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SubjectRe: [PATCH 3/3] mm: Fix missing mem cgroup soft limit tree updates
On Tue 09-02-21 12:29:47, Tim Chen wrote:
> On a per node basis, the mem cgroup soft limit tree on each node tracks
> how much a cgroup has exceeded its soft limit memory limit and sorts
> the cgroup by its excess usage. On page release, the trees are not
> updated right away, until we have gathered a batch of pages belonging to
> the same cgroup. This reduces the frequency of updating the soft limit tree
> and locking of the tree and associated cgroup.
>
> However, the batch of pages could contain pages from multiple nodes but
> only the soft limit tree from one node would get updated. Change the
> logic so that we update the tree in batch of pages, with each batch of
> pages all in the same mem cgroup and memory node. An update is issued for
> the batch of pages of a node collected till now whenever we encounter
> a page belonging to a different node.

I do agree with Johannes here. This shouldn't be done unconditionally
for all memcgs. Wouldn't it be much better to do the fix up in the
mem_cgroup_soft_reclaim path instead. Simply check the excess before
doing any reclaim?

Btw. have you seen this triggering a noticeable misbehaving? I would
expect this to have a rather small effect considering how many sources
of memcg_check_events we have.

Unless I have missed something this has been introduced by 747db954cab6
("mm: memcontrol: use page lists for uncharge batching"). Please add
Fixes tag as well if this is really worth fixing.

> Reviewed-by: Ying Huang <ying.huang@intel.com>
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> ---
> mm/memcontrol.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d72449eeb85a..f5a4a0e4e2ec 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6804,6 +6804,7 @@ struct uncharge_gather {
> unsigned long pgpgout;
> unsigned long nr_kmem;
> struct page *dummy_page;
> + int nid;
> };
>
> static inline void uncharge_gather_clear(struct uncharge_gather *ug)
> @@ -6849,7 +6850,9 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
> * exclusive access to the page.
> */
>
> - if (ug->memcg != page_memcg(page)) {
> + if (ug->memcg != page_memcg(page) ||
> + /* uncharge batch update soft limit tree on a node basis */
> + (ug->dummy_page && ug->nid != page_to_nid(page))) {
> if (ug->memcg) {
> uncharge_batch(ug);
> uncharge_gather_clear(ug);
> @@ -6869,6 +6872,7 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
> ug->pgpgout++;
>
> ug->dummy_page = page;
> + ug->nid = page_to_nid(page);
> page->memcg_data = 0;
> css_put(&ug->memcg->css);
> }
> --
> 2.20.1

--
Michal Hocko
SUSE Labs

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