[PATCH] mm, memcg: Optionally disable memcg by default using Kconfig

Mel Gorman
2015-05-19T10:41:06Z

memcg was reported years ago to have significant overhead when unused. It
has improved but it's still the case that users that have no knowledge of
memcg pay a performance penalty.

This patch adds a Kconfig that controls whether memcg is enabled by default
and a kernel parameter cgroup_enable= to enable it if desired. Anyone using
oldconfig will get the historical behaviour. It is not an option for most
distributions to simply disable MEMCG as there are users that require it
but they should also be knowledgable enough to use cgroup_enable=.

This was evaluated using aim9, a page fault microbenchmark and ebizzy
but I'll focus on the page fault microbenchmark. It can be reproduced
using pft from mmtests (https://github.com/gormanm/mmtests). Edit
configs/config-global-dhp__pagealloc-performance and update MMTESTS to
only contain pft. This is the relevant part of the profile summary

/usr/src/linux-4.0-vanilla/mm/memcontrol.c 6.6441 395842
mem_cgroup_try_charge 2.950% 175781
__mem_cgroup_count_vm_event 1.431% 85239
mem_cgroup_page_lruvec 0.456% 27156
mem_cgroup_commit_charge 0.392% 23342
uncharge_list 0.323% 19256
mem_cgroup_update_lru_size 0.278% 16538
memcg_check_events 0.216% 12858
mem_cgroup_charge_statistics.isra.22 0.188% 11172
try_charge 0.150% 8928
commit_charge 0.141% 8388
get_mem_cgroup_from_mm 0.121% 7184

It's showing 6.64% overhead in memcontrol.c when no memcgs are in
use. Applying the patch and disabling memcg reduces this to 0.48%

/usr/src/linux-4.0-nomemcg-v1r1/mm/memcontrol.c 0.4834 27511
mem_cgroup_page_lruvec 0.161% 9172
mem_cgroup_update_lru_size 0.154% 8794
mem_cgroup_try_charge 0.126% 7194
mem_cgroup_commit_charge 0.041% 2351

Note that it's not very visible from headline performance figures

pft faults
4.0.0 4.0.0
vanilla nomemcg-v1
Hmean faults/cpu-1 1443258.1051 ( 0.00%) 1530574.6033 ( 6.05%)
Hmean faults/cpu-3 1340385.9270 ( 0.00%) 1375156.5834 ( 2.59%)
Hmean faults/cpu-5 875599.0222 ( 0.00%) 876217.9211 ( 0.07%)
Hmean faults/cpu-7 601146.6726 ( 0.00%) 599068.4360 ( -0.35%)
Hmean faults/cpu-8 510728.2754 ( 0.00%) 509887.9960 ( -0.16%)
Hmean faults/sec-1 1432084.7845 ( 0.00%) 1518566.3541 ( 6.04%)
Hmean faults/sec-3 3943818.1437 ( 0.00%) 4036918.0217 ( 2.36%)
Hmean faults/sec-5 3877573.5867 ( 0.00%) 3922745.9207 ( 1.16%)
Hmean faults/sec-7 3991832.0418 ( 0.00%) 3990670.8481 ( -0.03%)
Hmean faults/sec-8 3987189.8167 ( 0.00%) 3978842.8107 ( -0.21%)

Low thread counts get a boost but it's within noise as memcg overhead does
not dominate. It's not obvious at all at higher thread counts as other
factors cause more problems. The overall breakdown of CPU usage looks like

4.0.0 4.0.0
vanilla nomemcg-v1
User 41.45 41.11
System 410.19 404.76
Elapsed 130.33 126.30

Despite the relative unimportance, there is at least some justification
for disabling memcg by default.

Signed-off-by: Mel Gorman <mgorman@suse.de>
---
Documentation/kernel-parameters.txt | 4 ++++
init/Kconfig | 15 +++++++++++++++
kernel/cgroup.c | 20 ++++++++++++++++----
mm/memcontrol.c | 3 +++
4 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index bfcb1a62a7b4..4f264f906816 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -591,6 +591,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
cut the overhead, others just disable the usage. So
only cgroup_disable=memory is actually worthy}

+ cgroup_enable= [KNL] Enable a particular controller
+ Similar to cgroup_disable except that it enables
+ controllers that are disabled by default.
+
checkreqprot [SELINUX] Set initial checkreqprot flag value.
Format: { "0" | "1" }
See security/selinux/Kconfig help text.
diff --git a/init/Kconfig b/init/Kconfig
index f5dbc6d4261b..819b6cc05cba 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -990,6 +990,21 @@ config MEMCG
Provides a memory resource controller that manages both anonymous
memory and page cache. (See Documentation/cgroups/memory.txt)

+config MEMCG_DEFAULT_ENABLED
+ bool "Automatically enable memory resource controller"
+ default y
+ depends on MEMCG
+ help
+ The memory controller has some overhead even if idle as resource
+ usage must be tracked in case a group is created and a process
+ migrated. As users may not be aware of this and the cgroup_disable=
+ option, this config option controls whether it is enabled by
+ default. It is assumed that someone that requires the controller
+ can find the cgroup_enable= switch.
+
+ Say N if unsure. This is default Y to preserve oldconfig and
+ historical behaviour.
+
config MEMCG_SWAP
bool "Memory Resource Controller Swap Extension"
depends on MEMCG && SWAP
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 29a7b2cc593e..0e79db55bf1a 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -5370,7 +5370,7 @@ out_free:
kfree(pathbuf);
}

-static int __init cgroup_disable(char *str)
+static int __init __cgroup_set_state(char *str, bool disabled)
{
struct cgroup_subsys *ss;
char *token;
@@ -5382,16 +5382,28 @@ static int __init cgroup_disable(char *str)

for_each_subsys(ss, i) {
if (!strcmp(token, ss->name)) {
- ss->disabled = 1;
- printk(KERN_INFO "Disabling %s control group"
- " subsystem\n", ss->name);
+ ss->disabled = disabled;
+ printk(KERN_INFO "Setting %s control group"
+ " subsystem %s\n", ss->name,
+ disabled ? "disabled" : "enabled");
break;
}
}
}
return 1;
}
+
+static int __init cgroup_disable(char *str)
+{
+ return __cgroup_set_state(str, true);
+}
+
+static int __init cgroup_enable(char *str)
+{
+ return __cgroup_set_state(str, false);
+}
__setup("cgroup_disable=", cgroup_disable);
+__setup("cgroup_enable=", cgroup_enable);

static int __init cgroup_set_legacy_files_on_dfl(char *str)
{
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b34ef4a32a3b..ce171ba16949 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5391,6 +5391,9 @@ struct cgroup_subsys memory_cgrp_subsys = {
.dfl_cftypes = memory_files,
.legacy_cftypes = mem_cgroup_legacy_files,
.early_init = 0,
+#ifndef CONFIG_MEMCG_DEFAULT_ENABLED
+ .disabled = 1,
+#endif
};

/**