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* [RFC] Make need_resched() return true when rcu_urgent_qs requested
@ 2018-07-06 14:53 David Woodhouse
  2018-07-06 16:29 ` Peter Zijlstra
  0 siblings, 1 reply; 93+ messages in thread
From: David Woodhouse @ 2018-07-06 14:53 UTC (permalink / raw)
  To: Paul McKenney, Peter Zijlstra, mhillenb; +Cc: linux-kernel

In 4.15 without CONFIG_PREEMPT we observed expand_fdtable() taking
about 10 seconds for synchronize_sched() to complete, when most of the
other threads were running KVM guests.

In vcpu_run() there's a loop with the fairly common construct:

  if (need_resched()) {
      … local unlocks …
      cond_resched();
      … local locks …
  }

But because need_resched() wasn't true (until half the RCU warning time
was completed and rcu_implicit_dynticks_qs() calls resched_cpu()), that
never happens and cond_resched() is never called. In cond_resched()
there is an unconditional call to rcu_all_qs() which would DTRT.

Now, there's a simple way to fix it for the specific case of KVM — we
can find a place we can just call rcu_all_qs(), something like this:

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 00520711..a304693 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7214,6 +7214,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	 */
 	smp_mb__after_srcu_read_unlock();
 
+	/* Force quiescent state (if requested) before entering guest mode */
+	rcu_all_qs();
+
 	/*
 	 * This handles the case where a posted interrupt was
 	 * notified with kvm_vcpu_kick.


But I wonder if we should attempt to fix the general case by making
need_resched() return true when an RCU quiescent state is needed. To do
that without having an out-of-line function call in kernel/rcu/tree.c
would look something like the patch below. Paul, did you say you had
other ideas about how to export/inline it?

Alternatively — or perhaps additionally — shouldn't CPUs which are
currently in guest mode be counted as quiescent anyway? Or is that
something we'll only ever want to do in full NOHZ mode?

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index b3dbf95..2f8a3bd 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -126,6 +126,7 @@ static inline bool rcu_is_watching(void) { return true; }
 
 /* Avoid RCU read-side critical sections leaking across. */
 static inline void rcu_all_qs(void) { barrier(); }
+static inline bool rcu_urgent_qs_requested(void) { return false; }
 
 /* RCUtree hotplug events */
 #define rcutree_prepare_cpu      NULL
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 37d6fd3..d20b987 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -30,6 +30,36 @@
 #ifndef __LINUX_RCUTREE_H
 #define __LINUX_RCUTREE_H
 
+/*
+ * Dynticks per-CPU state.
+ */
+struct rcu_dynticks {
+	long long dynticks_nesting; /* Track irq/process nesting level. */
+				    /* Process level is worth LLONG_MAX/2. */
+	int dynticks_nmi_nesting;   /* Track NMI nesting level. */
+	atomic_t dynticks;	    /* Even value for idle, else odd. */
+	bool rcu_need_heavy_qs;     /* GP old, need heavy quiescent state. */
+	unsigned long rcu_qs_ctr;   /* Light universal quiescent state ctr. */
+	bool rcu_urgent_qs;	    /* GP old need light quiescent state. */
+#ifdef CONFIG_RCU_FAST_NO_HZ
+	bool all_lazy;		    /* Are all CPU's CBs lazy? */
+	unsigned long nonlazy_posted;
+				    /* # times non-lazy CBs posted to CPU. */
+	unsigned long nonlazy_posted_snap;
+				    /* idle-period nonlazy_posted snapshot. */
+	unsigned long last_accelerate;
+				    /* Last jiffy CBs were accelerated. */
+	unsigned long last_advance_all;
+				    /* Last jiffy CBs were all advanced. */
+	int tick_nohz_enabled_snap; /* Previously seen value from sysfs. */
+#endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */
+};
+DECLARE_PER_CPU(struct rcu_dynticks, rcu_dynticks);
+static __always_inline bool rcu_urgent_qs_requested(void)
+{
+	return unlikely(raw_cpu_read(rcu_dynticks.rcu_urgent_qs));
+}
+
 void rcu_note_context_switch(bool preempt);
 int rcu_needs_cpu(u64 basem, u64 *nextevt);
 void rcu_cpu_stall_reset(void);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index e4d4e60..89f5814 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1616,7 +1616,8 @@ static inline int spin_needbreak(spinlock_t *lock)
 
 static __always_inline bool need_resched(void)
 {
-	return unlikely(tif_need_resched());
+	return unlikely(tif_need_resched()) ||
+		rcu_urgent_qs_requested();
 }
 
