Merge branch 'nohz/guest' of git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks into timers/nohz

Pull full dynticks support for virt guests from Frederic Weisbecker:

 "Some measurements showed that disabling the tick on the host while the
  guest is running can be interesting on some workloads. Indeed the
  host tick is irrelevant while a vcpu runs, it consumes CPU time and cache
  footprint for no good reasons.

  Full dynticks already works in every context, but RCU prevents it to
  be effective outside userspace, because the CPU needs to take part of
  RCU grace period completion as long as RCU may be used on it, which is
  the case in kernel context.

  However guest is similar to userspace and idle in that we know RCU is
  unused on such context. Therefore a CPU in guest/userspace/idle context
  can let other CPUs report its own RCU quiescent state on its behalf
  and shut down the tick safely, provided it isn't needed for other
  reasons than RCU. This is called RCU extended quiescent state.

  This was already implemented for idle and userspace. This patchset now
  brings it for guest contexts through the following steps:

  - Generalize the context tracking APIs to also track guest state
  - Rename/sanitize a few CPP symbols accordingly
  - Report guest entry/exit to RCU and define this context area as an RCU
    extended quiescent state."

Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/powerpc/include/asm/kvm_book3s.h

@@ -106,10 +106,6 @@ struct kvmppc_vcpu_book3s {
 	spinlock_t mmu_lock;
 };
 
-#define CONTEXT_HOST		0
-#define CONTEXT_GUEST		1
-#define CONTEXT_GUEST_END	2
-
 #define VSID_REAL	0x07ffffffffc00000ULL
 #define VSID_BAT	0x07ffffffffb00000ULL
 #define VSID_64K	0x0800000000000000ULL

arch/x86/kernel/traps.c

@@ -123,7 +123,7 @@ enum ctx_state ist_enter(struct pt_regs *regs)
 		 * but we need to notify RCU.
 		 */
 		rcu_nmi_enter();
-		prev_state = IN_KERNEL;  /* the value is irrelevant. */
+		prev_state = CONTEXT_KERNEL;  /* the value is irrelevant. */
 	}
 
 	/*

include/linux/context_tracking.h

@@ -10,6 +10,8 @@
 #ifdef CONFIG_CONTEXT_TRACKING
 extern void context_tracking_cpu_set(int cpu);
 
+extern void context_tracking_enter(enum ctx_state state);
+extern void context_tracking_exit(enum ctx_state state);
 extern void context_tracking_user_enter(void);
 extern void context_tracking_user_exit(void);
 extern void __context_tracking_task_switch(struct task_struct *prev,
@@ -35,7 +37,8 @@ static inline enum ctx_state exception_enter(void)
 		return 0;
 
 	prev_ctx = this_cpu_read(context_tracking.state);
-	context_tracking_user_exit();
+	if (prev_ctx != CONTEXT_KERNEL)
+		context_tracking_exit(prev_ctx);
 
 	return prev_ctx;
 }
@@ -43,8 +46,8 @@ static inline enum ctx_state exception_enter(void)
 static inline void exception_exit(enum ctx_state prev_ctx)
 {
 	if (context_tracking_is_enabled()) {
-		if (prev_ctx == IN_USER)
-			context_tracking_user_enter();
+		if (prev_ctx != CONTEXT_KERNEL)
+			context_tracking_enter(prev_ctx);
 	}
 }
 
@@ -78,10 +81,16 @@ static inline void guest_enter(void)
 		vtime_guest_enter(current);
 	else
 		current->flags |= PF_VCPU;
+
+	if (context_tracking_is_enabled())
+		context_tracking_enter(CONTEXT_GUEST);
 }
 
 static inline void guest_exit(void)
 {
+	if (context_tracking_is_enabled())
+		context_tracking_exit(CONTEXT_GUEST);
+
 	if (vtime_accounting_enabled())
 		vtime_guest_exit(current);
 	else

include/linux/context_tracking_state.h

@@ -13,8 +13,9 @@ struct context_tracking {
 	 */
 	bool active;
 	enum ctx_state {
-		IN_KERNEL = 0,
-		IN_USER,
+		CONTEXT_KERNEL = 0,
+		CONTEXT_USER,
+		CONTEXT_GUEST,
 	} state;
 };
 
@@ -34,11 +35,13 @@ static inline bool context_tracking_cpu_is_enabled(void)
 
 static inline bool context_tracking_in_user(void)
 {
-	return __this_cpu_read(context_tracking.state) == IN_USER;
+	return __this_cpu_read(context_tracking.state) == CONTEXT_USER;
 }
 #else
 static inline bool context_tracking_in_user(void) { return false; }
 static inline bool context_tracking_active(void) { return false; }
+static inline bool context_tracking_is_enabled(void) { return false; }
+static inline bool context_tracking_cpu_is_enabled(void) { return false; }
 #endif /* CONFIG_CONTEXT_TRACKING */
 
