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Date: Wed, 21 May 2014 20:57:41 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: akpm@...ux-foundation.org, riel@...hat.com, dchinner@...hat.com,
kosaki.motohiro@...fujitsu.com
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH] mm/vmscan: Kill shrinker's global semaphore.
I'm trying to identify the cause of stalls with 100% CPU usage when
a certain type of memory pressure is given. I noticed that some of
shrinker functions may take unpredictably long duration to complete.
Since shrinker list is protected by a global semaphore, I came to worry
that (e.g.) umount operation which involves deactivate_locked_super()
might become unresponding for very long time. Maybe we want to kill
global semaphore?
----------
>>From ba9c6a433377b92ded32217176a77e00c4ca488b Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Date: Wed, 21 May 2014 15:05:17 +0900
Subject: [PATCH] mm/vmscan: Kill shrinker's global semaphore.
Currently register_shrinker()/unregister_shrinker() calls
down_write(&shrinker_rwsem) while shrink_slab() calls
down_read_trylock(&shrinker_rwsem).
While it is expected that shrinker functions do not allocate memory,
there are shrinker functions that allocate memory and/or hold mutex
which may take unpredictably long duration to complete.
Therefore, if one of shrinkers takes too long time (maybe due to a bug),
other shrinkers cannot be registered or unregistered due to use of
global semaphore.
This patch replaces global semaphore with per a shrinker refcounter.
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 RCU grace period otherwise.
Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
---
include/linux/shrinker.h | 4 ++
mm/vmscan.c | 100 ++++++++++++++++++++++++++++++++++++-----------
2 files changed, 82 insertions(+), 22 deletions(-)
diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 68c0970..c16b0aa 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 GC 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 32c661d..c0db2fc 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -133,7 +133,8 @@ int vm_swappiness = 60;
unsigned long vm_total_pages; /* The total number of pages which the VM controls */
static LIST_HEAD(shrinker_list);
-static DECLARE_RWSEM(shrinker_rwsem);
+static LIST_HEAD(shrinker_gc_list);
+static DEFINE_SPINLOCK(shrinker_list_lock);
#ifdef CONFIG_MEMCG
static bool global_reclaim(struct scan_control *sc)
@@ -196,9 +197,17 @@ 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);
+ list_add_tail(&shrinker->gc_list, &shrinker_gc_list);
+ spin_unlock(&shrinker_list_lock);
return 0;
}
EXPORT_SYMBOL(register_shrinker);
@@ -208,9 +217,61 @@ EXPORT_SYMBOL(register_shrinker);
*/
void unregister_shrinker(struct shrinker *shrinker)
{
- down_write(&shrinker_rwsem);
- list_del(&shrinker->list);
- up_write(&shrinker_rwsem);
+ struct shrinker *gc;
+ unsigned int i = 0;
+
+ /*
+ * For explanation, this function refers shrinker objects like
+ * shrinker[x-2], shrinker[x-1], shrinker[x] and assumes that
+ *
+ * shrinker_list.next == &shrinker[x-2].list
+ * shrinker[x-2].list.prev == &shrinker_list
+ * shrinker[x-2].list.next == &shrinker[x].list
+ * shrinker[x].list.prev == &shrinker[x-2].list
+ * shrinker[x].list.next == &shrinker_list
+ * shrinker_list.prev == &shrinker[x].list
+ * shrinker[x-1].list.prev == LIST_POISON2
+ * shrinker[x-1].list.next == &shrinker[x].list
+ *
+ * when this function is called for deleting shrinker[x] after
+ * this function is called for deleting shrinker[x-1].
+ *
+ * First, make it impossible for list_for_each_entry_rcu(shrinker,
+ * &shrinker_list, list) in shrink_slab() to find shrinker[x]
+ * after RCU grace period. Note that we need to do
+ *
+ * shrinker[x-1].list.next = shrinker[x].list.next
+ *
+ * when we do
+ *
+ * shrinker[x-2].list.next = shrinker[x].list.next
+ *
+ * because shrinker[x-1] may be still in use.
+ */
+ spin_lock(&shrinker_list_lock);
+ list_del_rcu(&shrinker->list);
+ list_for_each_entry(gc, &shrinker_gc_list, list) {
+ if (gc->list.next == &shrinker->list)
+ gc->list.next = shrinker->list.next;
+ }
+ spin_unlock(&shrinker_list_lock);
+ synchronize_rcu();
+ /*
+ * Wait for readers who acquired a reference to shrinker[x]
+ * before RCU grace period.
+ */
+ while (atomic_read(&shrinker->usage)) {
+ msleep(100);
+ if (++i % 600)
+ continue;
+ pr_info("Process %d (%s) blocked at %s for %u seconds\n",
+ task_pid_nr(current), current->comm, __func__,
+ i / 10);
+ }
+ /* 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);
@@ -357,23 +418,15 @@ 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) {
@@ -382,9 +435,12 @@ 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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