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 <hannes@xxxxxxxxxxx> > 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); > } -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
