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

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Message-ID: <ZfN0wY41pU5UjP8T@boqun-archlinux>
Date: Thu, 14 Mar 2024 15:05:53 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Florian Fainelli <f.fainelli@...il.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	"Russell King (Oracle)" <linux@...linux.org.uk>,
	Joel Fernandes <joel@...lfernandes.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, kernel-team@...a.com,
	paulmck@...nel.org, mingo@...nel.org, rcu@...r.kernel.org,
	neeraj.upadhyay@....com, urezki@...il.com,
	qiang.zhang1211@...il.com, bigeasy@...utronix.de,
	chenzhongjin@...wei.com, yangjihong1@...wei.com,
	rostedt@...dmis.org, Justin Chen <justin.chen@...adcom.com>
Subject: Re: Unexplained long boot delays [Was Re: [GIT PULL] RCU changes for
 v6.9]

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.