linux-kernel - Re: [PATCH 02/11] sched, rcu: Make RCU use resched

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Message-ID: <20130917144012.GC29354@twins.programming.kicks-ass.net>
Date:	Tue, 17 Sep 2013 16:40:12 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>
Cc:	Andi Kleen <ak@...ux.intel.com>, Peter Anvin <hpa@...or.com>,
	Mike Galbraith <bitbucket@...ine.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Thomas Meyer <thomas@...3r.de>
Subject: Re: [PATCH 02/11] sched, rcu: Make RCU use resched_cpu()

Thomas Meyer reported a UP build fail, should be fixed.

---
Subject: sched, rcu: Make RCU use resched_cpu()
From: Peter Zijlstra <peterz@...radead.org>
Date: Tue Sep 17 09:30:55 CEST 2013

We're going to deprecate and remove set_need_resched() for it will do
the wrong thing. Make an exception for RCU and allow it to use
resched_cpu() which will do the right thing.

Cc: Paul McKenney <paulmck@...ux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
---
 kernel/rcutree.c    |   15 ++++++++++++++-
 kernel/sched/core.c |   10 ++--------
 2 files changed, 16 insertions(+), 9 deletions(-)

--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -898,6 +898,12 @@ static void print_other_cpu_stall(struct
 	force_quiescent_state(rsp);  /* Kick them all. */
 }
 
+/*
+ * This function really isn't for public consumption, but RCU is special in
+ * that context switches can allow the state machine to make progress.
+ */
+extern void resched_cpu(int cpu);
+
 static void print_cpu_stall(struct rcu_state *rsp)
 {
 	int cpu;
@@ -927,7 +933,14 @@ static void print_cpu_stall(struct rcu_s
 				     3 * rcu_jiffies_till_stall_check() + 3;
 	raw_spin_unlock_irqrestore(&rnp->lock, flags);
 
-	set_need_resched();  /* kick ourselves to get things going. */
+	/*
+	 * Attempt to revive the RCU machinery by forcing a context switch.
+	 *
+	 * A context switch would normally allow the RCU state machine to make
+	 * progress and it could be we're stuck in kernel space without context
+	 * switches for an entirely unreasonable amount of time.
+	 */
+	resched_cpu(smp_processor_id());
 }
 
 static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -513,12 +513,11 @@ static inline void init_hrtick(void)
  * might also involve a cross-CPU call to trigger the scheduler on
  * the target CPU.
  */
-#ifdef CONFIG_SMP
 void resched_task(struct task_struct *p)
 {
 	int cpu;
 
-	assert_raw_spin_locked(&task_rq(p)->lock);
+	lockdep_assert_held(&task_rq(p)->lock);
 
 	if (test_tsk_need_resched(p))
 		return;
@@ -546,6 +545,7 @@ void resched_cpu(int cpu)
 	raw_spin_unlock_irqrestore(&rq->lock, flags);
 }
 
+#ifdef CONFIG_SMP
 #ifdef CONFIG_NO_HZ_COMMON
 /*
  * In the semi idle case, use the nearest busy cpu for migrating timers
@@ -693,12 +693,6 @@ void sched_avg_update(struct rq *rq)
 	}
 }
 
-#else /* !CONFIG_SMP */
-void resched_task(struct task_struct *p)
-{
-	assert_raw_spin_locked(&task_rq(p)->lock);
-	set_tsk_need_resched(p);
-}
 #endif /* CONFIG_SMP */
 
 #if defined(CONFIG_RT_GROUP_SCHED) || (defined(CONFIG_FAIR_GROUP_SCHED) && \


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