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Date: Thu, 3 Dec 2015 12:12:09 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Waiman Long <Waiman.Long@....com>
Cc: Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
Yuyang Du <yuyang.du@...el.com>, Paul Turner <pjt@...gle.com>,
Ben Segall <bsegall@...gle.com>,
Morten Rasmussen <morten.rasmussen@....com>,
Scott J Norton <scott.norton@....com>,
Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own
cacheline
I made this:
---
Subject: sched/fair: Move hot load_avg into its own cacheline
From: Waiman Long <Waiman.Long@....com>
Date: Wed, 2 Dec 2015 13:41:49 -0500
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
Cc: Scott J Norton <scott.norton@....com>
Cc: Douglas Hatch <doug.hatch@....com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Yuyang Du <yuyang.du@...el.com>
Cc: Paul Turner <pjt@...gle.com>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Morten Rasmussen <morten.rasmussen@....com>
Signed-off-by: Waiman Long <Waiman.Long@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: http://lkml.kernel.org/r/1449081710-20185-3-git-send-email-Waiman.Long@hpe.com
---
kernel/sched/core.c | 10 +++++++---
kernel/sched/sched.h | 7 ++++++-
2 files changed, 13 insertions(+), 4 deletions(-)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7345,6 +7345,9 @@ int in_sched_functions(unsigned long add
*/
struct task_group root_task_group;
LIST_HEAD(task_groups);
+
+/* Cacheline aligned slab cache for task_group */
+static struct kmem_cache *task_group_cache __read_mostly;
#endif
DECLARE_PER_CPU(cpumask_var_t, load_balance_mask);
@@ -7402,11 +7405,12 @@ void __init sched_init(void)
#endif /* CONFIG_RT_GROUP_SCHED */
#ifdef CONFIG_CGROUP_SCHED
+ task_group_cache = KMEM_CACHE(task_group, 0);
+
list_add(&root_task_group.list, &task_groups);
INIT_LIST_HEAD(&root_task_group.children);
INIT_LIST_HEAD(&root_task_group.siblings);
autogroup_init(&init_task);
-
#endif /* CONFIG_CGROUP_SCHED */
for_each_possible_cpu(i) {
@@ -7687,7 +7691,7 @@ static void free_sched_group(struct task
free_fair_sched_group(tg);
free_rt_sched_group(tg);
autogroup_free(tg);
- kfree(tg);
+ kmem_cache_free(task_group_cache, tg);
}
/* allocate runqueue etc for a new task group */
@@ -7695,7 +7699,7 @@ struct task_group *sched_create_group(st
{
struct task_group *tg;
- tg = kzalloc(sizeof(*tg), GFP_KERNEL);
+ tg = kmem_cache_alloc(task_group_cache, GFP_KERNEL | __GFP_ZERO);
if (!tg)
return ERR_PTR(-ENOMEM);
--- 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
--
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