On systems that run FIFO:1 applications that busy loop,
any SCHED_OTHER task that attempts to execute
on such a CPU (such as work threads) will not
be scheduled, which leads to system hangs.
Commit d479960e44f27e0e52ba31b21740b703c538027c ("mm: disable LRU
pagevec during the migration temporarily") relies on
queueing work items on all online CPUs to ensure visibility
of lru_disable_count.
To fix this, replace the usage of work items with synchronize_rcu,
which provides the same guarantees.
Readers of lru_disable_count are protected by either disabling
preemption or rcu_read_lock:
preempt_disable, local_irq_disable [bh_lru_lock()]
rcu_read_lock [rt_spin_lock CONFIG_PREEMPT_RT]
preempt_disable [local_lock !CONFIG_PREEMPT_RT]
Since v5.1 kernel, synchronize_rcu() is guaranteed to wait on
preempt_disable() regions of code. So any CPU which sees
lru_disable_count = 0 will have exited the critical
section when synchronize_rcu() returns.
Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
Reviewed-by: Nicolas Saenz Julienne <nsaenzju@xxxxxxxxxx>
Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
---
v5: changelog improvements (Andrew Morton)
v4: improve comment clarity, mention synchronize_rcu guarantees
on v5.1 (Andrew Morton /
Paul E. McKenney)
v3: update stale comment (Nicolas Saenz Julienne)
v2: rt_spin_lock calls rcu_read_lock, no need
to add it before local_lock on swap.c (Nicolas Saenz Julienne)
diff --git a/mm/swap.c b/mm/swap.c
index bcf3ac288b56..b5ee163daa66 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -831,8 +831,7 @@ inline void __lru_add_drain_all(bool force_all_cpus)
for_each_online_cpu(cpu) {
struct work_struct *work = &per_cpu(lru_add_drain_work, cpu);
- if (force_all_cpus ||
- pagevec_count(&per_cpu(lru_pvecs.lru_add, cpu)) ||
+ if (pagevec_count(&per_cpu(lru_pvecs.lru_add, cpu)) ||
data_race(pagevec_count(&per_cpu(lru_rotate.pvec, cpu))) ||
pagevec_count(&per_cpu(lru_pvecs.lru_deactivate_file, cpu)) ||
pagevec_count(&per_cpu(lru_pvecs.lru_deactivate, cpu)) ||
@@ -876,15 +875,21 @@ atomic_t lru_disable_count = ATOMIC_INIT(0);
void lru_cache_disable(void)
{
atomic_inc(&lru_disable_count);
-#ifdef CONFIG_SMP
/*
- * lru_add_drain_all in the force mode will schedule draining on
- * all online CPUs so any calls of lru_cache_disabled wrapped by
- * local_lock or preemption disabled would be ordered by that.
- * The atomic operation doesn't need to have stronger ordering
- * requirements because that is enforced by the scheduling
- * guarantees.
+ * Readers of lru_disable_count are protected by either disabling
+ * preemption or rcu_read_lock:
+ *
+ * preempt_disable, local_irq_disable [bh_lru_lock()]
+ * rcu_read_lock [rt_spin_lock CONFIG_PREEMPT_RT]
+ * preempt_disable [local_lock !CONFIG_PREEMPT_RT]
+ *
+ * Since v5.1 kernel, synchronize_rcu() is guaranteed to wait on
+ * preempt_disable() regions of code. So any CPU which sees
+ * lru_disable_count = 0 will have exited the critical
+ * section when synchronize_rcu() returns.
*/
+ synchronize_rcu();
+#ifdef CONFIG_SMP
__lru_add_drain_all(true);
#else
lru_add_and_bh_lrus_drain();
