempt-depth
> #                              ||| / _-=> migrate-disable
> #                              |||| /     delay
> #           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
> #              | |         |   |||||     |         |
>    systemd-udevd-669     [002] .....     2.338739: e1000_probe: Intel(R) PRO/1000 Network Connection
>    systemd-udevd-665     [061] .....     2.338952: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....     2.338962: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>    systemd-udevd-665     [061] .....     2.338965: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-645     [053] .....     2.338968: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>    systemd-udevd-665     [061] .....     2.338987: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>    systemd-udevd-645     [053] .....     2.338989: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-645     [053] .....     2.338999: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>    systemd-udevd-645     [053] .....     2.339002: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-645     [053] .....     2.339024: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>      kworker/0:3-546     [000] d..1.     6.329516: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70effe40a0
>          rcuop/0-17      [000] b....     6.337320: __call_rcu_common: -> 0x0: exit_creds+0x63/0x70 <- 0x0
>     kworker/38:1-744     [038] d..1.     6.841479: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f97e40a0
>            <...>-739     [035] d..1.     6.841479: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f8be40a0
>            <...>-732     [021] d..1.     6.841486: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f53e40a0
>     kworker/36:1-740     [036] d..1.     6.841487: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f8fe40a0
>         rcuop/21-170     [023] b....     6.849276: __call_rcu_common: -> 0x0: exit_creds+0x63/0x70 <- 0x0
>         rcuop/38-291     [052] b....     6.849950: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/38-291     [052] b....     6.849957: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>      kworker/5:1-712     [005] d..1.     7.097392: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f13e40a0
>     kworker/19:1-727     [019] d..1.     7.097392: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f4be40a0
>            <...>-719     [007] d..1.     7.097392: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f1be40a0
>     kworker/13:1-721     [013] d..1.     7.097392: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f33e40a0
>     kworker/52:1-756     [052] d..1.     7.097395: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70fcfe40a0
>     kworker/29:1-611     [029] d..1.     7.097395: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f73e40a0
>            <...>-754     [049] d..1.     7.097405: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70fc3e40a0
>     kworker/12:1-726     [012] d..1.     7.097405: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f2fe40a0
>     kworker/53:1-710     [053] d..1.     7.097405: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70fd3e40a0
>            <...>-762     [061] d..1.     7.097405: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70ff3e40a0
>            <...>-757     [054] d..1.     7.097408: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70fd7e40a0
>     kworker/25:1-537     [025] d..1.     7.097408: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f63e40a0
>            <...>-714     [004] d..1.     7.097408: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f0fe40a0
>            <...>-749     [044] d..1.     7.097413: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70fafe40a0
>     kworker/51:1-755     [051] d..1.     7.097413: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70fcbe40a0
>            <...>-764     [063] d..1.     7.097415: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70ffbe40a0
>            <...>-753     [045] d..1.     7.097416: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70fb3e40a0
>     kworker/43:1-748     [043] d..1.     7.097416: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70fabe40a0
>     kworker/41:1-747     [041] d..1.     7.097416: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70fa3e40a0
>     kworker/57:1-760     [057] d..1.     7.097416: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70fe3e40a0
>            <...>-720     [008] d..1.     7.097418: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f1fe40a0
>     kworker/58:1-759     [058] d..1.     7.097421: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70fe7e40a0
>     kworker/16:1-728     [016] d..1.     7.097424: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f3fe40a0
>            <...>-722     [010] d..1.     7.097427: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f27e40a0
>     kworker/22:1-733     [022] d..1.     7.097432: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f57e40a0
>            <...>-731     [026] d..1.     7.097432: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f67e40a0
>            <...>-752     [048] d..1.     7.097437: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70fbfe40a0
>     kworker/18:0-147     [018] d..1.     7.097437: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f47e40a0
>     kworker/39:1-745     [039] d..1.     7.097437: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f9be40a0
>            <...>-716     [003] d..1.     7.097437: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70f0be40a0
>            <...>-703     [050] d..1.     7.097437: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70fc7e40a0
>     kworker/42:1-746     [042] d..1.     7.097444: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70fa7e40a0
>         rcuop/13-113     [013] b....     7.105592: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/13-113     [013] b....     7.105595: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/10-92      [040] b....     7.105608: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/10-92      [040] b....     7.105610: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/16-135     [023] b....     7.105613: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>          rcuop/8-78      [039] b....     7.105636: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>          rcuop/8-78      [039] b....     7.105640: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/12-106     [040] b....     7.105651: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/12-106     [040] b....     7.105652: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/19-156     [000] b....     7.105727: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/19-156     [000] b....     7.105730: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>          rcuop/5-56      [058] b....     7.105808: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>          rcuop/5-56      [058] b....     7.105814: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/20-163     [023] b....    17.345648: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/20-163     [023] b....    17.345655: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/14-120     [013] b....    17.345675: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/14-120     [013] b....    17.345681: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>          rcuop/6-63      [013] b....    17.345714: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>          rcuop/6-63      [013] b....    17.345715: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>          rcuop/9-85      [000] b....    17.345753: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>          rcuop/9-85      [000] b....    17.345758: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/17-142     [000] b....    17.345775: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/17-142     [000] b....    17.345776: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/17-142     [000] b....    17.345777: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/11-99      [000] b....    17.345810: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/11-99      [000] b....    17.345811: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/15-127     [013] b....    17.345832: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/15-127     [013] b....    17.345834: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>          rcuop/1-28      [000] b....    17.345834: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>          rcuop/1-28      [000] b....    17.345835: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/15-127     [013] b....    17.345835: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>         rcuop/15-127     [013] b....    17.345837: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>    systemd-udevd-633     [035] .....    