Re: [PATCH tip/core/rcu 15/24] rcu: Directly drive RCU_USER_QS from Kconfig

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, May 13, 2015 at 02:37:52AM +0200, Frederic Weisbecker wrote:
> On Tue, May 12, 2015 at 03:30:45PM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > 
> > Currently, Kconfig will ask the user whether RCU_USER_QS should be set.
> > This is silly because Kconfig already has all the information that it
> > needs to set this parameter.  This commit therefore directly drives
> > the value of RCU_USER_QS via NO_HZ_FULL's "select" statement.
> > 
> > Reported-by: Ingo Molnar <mingo@kernel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Frederic Weisbecker <fweisbec@gmail.com>
> 
> ACK. And we should remove it completely and use NO_HZ_FULL instead.
> There won't seem to be more users.

Good point!  I have queued the patch shown below for 4.3.

							Thanx, Paul

------------------------------------------------------------------------

    rcu: Drop RCU_USER_QS in favor of NO_HZ_FULL
    
    The RCU_USER_QS Kconfig parameter is now just a synonym for NO_HZ_FULL,
    so this commit eliminates RCU_USER_QS, replacing all uses with NO_HZ_FULL.
    
    Reported-by: Frederic Weisbecker <fweisbec@gmail.com>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 03a899aabd17..18e377b92875 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -307,7 +307,7 @@ static inline void rcu_sysrq_end(void)
 }
 #endif /* #else #ifdef CONFIG_RCU_STALL_COMMON */
 
-#ifdef CONFIG_RCU_USER_QS
+#ifdef CONFIG_NO_HZ_FULL
 void rcu_user_enter(void);
 void rcu_user_exit(void);
 #else
@@ -315,7 +315,7 @@ static inline void rcu_user_enter(void) { }
 static inline void rcu_user_exit(void) { }
 static inline void rcu_user_hooks_switch(struct task_struct *prev,
 					 struct task_struct *next) { }
-#endif /* CONFIG_RCU_USER_QS */
+#endif /* CONFIG_NO_HZ_FULL */
 
 #ifdef CONFIG_RCU_NOCB_CPU
 void rcu_init_nohz(void);
diff --git a/init/Kconfig b/init/Kconfig
index 4c08197044f1..5b8726c10685 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -537,15 +537,6 @@ config RCU_STALL_COMMON
 config CONTEXT_TRACKING
        bool
 
-config RCU_USER_QS
-	bool
-	help
-	  This option sets hooks on kernel / userspace boundaries and
-	  puts RCU in extended quiescent state when the CPU runs in
-	  userspace. It means that when a CPU runs in userspace, it is
-	  excluded from the global RCU state machine and thus doesn't
-	  try to keep the timer tick on for RCU.
-
 config CONTEXT_TRACKING_FORCE
 	bool "Force context tracking"
 	depends on CONTEXT_TRACKING
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 7651d7dd982c..012cbee9d354 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -696,7 +696,7 @@ void rcu_idle_enter(void)
 }
 EXPORT_SYMBOL_GPL(rcu_idle_enter);
 
-#ifdef CONFIG_RCU_USER_QS
+#ifdef CONFIG_NO_HZ_FULL
 /**
  * rcu_user_enter - inform RCU that we are resuming userspace.
  *
@@ -709,7 +709,7 @@ void rcu_user_enter(void)
 {
 	rcu_eqs_enter(1);
 }
-#endif /* CONFIG_RCU_USER_QS */
+#endif /* CONFIG_NO_HZ_FULL */
 
 /**
  * rcu_irq_exit - inform RCU that current CPU is exiting irq towards idle
@@ -823,7 +823,7 @@ void rcu_idle_exit(void)
 }
 EXPORT_SYMBOL_GPL(rcu_idle_exit);
 
-#ifdef CONFIG_RCU_USER_QS
+#ifdef CONFIG_NO_HZ_FULL
 /**
  * rcu_user_exit - inform RCU that we are exiting userspace.
  *
@@ -834,7 +834,7 @@ void rcu_user_exit(void)
 {
 	rcu_eqs_exit(1);
 }
-#endif /* CONFIG_RCU_USER_QS */
+#endif /* CONFIG_NO_HZ_FULL */
 
 /**
  * rcu_irq_enter - inform RCU that current CPU is entering irq away from idle
diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index 579ce1b929af..4008d9f95dd7 100644
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -92,12 +92,10 @@ config NO_HZ_FULL
 	depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS
 	# We need at least one periodic CPU for timekeeping
 	depends on SMP
-	# RCU_USER_QS dependency
 	depends on HAVE_CONTEXT_TRACKING
 	# VIRT_CPU_ACCOUNTING_GEN dependency
 	depends on HAVE_VIRT_CPU_ACCOUNTING_GEN
 	select NO_HZ_COMMON
-	select RCU_USER_QS
 	select RCU_NOCB_CPU
 	select VIRT_CPU_ACCOUNTING_GEN
 	select IRQ_WORK