 /*
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f9c0ca2..cf1c66c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -264,10 +264,11 @@ void rcu_bh_qs(void)
 #define rcu_eqs_special_exit() do { } while (0)
 #endif
 
-static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
+DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
 	.dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
 	.dynticks = ATOMIC_INIT(RCU_DYNTICK_CTRL_CTR),
 };
+EXPORT_SYMBOL(rcu_dynticks); /* for need_resched() */
 
 /*
  * There's a few places, currently just in the tracing infrastructure,
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 46a5d19..462b25b 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -34,31 +34,6 @@
 
 #include "rcu_segcblist.h"
 
-/*
- * Dynticks per-CPU state.
- */
-struct rcu_dynticks {
-	long long dynticks_nesting; /* Track irq/process nesting level. */
-				    /* Process level is worth LLONG_MAX/2. */
-	int dynticks_nmi_nesting;   /* Track NMI nesting level. */
-	atomic_t dynticks;	    /* Even value for idle, else odd. */
-	bool rcu_need_heavy_qs;     /* GP old, need heavy quiescent state. */
-	unsigned long rcu_qs_ctr;   /* Light universal quiescent state ctr. */
-	bool rcu_urgent_qs;	    /* GP old need light quiescent state. */
-#ifdef CONFIG_RCU_FAST_NO_HZ
-	bool all_lazy;		    /* Are all CPU's CBs lazy? */
-	unsigned long nonlazy_posted;
-				    /* # times non-lazy CBs posted to CPU. */
-	unsigned long nonlazy_posted_snap;
-				    /* idle-period nonlazy_posted snapshot. */
-	unsigned long last_accelerate;
-				    /* Last jiffy CBs were accelerated. */
-	unsigned long last_advance_all;
-				    /* Last jiffy CBs were all advanced. */
-	int tick_nohz_enabled_snap; /* Previously seen value from sysfs. */
-#endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */
-};
-
 /* RCU's kthread states for tracing. */
 #define RCU_KTHREAD_STOPPED  0
 #define RCU_KTHREAD_RUNNING  1

-- 
dwmw2

^ permalink raw reply related	[flat|nested] 93+ messages in thread

end of thread, other threads:[~2018-07-23 12:22 UTC | newest]