 #endif

include/linux/kvm_host.h

@@ -766,7 +766,8 @@ static inline void kvm_guest_enter(void)
 	 * one time slice). Lets treat guest mode as quiescent state, just like
 	 * we do with user-mode execution.
 	 */
-	rcu_virt_note_context_switch(smp_processor_id());
+	if (!context_tracking_cpu_is_enabled())
+		rcu_virt_note_context_switch(smp_processor_id());
 }
 
 static inline void kvm_guest_exit(void)

kernel/context_tracking.c

@@ -39,15 +39,15 @@ void context_tracking_cpu_set(int cpu)
 }
 
 /**
- * context_tracking_user_enter - Inform the context tracking that the CPU is going to
- *                               enter userspace mode.
+ * context_tracking_enter - Inform the context tracking that the CPU is going
+ *                          enter user or guest space mode.
  *
  * This function must be called right before we switch from the kernel
- * to userspace, when it's guaranteed the remaining kernel instructions
- * to execute won't use any RCU read side critical section because this
- * function sets RCU in extended quiescent state.
+ * to user or guest space, when it's guaranteed the remaining kernel
+ * instructions to execute won't use any RCU read side critical section
+ * because this function sets RCU in extended quiescent state.
  */
-void context_tracking_user_enter(void)
+void context_tracking_enter(enum ctx_state state)
 {
 	unsigned long flags;
 
@@ -75,9 +75,8 @@ void context_tracking_user_enter(void)
 	WARN_ON_ONCE(!current->mm);
 
 	local_irq_save(flags);
-	if ( __this_cpu_read(context_tracking.state) != IN_USER) {
+	if ( __this_cpu_read(context_tracking.state) != state) {
 		if (__this_cpu_read(context_tracking.active)) {
-			trace_user_enter(0);
 			/*
 			 * At this stage, only low level arch entry code remains and
 			 * then we'll run in userspace. We can assume there won't be
@@ -85,7 +84,10 @@ void context_tracking_user_enter(void)
 			 * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
 			 * on the tick.
 			 */
-			vtime_user_enter(current);
+			if (state == CONTEXT_USER) {
+				trace_user_enter(0);
+				vtime_user_enter(current);
+			}
 			rcu_user_enter();
 		}
 		/*
@@ -101,24 +103,32 @@ void context_tracking_user_enter(void)
 		 * OTOH we can spare the calls to vtime and RCU when context_tracking.active
 		 * is false because we know that CPU is not tickless.
 		 */
-		__this_cpu_write(context_tracking.state, IN_USER);
+		__this_cpu_write(context_tracking.state, state);
 	}
 	local_irq_restore(flags);
 }
+NOKPROBE_SYMBOL(context_tracking_enter);
+EXPORT_SYMBOL_GPL(context_tracking_enter);
+
+void context_tracking_user_enter(void)
+{
+	context_tracking_enter(CONTEXT_USER);
+}
 NOKPROBE_SYMBOL(context_tracking_user_enter);
 
 /**
- * context_tracking_user_exit - Inform the context tracking that the CPU is
- *                              exiting userspace mode and entering the kernel.
+ * context_tracking_exit - Inform the context tracking that the CPU is
+ *                         exiting user or guest mode and entering the kernel.
  *
- * This function must be called after we entered the kernel from userspace
- * before any use of RCU read side critical section. This potentially include
- * any high level kernel code like syscalls, exceptions, signal handling, etc...
+ * This function must be called after we entered the kernel from user or
+ * guest space before any use of RCU read side critical section. This
+ * potentially include any high level kernel code like syscalls, exceptions,
+ * signal handling, etc...
  *
  * This call supports re-entrancy. This way it can be called from any exception
  * handler without needing to know if we came from userspace or not.
  */
-void context_tracking_user_exit(void)
+void context_tracking_exit(enum ctx_state state)
 {
 	unsigned long flags;
 
@@ -129,20 +139,29 @@ void context_tracking_user_exit(void)
 		return;
 
 	local_irq_save(flags);
-	if (__this_cpu_read(context_tracking.state) == IN_USER) {
+	if (__this_cpu_read(context_tracking.state) == state) {
 		if (__this_cpu_read(context_tracking.active)) {
 			/*
 			 * We are going to run code that may use RCU. Inform
 			 * RCU core about that (ie: we may need the tick again).
 			 */
 			rcu_user_exit();
-			vtime_user_exit(current);
-			trace_user_exit(0);
+			if (state == CONTEXT_USER) {
+				vtime_user_exit(current);
+				trace_user_exit(0);
+			}
 		}
-		__this_cpu_write(context_tracking.state, IN_KERNEL);
+		__this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
 	}
 	local_irq_restore(flags);
 }
+NOKPROBE_SYMBOL(context_tracking_exit);
+EXPORT_SYMBOL_GPL(context_tracking_exit);
+
+void context_tracking_user_exit(void)
+{
+	context_tracking_exit(CONTEXT_USER);
+}
 NOKPROBE_SYMBOL(context_tracking_user_exit);
 
 /**

kernel/sched/core.c

@@ -2818,7 +2818,7 @@ asmlinkage __visible void __sched schedule_user(void)
 	 * we find a better solution.
 	 *
 	 * NB: There are buggy callers of this function.  Ideally we
-	 * should warn if prev_state != IN_USER, but that will trigger
+	 * should warn if prev_state != CONTEXT_USER, but that will trigger
 	 * too frequently to make sense yet.
 	 */
 	enum ctx_state prev_state = exception_enter();