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

[Waiman Long]

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
was enabled which was true for most distributions.

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. 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@xxxxxxx>
---
 kernel/sched/sched.h |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

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

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