I'm seeing a pretty large regression in 3.17-rc2 vs 3.16 coming from the
memory cgroups code. This is on a kernel with cgroups enabled at
compile time, but not _used_ for anything. See the green lines in the
graph:
https://www.sr71.net/~dave/intel/regression-from-05b843012.png
The workload is a little parallel microbenchmark doing page faults:
> https://github.com/antonblanchard/will-it-scale/blob/master/tests/page_fault2.c
The hardware is an 8-socket Westmere box with 160 hardware threads. For
some reason, this does not affect the version of the microbenchmark
which is doing completely anonymous page faults.
I bisected it down to this commit:
> commit 05b8430123359886ef6a4146fba384e30d771b3f
> Author: Johannes Weiner <han...@cmpxchg.org>
> Date: Wed Aug 6 16:05:59 2014 -0700
>
> mm: memcontrol: use root_mem_cgroup res_counter
>
> Due to an old optimization to keep expensive res_counter changes at a
> minimum, the root_mem_cgroup res_counter is never charged; there is no
> limit at that level anyway, and any statistics can be generated on
> demand by summing up the counters of all other cgroups.
>
> However, with per-cpu charge caches, res_counter operations do not even
> show up in profiles anymore, so this optimization is no longer
> necessary.
>
> Remove it to simplify the code.
It does not revert cleanly because of the hunks below. The code in
those hunks was removed, so I tried running without properly merging
them and it spews warnings because counter->usage is seen going negative.
So, it doesn't appear we can quickly revert this.
> --- mm/memcontrol.c
> +++ mm/memcontrol.c
> @@ -3943,7 +3947,7 @@
> * replacement page, so leave it alone when phasing out the
> * page that is unused after the migration.
> */
> - if (!end_migration)
> + if (!end_migration && !mem_cgroup_is_root(memcg))
> mem_cgroup_do_uncharge(memcg, nr_pages, ctype);
>
> return memcg;
> @@ -4076,7 +4080,8 @@
> * We uncharge this because swap is freed. This memcg can
> * be obsolete one. We avoid calling css_tryget_online().
> */
> - res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
> + if (!mem_cgroup_is_root(memcg))
> + res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
> mem_cgroup_swap_statistics(memcg, false);
> css_put(&memcg->css);
> }
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