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Date:   Wed, 22 Jun 2022 17:29:35 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com, rostedt@goodmis.org,
        Patrick Wang <patrick.wang.shcn@gmail.com>
Subject: Re: [PATCH rcu 02/12] rcu: Avoid tracing a few functions executed in
 stop machine
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On Wed, Jun 22, 2022 at 09:19:25PM +0530, Neeraj Upadhyay wrote:
> 
> 
> On 6/22/2022 9:05 PM, Paul E. McKenney wrote:
> > On Wed, Jun 22, 2022 at 05:20:35PM +0530, Neeraj Upadhyay wrote:
> > > 
> > > 
> > > On 6/22/2022 3:51 AM, Paul E. McKenney wrote:
> > > > On Tue, Jun 21, 2022 at 11:17:10AM +0530, Neeraj Upadhyay wrote:
> > > > > 
> > > > > 
> > > > > On 6/21/2022 3:50 AM, Paul E. McKenney wrote:
> > > > > > From: Patrick Wang <patrick.wang.shcn@gmail.com>
> > > > > > 
> > > > > > Stop-machine recently started calling additional functions while waiting:
> > > > > > 
> > > > > > ----------------------------------------------------------------
> > > > > > 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 (and the functions that they call) must be marked
> > > > > > notrace to prevent them from being updated while they are executing.
> > > > > > The consequences of failing to mark these functions can be severe:
> > > > > > 
> > > > > >      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:
> > > > > > 
> > > > > > This commit therefore marks these functions notrace:
> > > > > >     rcu_preempt_deferred_qs()
> > > > > >     rcu_preempt_need_deferred_qs()
> > > > > >     rcu_preempt_deferred_qs_irqrestore()
> > > > > > 
> > > > > 
> > > > > Only the preemptible RCU definitions are updated; so, this change is not
> > > > > required for non-preemptible RCU case?
> > > > 
> > > > It appears to me to be required.  How about as shown below?
> > > > 
> > > 
> > > Looks good to me.
> > 
> > Thank you!  May I apply your Reviewed-by to both of these?  (1/12 and
> > 2/12.)
> > 
> 
> Yes, sure.
> 
> Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>

Very good, thank you!  I will apply these on my next rebase.

							Thanx, Paul

> Thanks
> Neeraj
> 
> > 							Thanx, Paul
> > 
> > > Thanks
> > > Neeraj
> > > 
> > > > 							Thanx, Paul
> > > > 
> > > > ------------------------------------------------------------------------
> > > > 
> > > > commit 06cfe0c675c93884c3ffc75ec24ece7d0acd7a32
> > > > Author: Patrick Wang <patrick.wang.shcn@gmail.com>
> > > > Date:   Tue Apr 26 18:45:02 2022 +0800
> > > > 
> > > >       rcu: Avoid tracing a few functions executed in stop machine
> > > >       Stop-machine recently started calling additional functions while waiting:
> > > >       ----------------------------------------------------------------
> > > >       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 (and the functions that they call) must be marked
> > > >       notrace to prevent them from being updated while they are executing.
> > > >       The consequences of failing to mark these functions can be severe:
> > > >         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:
> > > >       This commit therefore marks these functions notrace:
> > > >        rcu_preempt_deferred_qs()
> > > >        rcu_preempt_need_deferred_qs()
> > > >        rcu_preempt_deferred_qs_irqrestore()
> > > >       [ paulmck: Apply feedback from Neeraj Upadhyay. ]
> > > >       Signed-off-by: Patrick Wang <patrick.wang.shcn@gmail.com>
> > > >       Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> > > >       Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > > > 
> > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > > index c8ba0fe17267c..7a07f2ca153e2 100644
> > > > --- a/kernel/rcu/tree_plugin.h
> > > > +++ b/kernel/rcu/tree_plugin.h
> > > > @@ -460,7 +460,7 @@ static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
> > > >     * be quite short, for example, in the case of the call from
> > > >     * rcu_read_unlock_special().
> > > >     */
> > > > -static void
> > > > +static notrace void
> > > >    rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
> > > >    {
> > > >    	bool empty_exp;
> > > > @@ -581,7 +581,7 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
> > > >     * is disabled.  This function cannot be expected to understand these
> > > >     * nuances, so the caller must handle them.
> > > >     */
> > > > -static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
> > > > +static notrace bool rcu_preempt_need_deferred_qs(struct task_struct *t)
> > > >    {
> > > >    	return (__this_cpu_read(rcu_data.cpu_no_qs.b.exp) ||
> > > >    		READ_ONCE(t->rcu_read_unlock_special.s)) &&
> > > > @@ -595,7 +595,7 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
> > > >     * evaluate safety in terms of interrupt, softirq, and preemption
> > > >     * disabling.
> > > >     */
> > > > -static void rcu_preempt_deferred_qs(struct task_struct *t)
> > > > +static notrace void rcu_preempt_deferred_qs(struct task_struct *t)
> > > >    {
> > > >    	unsigned long flags;
> > > > @@ -926,7 +926,7 @@ static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
> > > >     * Because there is no preemptible RCU, there can be no deferred quiescent
> > > >     * states.
> > > >     */
> > > > -static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
> > > > +static notrace bool rcu_preempt_need_deferred_qs(struct task_struct *t)
> > > >    {
> > > >    	return false;
> > > >    }
> > > > @@ -935,7 +935,7 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
> > > >    // period for a quiescent state from this CPU.  Note that requests from
> > > >    // tasks are handled when removing the task from the blocked-tasks list
> > > >    // below.
> > > > -static void rcu_preempt_deferred_qs(struct task_struct *t)
> > > > +static notrace void rcu_preempt_deferred_qs(struct task_struct *t)
> > > >    {
> > > >    	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);