mm: lru_cache_disable: replace work queue synchronization with synchronize_rcu

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 d479960e44f27e0e5 ("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.

Link: https://lkml.kernel.org/r/Yin7hDxdt0s/x+fp@fuller.cnet
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Reviewed-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/swap.c b/mm/swap.c
index e9786c4..754520b 100644 (file)
--- 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();