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, 10 insertions, 0 deletions

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 6558b7ac112..f784d83e29f 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -154,6 +154,7 @@ static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)
 
 #ifdef CONFIG_NO_HZ_FULL
 cpumask_var_t tick_nohz_full_mask;
+cpumask_var_t housekeeping_mask;
 bool tick_nohz_full_running;
 
 static bool can_stop_full_tick(void)
@@ -281,6 +282,7 @@ static int __init tick_nohz_full_setup(char *str)
 	int cpu;
 
 	alloc_bootmem_cpumask_var(&tick_nohz_full_mask);
+	alloc_bootmem_cpumask_var(&housekeeping_mask);
 	if (cpulist_parse(str, tick_nohz_full_mask) < 0) {
 		pr_warning("NOHZ: Incorrect nohz_full cpumask\n");
 		return 1;
@@ -291,6 +293,8 @@ static int __init tick_nohz_full_setup(char *str)
 		pr_warning("NO_HZ: Clearing %d from nohz_full range for timekeeping\n", cpu);
 		cpumask_clear_cpu(cpu, tick_nohz_full_mask);
 	}
+	cpumask_andnot(housekeeping_mask,
+		       cpu_possible_mask, tick_nohz_full_mask);
 	tick_nohz_full_running = true;
 
 	return 1;
@@ -332,9 +336,15 @@ static int tick_nohz_init_all(void)
 		pr_err("NO_HZ: Can't allocate full dynticks cpumask\n");
 		return err;
 	}
+	if (!alloc_cpumask_var(&housekeeping_mask, GFP_KERNEL)) {
+		pr_err("NO_HZ: Can't allocate not-full dynticks cpumask\n");
+		return err;
+	}
 	err = 0;
 	cpumask_setall(tick_nohz_full_mask);
 	cpumask_clear_cpu(smp_processor_id(), tick_nohz_full_mask);
+	cpumask_clear(housekeeping_mask);
+	cpumask_set_cpu(smp_processor_id(), housekeeping_mask);
 	tick_nohz_full_running = true;
 #endif
 	return err;