Re: [PATCH v2 3/4] mm/memcg: Protect per-CPU counter by disabling preemption on PREEMPT_RT where needed.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2022-02-14 11:46:00 [-0500], Johannes Weiner wrote:
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index c1caa662946dc..466466f285cea 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -705,6 +705,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
> >  	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
> >  	memcg = pn->memcg;
> >  
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > +		preempt_disable();
> >  	/* Update memcg */
> >  	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
> >  
> > @@ -712,6 +714,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
> >  	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
> >  
> >  	memcg_rstat_updated(memcg, val);
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > +		preempt_enable();
> >  }
> 
> I notice you didn't annoate __mod_memcg_state(). I suppose that is
> because it's called with explicit local_irq_disable(), and that
> disables preemption on rt? And you only need another preempt_disable()
> for stacks that rely on coming from spin_lock_irq(save)?

Correct. The code is not used in_hardirq() on PREEMPT_RT so
preempt_disable() is sufficient. I didn't bother to replace all
local_irq_save() with preempt_disable() since it is probably not worth
it.
And yes: spin_lock_irq() does not disable interrupts so I need something
here to ensure that the RMW operation is not interrupted.

> That makes sense, but it's difficult to maintain. It'll easily break
> if somebody adds more memory accounting sites that may also rely on an
> irq-disabled spinlock somewhere.
> 
> So better to make this an unconditional locking protocol:
> 
> static void memcg_stats_lock(void)
> {
> #ifdef CONFIG_PREEMPT_RT
> 	preempt_disable();
> #else
> 	VM_BUG_ON(!irqs_disabled());
> #endif
> }
> 
> static void memcg_stats_unlock(void)
> {
> #ifdef CONFIG_PREEMPT_RT
> 	preempt_enable();
> #endif
> }
> 
> and always use these around the counter updates.

Something like the following perhaps? I didn't add anything to
__mod_memcg_state() since it has no users besides the one which does
local_irq_save().

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c1caa662946dc..69130a5fe3d51 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -629,6 +629,28 @@ static DEFINE_SPINLOCK(stats_flush_lock);
 static DEFINE_PER_CPU(unsigned int, stats_updates);
 static atomic_t stats_flush_threshold = ATOMIC_INIT(0);
 
+/*
+ * Accessors to ensure that preemption is disabled on PREEMPT_RT because it can
+ * not rely on this as part of an acquired spinlock_t lock. These functions are
+ * never used in hardirq context on PREEMPT_RT and therefore disabling preemtion
+ * is sufficient.
+ */
+static void memcg_stats_lock(void)
+{
+#ifdef CONFIG_PREEMPT_RT
+      preempt_disable();
+#else
+      VM_BUG_ON(!irqs_disabled());
+#endif
+}
+
+static void memcg_stats_unlock(void)
+{
+#ifdef CONFIG_PREEMPT_RT
+      preempt_enable();
+#endif
+}
+
 static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
 {
 	unsigned int x;
@@ -705,6 +727,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
 	memcg = pn->memcg;
 
+	memcg_stats_lock();
 	/* Update memcg */
 	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
 
@@ -712,6 +735,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
 
 	memcg_rstat_updated(memcg, val);
+	memcg_stats_unlock();
 }
 
 /**
@@ -794,8 +818,10 @@ void __count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
 	if (mem_cgroup_disabled())
 		return;
 
+	memcg_stats_lock();
 	__this_cpu_add(memcg->vmstats_percpu->events[idx], count);
 	memcg_rstat_updated(memcg, count);
+	memcg_stats_unlock();
 }
 
 static unsigned long memcg_events(struct mem_cgroup *memcg, int event)
@@ -7149,8 +7175,9 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
 	 * important here to have the interrupts disabled because it is the
 	 * only synchronisation we have for updating the per-CPU variables.
 	 */
-	VM_BUG_ON(!irqs_disabled());
+	memcg_stats_lock();
 	mem_cgroup_charge_statistics(memcg, -nr_entries);
+	memcg_stats_unlock();
 	memcg_check_events(memcg, page_to_nid(page));
 
 	css_put(&memcg->css);
-- 
2.34.1