[PATCH-next v3] mm/memcg: Properly handle memcg_stock access for PREEMPT_RT

Waiman Long
2021-12-14T14:44:12Z

Direct calls to local_irq_{save/restore}() and preempt_{enable/disable}()
are not appropriate for PREEMPT_RT. To provide better PREEMPT_RT support,
change local_irq_{save/restore}() to local_lock_irq{save/restore}() and
add a local_lock_t to struct memcg_stock_pcp to cover the whole
structure including the embedded obj_stock structures.

Also disable the task and interrupt context optimization for obj_stock as
there will be no performance gain in the case of PREEMPT_RT. In this case,
task obj_stock will be there but remain unused and preempt_{enable/disable}()
will not be called for PREEMPT_RT.

Note that preempt_enable() and preempt_disable() in get_obj_stock() and
put_obj_stock() are not replaced by local_lock() and local_unlock() as it
is possible that a task accessing task_obj may get interrupted and then
access irq_obj concurrently. So using local_lock for task_obj access
may cause lockdep splat. Using separate local locks will complicate the
interaction between obj_stock and the embedding memcg_stock_pcp
structures.

Signed-off-by: Waiman Long <longman@redhat.com>
---
mm/memcontrol.c | 51 +++++++++++++++++++++++++++++++------------------
1 file changed, 32 insertions(+), 19 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a09a7d2e0b1b..2c690eceda54 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2096,7 +2096,12 @@ struct obj_stock {
#endif
};

+/*
+ * The local_lock protects the whole memcg_stock_pcp structure including
+ * the embedded obj_stock structures.
+ */
struct memcg_stock_pcp {
+ local_lock_t lock;
struct mem_cgroup *cached; /* this never be root cgroup */
unsigned int nr_pages;
struct obj_stock task_obj;
@@ -2145,7 +2150,7 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
if (nr_pages > MEMCG_CHARGE_BATCH)
return ret;

- local_irq_save(flags);
+ local_lock_irqsave(&memcg_stock.lock, flags);

stock = this_cpu_ptr(&memcg_stock);
if (memcg == stock->cached && stock->nr_pages >= nr_pages) {
@@ -2153,7 +2158,7 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
ret = true;
}

- local_irq_restore(flags);
+ local_unlock_irqrestore(&memcg_stock.lock, flags);

return ret;
}
@@ -2189,7 +2194,7 @@ 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
*/
- local_irq_save(flags);
+ local_lock_irqsave(&memcg_stock.lock, flags);

stock = this_cpu_ptr(&memcg_stock);
drain_obj_stock(&stock->irq_obj);
@@ -2198,7 +2203,7 @@ static void drain_local_stock(struct work_struct *dummy)
drain_stock(stock);
clear_bit(FLUSHING_CACHED_CHARGE, &stock->flags);

- local_irq_restore(flags);
+ local_unlock_irqrestore(&memcg_stock.lock, flags);
}

/*
@@ -2210,7 +2215,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
struct memcg_stock_pcp *stock;
unsigned long flags;

- local_irq_save(flags);
+ local_lock_irqsave(&memcg_stock.lock, flags);

stock = this_cpu_ptr(&memcg_stock);
if (stock->cached != memcg) { /* reset if necessary */
@@ -2223,7 +2228,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
if (stock->nr_pages > MEMCG_CHARGE_BATCH)
drain_stock(stock);

- local_irq_restore(flags);
+ local_unlock_irqrestore(&memcg_stock.lock, flags);
}

/*
@@ -2779,29 +2784,34 @@ static struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg)
* which is cheap in non-preempt kernel. The interrupt context object stock
* can only be accessed after disabling interrupt. User context code can
* access interrupt object stock, but not vice versa.
+ *
+ * This task and interrupt context optimization is disabled for PREEMPT_RT
+ * as there is no performance gain in this case and changes will be made to
+ * irq_obj only.
+ *
+ * For non-PREEMPT_RT, we are not replacing preempt_disable() by local_lock()
+ * as nesting of task_obj and irq_obj are allowed which may cause lockdep
+ * splat if local_lock() is used. Using separate local locks will complicate
+ * the interaction between obj_stock and the broader memcg_stock object.
*/
static inline struct obj_stock *get_obj_stock(unsigned long *pflags)
{
- struct memcg_stock_pcp *stock;
-
- if (likely(in_task())) {
+ if (likely(in_task()) && !IS_ENABLED(CONFIG_PREEMPT_RT)) {
*pflags = 0UL;
preempt_disable();
- stock = this_cpu_ptr(&memcg_stock);
- return &stock->task_obj;
+ return this_cpu_ptr(&memcg_stock.task_obj);
}

- local_irq_save(*pflags);
- stock = this_cpu_ptr(&memcg_stock);
- return &stock->irq_obj;
+ local_lock_irqsave(&memcg_stock.lock, *pflags);
+ return this_cpu_ptr(&memcg_stock.irq_obj);
}

static inline void put_obj_stock(unsigned long flags)
{
- if (likely(in_task()))
+ if (likely(in_task()) && !IS_ENABLED(CONFIG_PREEMPT_RT))
preempt_enable();
else
- local_irq_restore(flags);
+ local_unlock_irqrestore(&memcg_stock.lock, flags);
}

/*
@@ -7088,9 +7098,12 @@ static int __init mem_cgroup_init(void)
cpuhp_setup_state_nocalls(CPUHP_MM_MEMCQ_DEAD, "mm/memctrl:dead", NULL,
memcg_hotplug_cpu_dead);

- for_each_possible_cpu(cpu)
- INIT_WORK(&per_cpu_ptr(&memcg_stock, cpu)->work,
- drain_local_stock);
+ for_each_possible_cpu(cpu) {
+ struct memcg_stock_pcp *stock = per_cpu_ptr(&memcg_stock, cpu);
+
+ INIT_WORK(&stock->work, drain_local_stock);
+ local_lock_init(&stock->lock);
+ }

for_each_node(node) {
struct mem_cgroup_tree_per_node *rtpn;
--
2.27.0