Re: Unexplained long boot delays [Was Re: [GIT PULL] RCU changes for v6.9]

Boqun Feng
2024-03-14T22:06:03Z

On Thu, Mar 14, 2024 at 10:21:21PM +0100, Thomas Gleixner wrote:
> On Thu, Mar 14 2024 at 21:45, Thomas Gleixner wrote:
> > On Thu, Mar 14 2024 at 12:09, Florian Fainelli wrote:
> >> https://gist.github.com/ffainelli/cb562c1a60ef8e0e69e7d42143c48e8f
> >>
> >> this one is does include the tmigr events. Thanks!
> >
> > You need 8ca1836769d758e4fbf5851bb81e181c52193f5d too.
>
> So from the above trace it's clear where it goes south:
>
> [ 236.318158] <idle>-0 3..s.. 2928466us : tmigr_handle_remote: group=aecb05cb lvl=0
> [ 236.326526] <idle>-0 3d.s.. 2928467us : tmigr_handle_remote_cpu: cpu=0 parent=aecb05cb wakeup=9223372036854775807
> [ 236.357809] <idle>-0 3d.s.. 2928469us : tmigr_update_events: child=00000000 group=aecb05cb group_lvl=0 child_active=0 group_active=8 nextevt=3103000000 next_expiry=2934000000 child_evt_expiry=0 child_evtcpu=0
>
> [ 236.377222] <idle>-0 0dn... 2928471us : tmigr_cpu_active: cpu=0 parent=aecb05cb wakeup=9223372036854775807
> [ 236.387765] <idle>-0 0dn... 2928471us : tmigr_group_set_cpu_active: group=aecb05cb lvl=0 numa=0 active=9 migrator=8 parent=00000000 childmask=1
>
> [ 236.401526] <idle>-0 0d.... 2928477us : tmigr_update_events: child=00000000 group=aecb05cb group_lvl=0 child_active=0 group_active=8 nextevt=3103000000 next_expiry=2934000000 child_evt_expiry=0 child_evtcpu=0
> [ 236.420940] <idle>-0 0d.... 2928478us : tmigr_group_set_cpu_inactive: group=aecb05cb lvl=0 numa=0 active=8 migrator=8 parent=00000000 childmask=1
> [ 236.434874] <idle>-0 0d.... 2928478us : tmigr_cpu_idle: cpu=0 parent=aecb05cb nextevt=3103000000 wakeup=9223372036854775807
>
> [ 236.446896] <idle>-0 3d.... 2929469us : tmigr_group_set_cpu_inactive: group=aecb05cb lvl=0 numa=0 active=0 migrator=ff parent=00000000 childmask=8
> [ 236.460916] <idle>-0 3d.... 2929470us : tmigr_cpu_idle: cpu=3 parent=aecb05cb nextevt=9223372036854775807 wakeup=9223372036854775807
> [ 236.473721] <idle>-0 3d.... 2934471us : tmigr_cpu_new_timer_idle: cpu=3 parent=aecb05cb nextevt=9223372036854775807 wakeup=9223372036854775807
>
> CPU3 is the last active CPU and goes idle. So it should take care of the
> pending events, but it does not.
>

I notice CPU3 didn't have its own non-deferrable timer queued (local or
global), so could the following happen?

timer_base_try_to_set_idle():
__get_next_timer_interrupt():
fetch_next_timer_interrupt():
// nextevt_local == nextevt_global == basej + NEXT_TIMER_MAX_DELTA
// tevt->local == tevt->gloabl = KTIME_MAX
timer_use_tmigr():
tmigr_cpu_deactivate():
__tmigr_cpu_deactivate():
// tmc->cpuevt.ignore untouched still == true
walk_groups(&tmigr_inactive_up, ...):
tmigr_inactive_up():
data->remote = true;
tmigr_update_events():
if (child) { // child is NULL
...
} else {
first_childevt = evt = data->evt;

if (evt->ignore && !remote)
return true; // no remote tick is picked.
...
}

Regards,
Boqun

> This is the next trace entry where CPU0 magically comes back to life.
>
> [ 236.487393] <idle>-0 0d.s.. 162001359us : timer_cancel: timer=8c725d84
>
> 8ca1836769d758e4fbf5851bb81e181c52193f5d is related, but dos not fully
> explain the fail. I haven't yet spotted where this goes into lala land.