rcu: Bind grace-period kthreads to non-NO_HZ_FULL CPUs

Binding the grace-period kthreads to the timekeeping CPU resulted in
significant performance decreases for some workloads.  For more detail,
see:

https://lkml.org/lkml/2014/6/3/395 for benchmark numbers

https://lkml.org/lkml/2014/6/4/218 for CPU statistics

It turns out that it is necessary to bind the grace-period kthreads
to the timekeeping CPU only when all but CPU 0 is a nohz_full CPU
on the one hand or if CONFIG_NO_HZ_FULL_SYSIDLE=y on the other.
In other cases, it suffices to bind the grace-period kthreads to the
set of non-nohz_full CPUs.

This commit therefore creates a tick_nohz_not_full_mask that is the
complement of tick_nohz_full_mask, and then binds the grace-period
kthread to the set of CPUs indicated by this new mask, which covers
the CONFIG_NO_HZ_FULL_SYSIDLE=n case.  The CONFIG_NO_HZ_FULL_SYSIDLE=y
case still binds the grace-period kthreads to the timekeeping CPU.
This commit also includes the tick_nohz_full_enabled() check suggested
by Frederic Weisbecker.

Reported-by: Jet Chen <jet.chen@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Created housekeeping_affine() and housekeeping_mask per
  fweisbec feedback. ]

1 files changed, 20 insertions, 0 deletions

diff --git a/include/linux/tick.h b/include/linux/tick.h
index b84773cb9f4c..06cc093ab7ad 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -12,6 +12,7 @@
 #include <linux/hrtimer.h>
 #include <linux/context_tracking_state.h>
 #include <linux/cpumask.h>
+#include <linux/sched.h>
 
 #ifdef CONFIG_GENERIC_CLOCKEVENTS
 
@@ -162,6 +163,7 @@ static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; }
 #ifdef CONFIG_NO_HZ_FULL
 extern bool tick_nohz_full_running;
 extern cpumask_var_t tick_nohz_full_mask;
+extern cpumask_var_t housekeeping_mask;
 
 static inline bool tick_nohz_full_enabled(void)
 {
@@ -194,6 +196,24 @@ static inline void tick_nohz_full_kick_all(void) { }
 static inline void __tick_nohz_task_switch(struct task_struct *tsk) { }
 #endif
 
+static inline bool is_housekeeping_cpu(int cpu)
+{
+#ifdef CONFIG_NO_HZ_FULL
+        if (tick_nohz_full_enabled())
+                return cpumask_test_cpu(cpu, housekeeping_mask);
+#endif
+        return true;
+}
+
+static inline void housekeeping_affine(struct task_struct *t)
+{
+#ifdef CONFIG_NO_HZ_FULL
+        if (tick_nohz_full_enabled())
+                set_cpus_allowed_ptr(t, housekeeping_mask);
+
+#endif
+}
+
 static inline void tick_nohz_full_check(void)
 {
         if (tick_nohz_full_enabled())