linux-kernel - [PATCH v2 5/5] mm/memcontrol: Remove flags from memcg_stock

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Date:   Wed, 25 Jan 2023 04:35:02 -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 5/5] mm/memcontrol: Remove flags from memcg_stock_pcp

The flags member of struct memcg_stock_pcp has only one used bit:
FLUSHING_CACHED_CHARGE

Both struct member and flag were created to avoid scheduling multiple
instances of kworkers running drain_local_stock() for a single cpu.

How could this scenario happen before:
- drain_all_stock() gets called, get ownership of percpu_charge_mutex,
  schedules a drain_local_stock() on cpu X, and drops ownership of
  percpu_charge_mutex.
- Another thread calls drain_all_stock(), get ownership of
  percpu_charge_mutex, schedules a drain_local_stock() on cpu X, ...

Since the stock draining is now performed by the thread running
drain_all_stock(), and happens before letting go of the
percpu_charge_mutex, there is no chance of another drain happening
between test_and_set_bit() and clear_bit(), so flags is now useless.

Remove the flags member of memcg_stock_pcp, its usages and the
FLUSHING_CACHED_CHARGE define.

Signed-off-by: Leonardo Bras <leobras@...hat.com>
---
 mm/memcontrol.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5b7f7c2e0232f..60712f69595e4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2183,9 +2183,6 @@ struct memcg_stock_pcp {
 	int nr_slab_reclaimable_b;
 	int nr_slab_unreclaimable_b;
 #endif
-
-	unsigned long flags;
-#define FLUSHING_CACHED_CHARGE	0
 };
 
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct memcg_stock_pcp, memcg_stock) = {
@@ -2281,7 +2278,6 @@ static void drain_stock_from(struct memcg_stock_pcp *stock)
 
 	old = drain_obj_stock(stock);
 	drain_stock(stock);
-	clear_bit(FLUSHING_CACHED_CHARGE, &stock->flags);
 
 	spin_unlock_irqrestore(&stock->stock_lock, flags);
 	if (old)
@@ -2351,8 +2347,7 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
 			flush = true;
 		rcu_read_unlock();
 
-		if (flush &&
-		    !test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
+		if (flush)
 			drain_stock_from(stock);
 	}
 	migrate_enable();
-- 
2.39.1