linux-kernel - Re: [PATCH v3] rcu: ftrace: avoid tracing a few functions executed in stop machine

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Date:   Tue, 26 Apr 2022 09:15:11 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Patrick Wang <patrick.wang.shcn@...il.com>, frederic@...nel.org,
        quic_neeraju@...cinc.com, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, joel@...lfernandes.org,
        jiangshanlai@...il.com, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] rcu: ftrace: avoid tracing a few functions executed
 in stop machine

On Tue, Apr 26, 2022 at 11:22:50AM -0400, Steven Rostedt wrote:
> On Tue, 26 Apr 2022 18:55:11 +0800
> Patrick Wang <patrick.wang.shcn@...il.com> wrote:
> 
> > Because of the change of stop machine implementation, there are functions
> > being called while waiting currently:
> > 
> > ----------------------------------------------------------------
> > Former stop machine wait loop:
> > do {
> >     cpu_relax(); => macro
> >     ...
> > } while (curstate != STOPMACHINE_EXIT);
> > -----------------------------------------------------------------
> > Current stop machine wait loop:
> > do {
> >     stop_machine_yield(cpumask); => function (notraced)
> >     ...
> >     touch_nmi_watchdog(); => function (notraced, inside calls also notraced)
> >     ...
> >     rcu_momentary_dyntick_idle(); => function (notraced, inside calls traced)
> > } while (curstate != MULTI_STOP_EXIT);
> > ------------------------------------------------------------------
> > 
> > These functions (including the calls inside) should be marked notrace to avoid
> > their codes being updated when they are being called. The calls inside
> > rcu_momentary_dyntick_idle() still remain traced, and will cause crash:
> > 
> >   rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> >   rcu: 	1-...!: (0 ticks this GP) idle=14f/1/0x4000000000000000 softirq=3397/3397 fqs=0
> >   rcu: 	3-...!: (0 ticks this GP) idle=ee9/1/0x4000000000000000 softirq=5168/5168 fqs=0
> >   	(detected by 0, t=8137 jiffies, g=5889, q=2 ncpus=4)
> >   Task dump for CPU 1:
> >   task:migration/1     state:R  running task     stack:    0 pid:   19 ppid:     2 flags:0x00000000
> >   Stopper: multi_cpu_stop+0x0/0x18c <- stop_machine_cpuslocked+0x128/0x174
> >   Call Trace:
> >   Task dump for CPU 3:
> >   task:migration/3     state:R  running task     stack:    0 pid:   29 ppid:     2 flags:0x00000000
> >   Stopper: multi_cpu_stop+0x0/0x18c <- stop_machine_cpuslocked+0x128/0x174
> >   Call Trace:
> >   rcu: rcu_preempt kthread timer wakeup didn't happen for 8136 jiffies! g5889 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402
> >   rcu: 	Possible timer handling issue on cpu=2 timer-softirq=594
> >   rcu: rcu_preempt kthread starved for 8137 jiffies! g5889 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402 ->cpu=2
> >   rcu: 	Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
> >   rcu: RCU grace-period kthread stack dump:
> >   task:rcu_preempt     state:I stack:    0 pid:   14 ppid:     2 flags:0x00000000
> >   Call Trace:
> >     schedule+0x56/0xc2
> >     schedule_timeout+0x82/0x184
> >     rcu_gp_fqs_loop+0x19a/0x318
> >     rcu_gp_kthread+0x11a/0x140
> >     kthread+0xee/0x118
> >     ret_from_exception+0x0/0x14
> >   rcu: Stack dump where RCU GP kthread last ran:
> >   Task dump for CPU 2:
> >   task:migration/2     state:R  running task     stack:    0 pid:   24 ppid:     2 flags:0x00000000
> >   Stopper: multi_cpu_stop+0x0/0x18c <- stop_machine_cpuslocked+0x128/0x174
> >   Call Trace:
> > 
> > Mark the calls inside rcu_momentary_dyntick_idle():
> >  rcu_preempt_deferred_qs()
> >  rcu_preempt_need_deferred_qs()
> >  rcu_preempt_deferred_qs_irqrestore()
> > as notrace to prevent this.
> > 
> > Signed-off-by: Patrick Wang <patrick.wang.shcn@...il.com>
> > ---
> > v1->v2:
> >  - Modify log message.
> 
> Acked-by: Steven Rostedt (Google) <rostedt@...dmis.org>

I have reverted Patrick's earlier commit and replaced it with this
new one, including Steve's ack.  Thank you both!

							Thanx, Paul