[PATCH v2 2/3] sched/fair: Move hot load_avg into its own cacheline

[Waiman Long <Waiman.Long@hpe.com>]

If a system with large number of sockets was driven to full
utilization, it was found that the clock tick handling occupied a
rather significant proportion of CPU time when fair group scheduling
and autogroup were enabled.

Running a java benchmark on a 16-socket IvyBridge-EX system, the perf
profile looked like:

  10.52%   0.00%  java   [kernel.vmlinux]  [k] smp_apic_timer_interrupt
   9.66%   0.05%  java   [kernel.vmlinux]  [k] hrtimer_interrupt
   8.65%   0.03%  java   [kernel.vmlinux]  [k] tick_sched_timer
   8.56%   0.00%  java   [kernel.vmlinux]  [k] update_process_times
   8.07%   0.03%  java   [kernel.vmlinux]  [k] scheduler_tick
   6.91%   1.78%  java   [kernel.vmlinux]  [k] task_tick_fair
   5.24%   5.04%  java   [kernel.vmlinux]  [k] update_cfs_shares

In particular, the high CPU time consumed by update_cfs_shares()
was mostly due to contention on the cacheline that contained the
task_group's load_avg statistical counter. This cacheline may also
contains variables like shares, cfs_rq & se which are accessed rather
frequently during clock tick processing.

This patch moves the load_avg variable into another cacheline
separated from the other frequently accessed variables. It also
creates a cacheline aligned kmemcache for task_group to make sure
that all the allocated task_group's are cacheline aligned.

By doing so, the perf profile became:

   9.44%   0.00%  java   [kernel.vmlinux]  [k] smp_apic_timer_interrupt
   8.74%   0.01%  java   [kernel.vmlinux]  [k] hrtimer_interrupt
   7.83%   0.03%  java   [kernel.vmlinux]  [k] tick_sched_timer
   7.74%   0.00%  java   [kernel.vmlinux]  [k] update_process_times
   7.27%   0.03%  java   [kernel.vmlinux]  [k] scheduler_tick
   5.94%   1.74%  java   [kernel.vmlinux]  [k] task_tick_fair
   4.15%   3.92%  java   [kernel.vmlinux]  [k] update_cfs_shares

The %cpu time is still pretty high, but it is better than before. The
benchmark results before and after the patch was as follows:

  Before patch - Max-jOPs: 907533    Critical-jOps: 134877
  After patch  - Max-jOPs: 916011    Critical-jOps: 142366

Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
---
 kernel/sched/core.c  |   36 ++++++++++++++++++++++++++++++++++--
 kernel/sched/sched.h |    7 ++++++-
 2 files changed, 40 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4d568ac..e39204f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7331,6 +7331,11 @@ int in_sched_functions(unsigned long addr)
  */
 struct task_group root_task_group;
 LIST_HEAD(task_groups);
+
+#ifdef CONFIG_FAIR_GROUP_SCHED
+/* Cacheline aligned slab cache for task_group */
+static struct kmem_cache *task_group_cache __read_mostly;
+#endif
 #endif
 
 DECLARE_PER_CPU(cpumask_var_t, load_balance_mask);
@@ -7356,6 +7361,7 @@ void __init sched_init(void)
 		root_task_group.cfs_rq = (struct cfs_rq **)ptr;
 		ptr += nr_cpu_ids * sizeof(void **);
 
+		task_group_cache = KMEM_CACHE(task_group, SLAB_HWCACHE_ALIGN);
 #endif /* CONFIG_FAIR_GROUP_SCHED */
 #ifdef CONFIG_RT_GROUP_SCHED
 		root_task_group.rt_se = (struct sched_rt_entity **)ptr;
@@ -7668,12 +7674,38 @@ void set_curr_task(int cpu, struct task_struct *p)
 /* task_group_lock serializes the addition/removal of task groups */
 static DEFINE_SPINLOCK(task_group_lock);
 
+/*
+ * Make sure that the task_group structure is cacheline aligned when
+ * fair group scheduling is enabled.
+ */
+#ifdef CONFIG_FAIR_GROUP_SCHED
+static inline struct task_group *alloc_task_group(void)
+{
+	return kmem_cache_alloc(task_group_cache, GFP_KERNEL | __GFP_ZERO);
+}
+
+static inline void free_task_group(struct task_group *tg)
+{
+	kmem_cache_free(task_group_cache, tg);
+}
+#else /* CONFIG_FAIR_GROUP_SCHED */
+static inline struct task_group *alloc_task_group(void)
+{
+	return kzalloc(sizeof(struct task_group), GFP_KERNEL);
+}
+
+static inline void free_task_group(struct task_group *tg)
+{
+	kfree(tg);
+}
+#endif /* CONFIG_FAIR_GROUP_SCHED */
+
 static void free_sched_group(struct task_group *tg)
 {
 	free_fair_sched_group(tg);
 	free_rt_sched_group(tg);
 	autogroup_free(tg);
-	kfree(tg);
+	free_task_group(tg);
 }
 
 /* allocate runqueue etc for a new task group */
@@ -7681,7 +7713,7 @@ struct task_group *sched_create_group(struct task_group *parent)
 {
 	struct task_group *tg;
 
-	tg = kzalloc(sizeof(*tg), GFP_KERNEL);
+	tg = alloc_task_group();
 	if (!tg)
 		return ERR_PTR(-ENOMEM);
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index efd3bfc..e679895 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -248,7 +248,12 @@ struct task_group {
 	unsigned long shares;
 
 #ifdef	CONFIG_SMP
-	atomic_long_t load_avg;
+	/*
+	 * load_avg can be heavily contended at clock tick time, so put
+	 * it in its own cacheline separated from the fields above which
+	 * will also be accessed at each tick.
+	 */
+	atomic_long_t load_avg ____cacheline_aligned;
 #endif
 #endif
 
-- 
1.7.1