[RFC 0/4] bypass charges if memcg is not used

[Glauber Costa]

For the special case that memcg is compiled in (a quite common case) but not in
use (also a common case), we are still seeing a huge performance impact. Mel
Gorman demonstrated in the Kernel Summit, mm mini summit, that depending on the
hardware, this can vary between 6 and 15 %.

I am proposing in this initial patch, that we take the strategy of bypassing
the memcg code with static branches. We can flip it on when the first memcg
gets created. Up to that moment, we'll mostly have a bunch of nops.

This patch also defers the call to page_cgroup_init() to that moment. This means
that the memory used by page_cgroup structure won't be wasted until we really
need it. We can do the same with the memory used for swap, if needed.

There are many edges to be trimmed, but I wanted to send this early to collect
feedback. I coded this enough to get numbers out of it. I tested it in a 24-way
2-socket Intel box, 24 Gb mem. I used Mel Gorman's pft test, that he used to
demonstrate this problem back in the Kernel Summit. There are three kernels:

nomemcg  : memcg compile disabled.
base     : memcg enabled, patch not applied.
bypassed : memcg enabled, with patch applied.

                base    bypassed
User          109.12      105.64
System       1646.84     1597.98
Elapsed       229.56      215.76

             nomemcg    bypassed
User          104.35      105.64
System       1578.19     1597.98
Elapsed       212.33      215.76

So as one can see, the difference between base and nomemcg in terms
of both system time and elapsed time is quite drastic, and consistent
with the figures shown by Mel Gorman in the Kernel summit. This is a
~ 7 % drop in performance, just by having memcg enabled. memcg functions
appear heavily in the profiles, even if all tasks lives in the root
memcg.

With bypassed kernel, we drop this down to 1.5 %, which starts to fall
in the acceptable range. More investigation is needed to see if we can
claim that last percent back, but I believe at last part of it should
be.


Glauber Costa (4):
  memcg: provide root figures from system totals
  memcg: make it suck faster
  memcg: do not call page_cgroup_init at system_boot
  memcg: do not walk all the way to the root for memcg

 include/linux/memcontrol.h  | 56 ++++++++++++++++++++++++++------
 include/linux/page_cgroup.h | 20 +++++++++---
 init/main.c                 |  1 -
 mm/memcontrol.c             | 78 ++++++++++++++++++++++++++++++++++++++++-----
 mm/page_cgroup.c            | 44 +++++++++++++++++++------
 5 files changed, 165 insertions(+), 34 deletions(-)

-- 
1.7.11.4

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