linux-kernel - Re: [entry] 47b8ff194c: will-it-scale.per_process

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Date:   Thu, 13 May 2021 10:19:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     kernel test robot <oliver.sang@...el.com>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        lkp@...el.com, ying.huang@...el.com, feng.tang@...el.com,
        zhengjun.xing@...el.com, Paul McKenney <paulmck@...nel.org>
Subject: Re: [entry]  47b8ff194c:  will-it-scale.per_process_ops -3.0%
 regression

On Wed, Apr 28, 2021 at 03:16:53PM +0800, kernel test robot wrote:
> 
> 
> Greeting,
> 
> FYI, we noticed a -3.0% regression of will-it-scale.per_process_ops due to commit:
> 
> 
> commit: 47b8ff194c1fd73d58dc339b597d466fe48c8958 ("entry: Explicitly flush pending rcuog wakeup before last rescheduling point")

So the RCU bits are in rcu_user_enter(), which is called from
__context_tracking_enter() aka user_enter(), which is under
context_tracking_enabled().

But the new code in entry is not; we now unconditionally call
rcu_nocb_flush_deferred_wakeup(). Did that want to be under
context_tracking_enabled() as well?

Frederic, Paul?

---

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 9455476c5ba2..f4df001410fc 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -114,7 +114,12 @@ static inline void rcu_user_exit(void) { }
 void rcu_init_nohz(void);
 int rcu_nocb_cpu_offload(int cpu);
 int rcu_nocb_cpu_deoffload(int cpu);
-void rcu_nocb_flush_deferred_wakeup(void);
+void __rcu_nocb_flush_deferred_wakeup(void);
+static inline void rcu_nocb_flush_deferred_wakeup(void)
+{
+	if (context_tracking_enabled())
+		__rcu_nocb_flush_deferred_wakeup();
+}
 #else /* #ifdef CONFIG_RCU_NOCB_CPU */
 static inline void rcu_init_nohz(void) { }
 static inline int rcu_nocb_cpu_offload(int cpu) { return -EINVAL; }
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index ad0156b86937..3cdbbf7fba01 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2378,7 +2378,7 @@ static bool do_nocb_deferred_wakeup(struct rcu_data *rdp)
 	return false;
 }
 
-void rcu_nocb_flush_deferred_wakeup(void)
+void __rcu_nocb_flush_deferred_wakeup(void)
 {
 	do_nocb_deferred_wakeup(this_cpu_ptr(&rcu_data));
 }