Kernel test robot reported [1] performance regression for will-it-scale
test suite's page_fault2 test case for the commit 70a64b7919 ("memcg:
dynamically allocate lruvec_stats"). After inspection it seems like the
commit has unintentionally introduced false cache sharing.
After the commit the fields of mem_cgroup_per_node which get read on the
performance critical path share the cacheline with the fields which get
updated often on LRU page allocations or deallocations. This has caused
contention on that cacheline and the workloads which manipulates a lot of
LRU pages are regressed as reported by the test report.
The solution is to rearrange the fields of mem_cgroup_per_node such that
the false sharing is eliminated. Let's move all the read only pointers at
the start of the struct, followed by memcg-v1 only fields and at the end
fields which get updated often.
Experiment setup: Ran fallocate1, fallocate2, page_fault1, page_fault2 and
page_fault3 from the will-it-scale test suite inside a three level memcg
with /tmp mounted as tmpfs on two different machines, one a single numa
node and the other one, two node machine.
$ ./[testcase]_processes -t $NR_CPUS -s 50
Results for single node, 52 CPU machine:
Testcase base with-patch
fallocate1 1031081 1431291 (38.80 %)
fallocate2 1029993 1421421 (38.00 %)
page_fault1 2269440 3405788 (50.07 %)
page_fault2 2375799 3572868 (50.30 %)
page_fault3 28641143 28673950 ( 0.11 %)
Results for dual node, 80 CPU machine:
Testcase base with-patch
fallocate1 2976288 3641185 (22.33 %)
fallocate2 2979366 3638181 (22.11 %)
page_fault1 6221790 7748245 (24.53 %)
page_fault2 6482854 7847698 (21.05 %)
page_fault3 28804324 28991870 ( 0.65 %)
Link: https://lkml.kernel.org/r/20240528164050.2625718-1-shakeel.butt@linux.dev
Fixes: 70a64b7919 ("memcg: dynamically allocate lruvec_stats")
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reported-by: kernel test robot <oliver.sang@intel.com>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Feng Tang <feng.tang@intel.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Yin Fengwei <fengwei.yin@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
fefc6e6631