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From:   Patrick Wang <patrick.wang.shcn@gmail.com>
To:     paulmck@kernel.org, rostedt@goodmis.org, frederic@kernel.org,
        quic_neeraju@quicinc.com, josh@joshtriplett.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
        joel@joelfernandes.org
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        patrick.wang.shcn@gmail.com
Subject: [PATCH v3] rcu: ftrace: avoid tracing a few functions executed in stop machine
Date:   Tue, 26 Apr 2022 18:45:02 +0800
Message-Id: <20220426104502.52663-1-patrick.wang.shcn@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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@gmail.com>
---
v1->v2:
 - Modify log message.

v2->v3:
 - Move "notrace" to behind "static" to keep the consistency with the rest of the code.

 kernel/rcu/tree_plugin.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index c8ba0fe17267..440d9e02a26e 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;
 
-- 
2.25.1