On Wed, May 22, 2024 at 8:48 PM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote:
>
> Kernel test robot reported [1] performance regression for will-it-scale
> test suite's page_fault2 test case for the commit 70a64b7919cb ("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 %)
Great analysis :)
>
> Fixes: 70a64b7919cb ("memcg: dynamically allocate lruvec_stats")
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-lkp/202405171353.b56b845-oliver.sang@xxxxxxxxx
> Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> ---
> include/linux/memcontrol.h | 18 ++++++++++--------
> 1 file changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 030d34e9d117..16efd9737be9 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -96,23 +96,25 @@ struct mem_cgroup_reclaim_iter {
> * per-node information in memory controller.
> */
> struct mem_cgroup_per_node {
> - struct lruvec lruvec;
> + /* Keep the read-only fields at the start */
> + struct mem_cgroup *memcg; /* Back pointer, we cannot */
> + /* use container_of */
>
> struct lruvec_stats_percpu __percpu *lruvec_stats_percpu;
> struct lruvec_stats *lruvec_stats;
> -
> - unsigned long lru_zone_size[MAX_NR_ZONES][NR_LRU_LISTS];
> -
> - struct mem_cgroup_reclaim_iter iter;
> -
> struct shrinker_info __rcu *shrinker_info;
>
> + /* memcg-v1 only stuff in middle */
> +
> struct rb_node tree_node; /* RB tree node */
> unsigned long usage_in_excess;/* Set to the value by which */
> /* the soft limit is exceeded*/
> bool on_tree;
> - struct mem_cgroup *memcg; /* Back pointer, we cannot */
> - /* use container_of */
Do we need CACHELINE_PADDING() here (or maybe make struct lruvec
cache-aligned) to make sure the false cacheline sharing doesn't happen
again with the fields below, or is the idea that the fields that get
read in hot paths (memcg, lruvec_stats_percpu, lruvec_stats) are far
at the top, and the memcg v1 elements in the middle act as a buffer?
IOW, is sharing between the fields below and memcg v1 fields okay
because they are not read in the hot path? If yes, I believe it's
worth a comment. It can be easily missed if the memcg v1 soft limit is
removed later for example.
> +
> + /* Fields which get updated often at the end. */
> + struct lruvec lruvec;
> + unsigned long lru_zone_size[MAX_NR_ZONES][NR_LRU_LISTS];
> + struct mem_cgroup_reclaim_iter iter;
> };
>
> struct mem_cgroup_threshold {
> --
> 2.43.0
>
