Re: [PATCH] mm/memcontrol: avoid stuck FLUSHING_CACHED_CHARGE bit on isolated cpus

[Michal Hocko]

On Fri 28-08-26 09:46:21, Rik van Riel wrote:
> drain_all_stock() can leave FLUSHING_CACHED_CHARGE set after the
> work is dropped.  It sets the bit before checking isolation and
> schedule_drain_work() checks isolation and queues in a separate RCU
> critical section, so housekeeping_update()'s synchronize_rcu() can
> race the second check.
> 
> drain_local_stock() only clears the bit for work that ran, so the bit
> remains set and the stock is never drained again.
> 
> Reorganize the drain_all_stock() loop, reducing nesting, splitting
> out local vs remote cpu handling, and skipping everything on isolated
> cpus, which solves the stuck FLUSHING_CACHED_CHARGE flag.

Is there any reason why we cannot simply clear the flag if the work is
not scheduled?
---

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 69b37f63a307..907ac63e5067 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2261,8 +2261,10 @@ static bool is_memcg_drain_needed(struct memcg_stock_pcp *stock,
 	return flush;
 }
 
-static void schedule_drain_work(int cpu, struct work_struct *work)
+static bool schedule_drain_work(int cpu, struct work_struct *work)
 {
+	int ret = false;
+
 	/*
 	 * Protect housekeeping cpumask read and work enqueue together
 	 * in the same RCU critical section so that later cpuset isolated
@@ -2270,8 +2272,12 @@ static void schedule_drain_work(int cpu, struct work_struct *work)
 	 * pending work on newly isolated CPUs.
 	 */
 	guard(rcu)();
-	if (!cpu_is_isolated(cpu))
-		queue_work_on(cpu, memcg_wq, work);
+	if (!cpu_is_isolated(cpu)) {
+		queue_work_on(cpu, memcg_wq, &memcg_st->work);
+		ret = true;
+	}
+
+	return ret;
 }
 
 /*
@@ -2303,8 +2309,8 @@ void drain_all_stock(struct mem_cgroup *root_memcg)
 				      &memcg_st->flags)) {
 			if (cpu == curcpu)
 				drain_local_memcg_stock(&memcg_st->work);
-			else
-				schedule_drain_work(cpu, &memcg_st->work);
+			else if (!schedule_drain_work(cpu, &memcg_st->work))
+				clear_bit(FLUSHING_CACHED_CHARGE, &memcg_st->flags)
 		}
 
 		if (!test_bit(FLUSHING_CACHED_CHARGE, &obj_st->flags) &&
@@ -2313,8 +2319,8 @@ void drain_all_stock(struct mem_cgroup *root_memcg)
 				      &obj_st->flags)) {
 			if (cpu == curcpu)
 				drain_local_obj_stock(&obj_st->work);
-			else
-				schedule_drain_work(cpu, &obj_st->work);
+			else if (!schedule_drain_work(cpu, &obj_st->work))
+				clear_bit(FLUSHING_CACHED_CHARGE, &obj_st->flags);
 		}
 	}
 	migrate_enable();
-- 
Michal Hocko
SUSE Labs