[RFC PATCH 3/3] mm/memcg: Allow the task_obj optimization only on non-PREEMPTIBLE kernels.

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: cgroups@vger.kernel.org, linux-mm@kvack.org
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@kernel.org>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Waiman Long <longman@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: [RFC PATCH 3/3] mm/memcg: Allow the task_obj optimization only on non-PREEMPTIBLE kernels.
Date: Wed, 22 Dec 2021 12:41:11 +0100	[thread overview]
Message-ID: <20211222114111.2206248-4-bigeasy@linutronix.de> (raw)
In-Reply-To: <20211222114111.2206248-1-bigeasy@linutronix.de>

Based on my understanding the optimisation with task_obj for in_task()
mask sense on non-PREEMPTIBLE kernels because preempt_disable()/enable()
is optimized away. This could be then restricted to !CONFIG_PREEMPTION kernel
instead to only PREEMPT_RT.
With CONFIG_PREEMPT_DYNAMIC a non-PREEMPTIBLE kernel can also be
configured but these kernels always have preempt_disable()/enable()
present so it probably makes no sense here for the optimisation.

Restrict the optimisation to !CONFIG_PREEMPTION kernels.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 mm/memcontrol.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1e76f26be2c15..92180f1aa9edc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2126,7 +2126,7 @@ struct memcg_stock_pcp {
 	local_lock_t stock_lock;
 	struct mem_cgroup *cached; /* this never be root cgroup */
 	unsigned int nr_pages;
-#ifndef CONFIG_PREEMPT_RT
+#ifndef CONFIG_PREEMPTION
 	/* Protects only task_obj */
 	local_lock_t task_obj_lock;
 	struct obj_stock task_obj;
@@ -2139,7 +2139,7 @@ struct memcg_stock_pcp {
 };
 static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock) = {
 	.stock_lock = INIT_LOCAL_LOCK(stock_lock),
-#ifndef CONFIG_PREEMPT_RT
+#ifndef CONFIG_PREEMPTION
 	.task_obj_lock = INIT_LOCAL_LOCK(task_obj_lock),
 #endif
 };
@@ -2228,7 +2228,7 @@ static void drain_local_stock(struct work_struct *dummy)
 	 * drain_stock races is that we always operate on local CPU stock
 	 * here with IRQ disabled
 	 */
-#ifndef CONFIG_PREEMPT_RT
+#ifndef CONFIG_PREEMPTION
 	local_lock(&memcg_stock.task_obj_lock);
 	old = drain_obj_stock(&this_cpu_ptr(&memcg_stock)->task_obj, NULL);
 	local_unlock(&memcg_stock.task_obj_lock);
@@ -2837,7 +2837,7 @@ static inline struct obj_stock *get_obj_stock(unsigned long *pflags,
 {
 	struct memcg_stock_pcp *stock;
 
-#ifndef CONFIG_PREEMPT_RT
+#ifndef CONFIG_PREEMPTION
 	if (likely(in_task())) {
 		*pflags = 0UL;
 		*stock_pcp = NULL;
@@ -2855,7 +2855,7 @@ static inline struct obj_stock *get_obj_stock(unsigned long *pflags,
 static inline void put_obj_stock(unsigned long flags,
 				 struct memcg_stock_pcp *stock_pcp)
 {
-#ifndef CONFIG_PREEMPT_RT
+#ifndef CONFIG_PREEMPTION
 	if (likely(!stock_pcp))
 		local_unlock(&memcg_stock.task_obj_lock);
 	else
@@ -3267,7 +3267,7 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 {
 	struct mem_cgroup *memcg;
 
-#ifndef CONFIG_PREEMPT_RT
+#ifndef CONFIG_PREEMPTION
 	if (in_task() && stock->task_obj.cached_objcg) {
 		memcg = obj_cgroup_memcg(stock->task_obj.cached_objcg);
 		if (memcg && mem_cgroup_is_descendant(memcg, root_memcg))
-- 
2.34.1


