[PATCH v3] mm/memcontrol: Avoid stuck FLUSHING_CACHED_CHARGE on isolated CPU

[PATCH v3] mm/memcontrol: Avoid stuck FLUSHING_CACHED_CHARGE on isolated CPU

[Hui Zhu]

From: Hui Zhu <zhuhui@kylinos.cn>

drain_all_stock() sets FLUSHING_CACHED_CHARGE before calling
schedule_drain_work() to queue per-CPU drain work.  When the target
CPU is isolated (cpu_is_isolated() == true), the work is silently
not queued, but FLUSHING_CACHED_CHARGE stays set.  Every subsequent
drain_all_stock() then sees the bit and skips this stock entirely,
so the entry is effectively pinned until something else on that CPU
runs drain_local_*_stock() and clears the bit -- which on a long-
isolated CPU may never happen.

The original idea was to actually perform the drain from the calling
CPU on behalf of the isolated one, by adding a lock around the
per-CPU stock so that a remote drainer could safely touch it.  In
practice this turned out to be intrusive: the stock data structures
and their fast paths (consume_stock(), refill_stock(), the obj_stock
helpers) are deliberately designed around current-CPU-only access,
and retrofitting cross-CPU serialisation onto them adds non-trivial
locking and PREEMPT_RT concerns for very little gain.

Looking at the actual amount of charge that can accumulate in a
single per-CPU stock, it is bounded and small, so leaving an
isolated CPU's stock undrained for a while is not a real problem.
The only real bug is that the stuck FLUSHING_CACHED_CHARGE bit
prevents future drain_all_stock() callers from re-attempting once
the CPU is no longer isolated.

Fix this minimally by clearing FLUSHING_CACHED_CHARGE when the work
could not be queued because the target CPU is isolated.  The cached
charge itself is left in place; it will be released the next time
the CPU runs drain_local_*_stock() (e.g. after leaving isolation,
or if the isolated CPU itself calls drain_all_stock() -- in that
case cpu == curcpu causes drain_local_memcg_stock() to be invoked
directly), and the next drain_all_stock() call is free to retry
instead of skipping the stock forever.

Fixes: 6a792697a53a ("memcg: do not drain charge pcp caches on remote isolated cpus")
Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
---
Changelog:
v3:
According to the comments of Shakeel Butt, keep the whole
schedule_drain_work() body under a single guard(rcu)() instead of
splitting it with scoped_guard(rcu){}.
v2:
According to the comments of Waiman Long, updated fixes.

 mm/memcontrol.c | 26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6dc4888a90f3..42a0d0d6284a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2256,7 +2256,8 @@ 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 void
+schedule_drain_work(int cpu, struct work_struct *work, unsigned long *flags)
 {
 	/*
 	 * Protect housekeeping cpumask read and work enqueue together
@@ -2265,8 +2266,21 @@ 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)) {
+		/*
+		 * The target CPU is isolated: the drain work was not
+		 * queued. Clear FLUSHING_CACHED_CHARGE so a future
+		 * drain_all_stock() call can retry instead of skipping
+		 * this stock forever.
+		 *
+		 * The cached charge itself is left untouched here; it
+		 * will be drained the next time drain_local_stock() runs
+		 * on this CPU (e.g. after it leaves isolation).
+		 */
+		clear_bit(FLUSHING_CACHED_CHARGE, flags);
+		return;
+	}
+	queue_work_on(cpu, memcg_wq, work);
 }
 
 /*
@@ -2299,7 +2313,8 @@ void drain_all_stock(struct mem_cgroup *root_memcg)
 			if (cpu == curcpu)
 				drain_local_memcg_stock(&memcg_st->work);
 			else
-				schedule_drain_work(cpu, &memcg_st->work);
+				schedule_drain_work(cpu, &memcg_st->work,
+						    &memcg_st->flags);
 		}
 
 		if (!test_bit(FLUSHING_CACHED_CHARGE, &obj_st->flags) &&
@@ -2309,7 +2324,8 @@ void drain_all_stock(struct mem_cgroup *root_memcg)
 			if (cpu == curcpu)
 				drain_local_obj_stock(&obj_st->work);
 			else
-				schedule_drain_work(cpu, &obj_st->work);
+				schedule_drain_work(cpu, &obj_st->work,
+						    &obj_st->flags);
 		}
 	}
 	migrate_enable();
-- 
2.43.0