Thread overview: 93+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2018-07-06 14:53 [RFC] Make need_resched() return true when rcu_urgent_qs requested David Woodhouse
2018-07-06 16:29 ` Peter Zijlstra
2018-07-06 17:11   ` Paul E. McKenney
2018-07-06 17:14     ` David Woodhouse
2018-07-06 21:12       ` Paul E. McKenney
2018-07-09  8:58       ` Peter Zijlstra
2018-07-09  8:53     ` Peter Zijlstra
2018-07-09  9:18       ` David Woodhouse
2018-07-09 10:44         ` Peter Zijlstra
2018-07-09 10:56           ` David Woodhouse
2018-07-09 11:06             ` Peter Zijlstra
2018-07-09 11:12               ` David Woodhouse
2018-07-09 11:31                 ` Peter Zijlstra
2018-07-09 12:34               ` Paul E. McKenney
2018-07-09 12:47                 ` David Woodhouse
2018-07-09 14:30                   ` Paul E. McKenney
2018-07-09 12:55                 ` Peter Zijlstra
2018-07-09 12:57                   ` David Woodhouse
2018-07-09 13:02                   ` Peter Zijlstra
2018-07-09 14:29                     ` Paul E. McKenney
2018-07-09 14:43                       ` Peter Zijlstra
2018-07-09 14:54                         ` Paul E. McKenney
2018-07-09 15:26                       ` Peter Zijlstra
2018-07-09 16:34                         ` Paul E. McKenney
2018-07-09 16:44                           ` Paul E. McKenney
2018-07-09 18:50                           ` David Woodhouse
2018-07-09 20:34                             ` Paul E. McKenney
2018-07-09 20:35                               ` David Woodhouse
2018-07-09 20:42                                 ` Paul E. McKenney
2018-07-09 20:45                                   ` David Woodhouse
2018-07-09 21:05                                     ` Paul E. McKenney
2018-07-09 22:08                                       ` Paul E. McKenney
2018-07-11 10:57                                         ` David Woodhouse
2018-07-11 12:51                                           ` Paul E. McKenney
2018-07-11 12:58                                             ` David Woodhouse
2018-07-11 14:25                                               ` Paul E. McKenney
2018-07-11 14:23                                         ` David Woodhouse
2018-07-11 14:43                                           ` Paul E. McKenney
2018-07-11 16:49                                             ` Paul E. McKenney
2018-07-11 17:03                                               ` David Woodhouse
2018-07-11 17:48                                                 ` Paul E. McKenney
2018-07-11 18:01                                                   ` [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode David Woodhouse
2018-07-11 18:20                                                     ` Paul E. McKenney
2018-07-11 18:36                                                       ` Paul E. McKenney
2018-07-11 18:39                                                         ` Christian Borntraeger
2018-07-11 20:27                                                           ` Paul E. McKenney
2018-07-11 20:54                                                             ` David Woodhouse
2018-07-11 21:09                                                               ` Paul E. McKenney
2018-07-11 21:11                                                             ` Christian Borntraeger
2018-07-11 21:32                                                               ` Paul E. McKenney
2018-07-11 21:39                                                                 ` Christian Borntraeger
2018-07-11 23:47                                                                   ` Paul E. McKenney
2018-07-12  8:31                                                                     ` David Woodhouse
2018-07-12 11:00                                                                       ` Christian Borntraeger
2018-07-12 11:10                                                                         ` David Woodhouse
2018-07-12 11:58                                                                           ` Christian Borntraeger
2018-07-12 12:04                                                                             ` Christian Borntraeger
2018-07-11 23:37                                                                 ` Paul E. McKenney
2018-07-12  2:15                                                                   ` Paul E. McKenney
2018-07-12  6:21                                                                   ` Christian Borntraeger
2018-07-12  9:52                                                                     ` David Woodhouse
2018-07-11 18:31                                                 ` [RFC] Make need_resched() return true when rcu_urgent_qs requested Christian Borntraeger
2018-07-11 20:17                                                   ` Paul E. McKenney
2018-07-11 20:19                                                     ` David Woodhouse
2018-07-11 21:08                                                       ` Paul E. McKenney
2018-07-12 12:00                                                         ` David Woodhouse
2018-07-12 12:53                                                           ` Paul E. McKenney
2018-07-12 16:17                                                             ` Paul E. McKenney
2018-07-16 15:40                                                               ` Paul E. McKenney
2018-07-17  8:19                                                                 ` David Woodhouse
2018-07-17 12:56                                                                   ` Paul E. McKenney
2018-07-18 15:36                                                                     ` Paul E. McKenney
2018-07-18 16:01                                                                       ` David Woodhouse
2018-07-18 16:37                                                                         ` Paul E. McKenney
2018-07-18 19:41                                                                           ` David Woodhouse
2018-07-18 20:17                                                                             ` Paul E. McKenney
2018-07-19  0:26                                                                               ` Frederic Weisbecker
2018-07-19  6:45                                                                               ` Christian Borntraeger
2018-07-19  7:20                                                                                 ` David Woodhouse
2018-07-19 10:23                                                                                   ` Christian Borntraeger
2018-07-19 12:55                                                                                     ` Paul E. McKenney
2018-07-19 13:14                                                                                   ` Frederic Weisbecker
2018-07-19 13:36                                                                                     ` David Woodhouse
2018-07-19 17:09                                                                           ` Paul E. McKenney
2018-07-23  8:08                                                                             ` David Woodhouse
2018-07-23 12:22                                                                               ` Paul E. McKenney
2018-07-19  0:32                                                 ` Frederic Weisbecker
2018-07-19  3:11                                                   ` Paul E. McKenney
2018-07-19  6:16                                                     ` David Woodhouse
2018-07-19 13:17                                                       ` Frederic Weisbecker
2018-07-19 13:15                                                     ` Frederic Weisbecker
2018-07-10  9:24                                   ` Peter Zijlstra
2018-07-10 16:26                                     ` Paul E. McKenney

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