linux-kernel - [PATCH v2 1/5] mm/memcontrol: Align percpu memcg

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Message-Id: <20230125073502.743446-2-leobras@redhat.com>
Date:   Wed, 25 Jan 2023 04:34:58 -0300
From:   Leonardo Bras <leobras@...hat.com>
To:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Leonardo Bras <leobras@...hat.com>, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/5] mm/memcontrol: Align percpu memcg_stock to cache

When a struct smaller than a cacheline has an instance that is not aligned
to cache block size, it could happen that data can be spread across two
cachelines.

For memcg_stock this could mean the need to fetch and get cache-exclusivity
in 2 cachelines instead of 1 when we bounce the cacheline between local
cpu functions and drain_all_stock(), which does remote read/write.

This could also mean some false-sharing costs being paid due to the
cacheline being shared between 2 unrelated structures.

Avoid this issue by getting memcg_stock cacheline-aligned.

Signed-off-by: Leonardo Bras <leobras@...hat.com>
---
 mm/memcontrol.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ab457f0394ab6..f8e86b88b3c7a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2188,7 +2188,8 @@ struct memcg_stock_pcp {
 	unsigned long flags;
 #define FLUSHING_CACHED_CHARGE	0
 };
-static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock) = {
+
+static DEFINE_PER_CPU_SHARED_ALIGNED(struct memcg_stock_pcp, memcg_stock) = {
 	.stock_lock = INIT_LOCAL_LOCK(stock_lock),
 };
 static DEFINE_MUTEX(percpu_charge_mutex);
-- 
2.39.1