17.346591: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>    systemd-udevd-633     [035] .....    17.346609: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>    systemd-udevd-633     [035] .....    17.346659: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>    systemd-udevd-633     [035] .....    17.346666: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>    systemd-udevd-669     [002] .....    17.347573: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>      kworker/2:2-769     [002] .....    17.347659: __call_rcu_common: -> 0x0: __wait_rcu_gp+0xff/0x120 <- 0x0
>    systemd-udevd-675     [012] .....    17.347981: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-675     [012] .....    17.348002: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-675     [012] .....    17.348037: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348098: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348117: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>    systemd-udevd-665     [061] .....    17.348120: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348156: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348166: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348176: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-642     [050] .....    17.348179: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348186: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348197: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348200: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348231: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348240: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348250: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348259: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348262: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348305: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348317: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348332: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348336: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348394: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348403: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348406: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-645     [053] .....    17.348503: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-645     [053] .....    17.348531: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>    systemd-udevd-645     [053] .....    17.348535: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348536: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-645     [053] .....    17.348563: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>    systemd-udevd-665     [061] .....    17.348575: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348628: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348704: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348828: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348884: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348904: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348954: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348983: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.348993: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349002: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349014: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349024: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349026: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349119: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349182: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349243: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349430: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349462: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349472: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349483: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349486: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349583: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349632: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349666: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349699: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-645     [053] .....    17.349727: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349733: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-645     [053] .....    17.349739: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>    systemd-udevd-645     [053] .....    17.349742: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349765: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-645     [053] .....    17.349766: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>    systemd-udevd-642     [050] .....    17.349780: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>    systemd-udevd-665     [061] .....    17.349800: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-642     [050] .....    17.349815: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-642     [050] .....    17.349829: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>    systemd-udevd-642     [050] .....    17.349832: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349834: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-675     [012] .....    17.349835: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-675     [012] .....    17.349853: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-642     [050] .....    17.349861: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>    systemd-udevd-675     [012] .....    17.349873: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.349879: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.350007: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.350011: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.350080: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.350175: __call_rcu_common: -> ____fput+0x0/0x10: task_work_run+0x5c/0x90 <- 0x0
>    systemd-udevd-665     [061] .....    17.350362: dev_change_name: --> renamed from eth0
> <snip>
> 
> First delay:
> 
> <snip>
> systemd-udevd-645     [053] .....     2.339024: __call_rcu_common: -> 0x0: __dentry_kill+0x140/0x180 <- 0x2
>   kworker/0:3-546     [000] d..1.     6.329516: __call_rcu_common: -> 0x0: queue_rcu_work+0x2b/0x40 <- 0xffff8c70effe40a0
> <snip>
> 
> __dentry_kill() function and after 4 seconds there is another one queue_rcu_work().
> I have checked the __dentry_kill() if it can do any sync talk with RCU but from the
> first glance i do not see anything critical. But more attention is required.

Can you log rcu_barrier() as well? It could be that the print is just a side
effect of something else that is not being printed.

If you follow the steps I shared, we should be able to get a full log of RCU
traces.

> Second delay:
> 
> <snip>
>   rcuop/5-56      [058] b....     7.105814: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
>  rcuop/20-163     [023] b....    17.345648: __call_rcu_common: -> 0x0: file_free_rcu+0x32/0x50 <- 0x0
> <snip>
> 
> 10 seconds. But please note that it could be that there were not any callbacks queued
> during 10 seconds.

True, there are things other than call_rcu() like rcu_barrier() that can
slow down the boot path. We fixed that and it is not an issue over here, but
I am not sure if you are triggering it some how.

thanks,

 - Joel