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

Neeraj Upadhyay
2022-06-22T11:50:48Z

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.

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);
>