Re: [PATCH rcu 02/12] rcu: Avoid tracing a few functions executed in stop machine

Paul E. McKenney
2022-06-22T15:35:12Z

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.)

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);