>From 92aec48e3b2e21c3716654670a24890f34c58683 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Date: Sun, 23 Nov 2014 13:39:25 +0900
Subject: [PATCH 2/5] mm: Kill shrinker's global semaphore.
Currently register_shrinker()/unregister_shrinker() calls down_write()
while shrink_slab() calls down_read_trylock(). This implies that the OOM
killer becomes disabled because shrink_slab() pretends "we reclaimed some
slab memory" even if "no slab memory can be reclaimed" when somebody calls
register_shrinker()/unregister_shrinker() while one of shrinker functions
allocates memory and/or holds mutex which may take unpredictably long
duration to complete.
This patch replaces global semaphore with per a shrinker refcounter
so that shrink_slab() can respond "we could not reclaim slab memory"
when out_of_memory() needs to be called.
Before this patch, response time of addition/removal are unpredictable
when one of shrinkers are in use by shrink_slab(), nearly 0 otherwise.
After this patch, response time of addition is nearly 0. Response time of
removal remains unpredictable when the shrinker to remove is in use by
shrink_slab(), nearly two RCU grace periods otherwise.
Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
---
include/linux/shrinker.h | 4 +++
mm/vmscan.c | 78 ++++++++++++++++++++++++++++++++++--------------
2 files changed, 60 insertions(+), 22 deletions(-)
diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 68c0970..745246a 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -59,6 +59,10 @@ struct shrinker {
struct list_head list;
/* objs pending delete, per node */
atomic_long_t *nr_deferred;
+ /* Number of users holding reference to this object. */
+ atomic_t usage;
+ /* Used for handling concurrent unregistration tracing. */
+ struct list_head gc_list;
};
#define DEFAULT_SEEKS 2 /* A good number if you don't know better. */
diff --git a/mm/vmscan.c b/mm/vmscan.c
index dcb4707..54d2638 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -144,7 +144,7 @@ int vm_swappiness = 60;
unsigned long vm_total_pages;
static LIST_HEAD(shrinker_list);
-static DECLARE_RWSEM(shrinker_rwsem);
+static DEFINE_SPINLOCK(shrinker_list_lock);
#ifdef CONFIG_MEMCG
static bool global_reclaim(struct scan_control *sc)
@@ -208,9 +208,16 @@ int register_shrinker(struct shrinker *shrinker)
if (!shrinker->nr_deferred)
return -ENOMEM;
- down_write(&shrinker_rwsem);
- list_add_tail(&shrinker->list, &shrinker_list);
- up_write(&shrinker_rwsem);
+ /*
+ * Make it possible for list_for_each_entry_rcu(shrinker,
+ * &shrinker_list, list) in shrink_slab() to find this shrinker.
+ * We assume that this shrinker is not under unregister_shrinker()
+ * call.
+ */
+ atomic_set(&shrinker->usage, 0);
+ spin_lock(&shrinker_list_lock);
+ list_add_tail_rcu(&shrinker->list, &shrinker_list);
+ spin_unlock(&shrinker_list_lock);
return 0;
}
EXPORT_SYMBOL(register_shrinker);
@@ -220,9 +227,41 @@ EXPORT_SYMBOL(register_shrinker);
*/
void unregister_shrinker(struct shrinker *shrinker)
{
- down_write(&shrinker_rwsem);
- list_del(&shrinker->list);
- up_write(&shrinker_rwsem);
+ static LIST_HEAD(shrinker_gc_list);
+ struct shrinker *gc;
+ unsigned int i = 0;
+ int usage;
+
+ /*
+ * Make it impossible for shrinkers on shrinker_list and shrinkers
+ * on shrinker_gc_list to call atomic_inc(&shrinker->usage) after
+ * RCU grace period expires.
+ */
+ spin_lock(&shrinker_list_lock);
+ list_del_rcu(&shrinker->list);
+ list_for_each_entry(gc, &shrinker_gc_list, gc_list) {
+ if (gc->list.next == &shrinker->list)
+ rcu_assign_pointer(gc->list.next, shrinker->list.next);
+ }
+ list_add_tail(&shrinker->gc_list, &shrinker_gc_list);
+ spin_unlock(&shrinker_list_lock);
+ synchronize_rcu();
+ /*
+ * Wait for readers until RCU grace period expires after the last
+ * atomic_dec(&shrinker->usage). Warn if it is taking too long.
+ */
+ while (1) {
+ usage = atomic_read(&shrinker->usage);
+ if (!usage)
+ break;
+ msleep(100);
+ WARN(++i % 600 == 0, "Shrinker usage=%d\n", usage);
+ }
+ synchronize_rcu();
+ /* Now, nobody is using this shrinker. */
+ spin_lock(&shrinker_list_lock);
+ list_del(&shrinker->gc_list);
+ spin_unlock(&shrinker_list_lock);
kfree(shrinker->nr_deferred);
}
EXPORT_SYMBOL(unregister_shrinker);
@@ -369,23 +408,15 @@ unsigned long shrink_slab(struct shrink_control *shrinkctl,
if (nr_pages_scanned == 0)
nr_pages_scanned = SWAP_CLUSTER_MAX;
- if (!down_read_trylock(&shrinker_rwsem)) {
- /*
- * If we would return 0, our callers would understand that we
- * have nothing else to shrink and give up trying. By returning
- * 1 we keep it going and assume we'll be able to shrink next
- * time.
- */
- freed = 1;
- goto out;
- }
-
- list_for_each_entry(shrinker, &shrinker_list, list) {
+ rcu_read_lock();
+ list_for_each_entry_rcu(shrinker, &shrinker_list, list) {
+ atomic_inc(&shrinker->usage);
+ rcu_read_unlock();
if (!(shrinker->flags & SHRINKER_NUMA_AWARE)) {
shrinkctl->nid = 0;
freed += shrink_slab_node(shrinkctl, shrinker,
nr_pages_scanned, lru_pages);
- continue;
+ goto next_entry;
}
for_each_node_mask(shrinkctl->nid, shrinkctl->nodes_to_scan) {
@@ -394,9 +425,12 @@ unsigned long shrink_slab(struct shrink_control *shrinkctl,
nr_pages_scanned, lru_pages);
}
+next_entry:
+ rcu_read_lock();
+ atomic_dec(&shrinker->usage);
}
- up_read(&shrinker_rwsem);
-out:
+ rcu_read_unlock();
+
cond_resched();
return freed;
}
--
1.8.3.1
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