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Date:   Wed, 25 Jan 2017 16:06:02 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Mike Galbraith <efault@....de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [rfc patch-rt] radix-tree: Partially disable memcg accounting in
 radix_tree_node_alloc()

On 2017-01-06 09:13:23 [+0100], Mike Galbraith wrote:
> radix-tree: Partially disable memcg accounting in radix_tree_node_alloc()
> 
> Having no preload, which turns accounting off for non-rt kernels, trying to
> allocate coming from shmem_fault() when memcg is full sends us scurrying off
> to pagefault_out_of_memory(), with dramatic (usually terminal) consequences.
> LTP's madvise06 testcase triggers this quite well, and per gitk, the below
> was the beginning of RT memcg woes.
> 
> 58e698af4c63 radix-tree: account radix_tree_node to memory cgroup
> 
> Turn memcg accounting off for RT in the problematic path.

According to the to description of radix_tree_preload() the return code
of 0 means that the following addition of a single element does not
fail. But in RT's case this requirement is not fulfilled. There is more
than just one user of that function. So instead adding an exception here
and maybe later one someplace else, what about the following patch?
That testcase you mentioned passes now:

|testcases/kernel/syscalls/madvise/madvise06
|tst_test.c:760: INFO: Timeout per run is 0h 05m 00s
|madvise06.c:65: INFO: dropping caches
|madvise06.c:139: INFO: SwapCached (before madvise): 304
|madvise06.c:153: INFO: SwapCached (after madvise): 309988
|madvise06.c:155: PASS: Regression test pass
|
|Summary:
|passed   1
|failed   0
|skipped  0
|warnings 0

diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index f87f87dec84c..277295039c8f 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -289,19 +289,11 @@ unsigned int radix_tree_gang_lookup(struct radix_tree_root *root,
 unsigned int radix_tree_gang_lookup_slot(struct radix_tree_root *root,
 			void ***results, unsigned long *indices,
 			unsigned long first_index, unsigned int max_items);
-#ifdef CONFIG_PREEMPT_RT_FULL
-static inline int radix_tree_preload(gfp_t gm) { return 0; }
-static inline int radix_tree_maybe_preload(gfp_t gfp_mask) { return 0; }
-static inline int radix_tree_maybe_preload_order(gfp_t gfp_mask, int order)
-{
-	return 0;
-};
-
-#else
 int radix_tree_preload(gfp_t gfp_mask);
 int radix_tree_maybe_preload(gfp_t gfp_mask);
 int radix_tree_maybe_preload_order(gfp_t gfp_mask, int order);
-#endif
+void radix_tree_preload_end(void);
+
 void radix_tree_init(void);
 void *radix_tree_tag_set(struct radix_tree_root *root,
 			unsigned long index, unsigned int tag);
@@ -324,11 +316,6 @@ unsigned long radix_tree_range_tag_if_tagged(struct radix_tree_root *root,
 int radix_tree_tagged(struct radix_tree_root *root, unsigned int tag);
 unsigned long radix_tree_locate_item(struct radix_tree_root *root, void *item);
 
-static inline void radix_tree_preload_end(void)
-{
-	preempt_enable_nort();
-}
-
 /**
  * struct radix_tree_iter - radix tree iterator state
  *
diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index 881cc195d85f..e96c6a99f25c 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -36,7 +36,7 @@
 #include <linux/bitops.h>
 #include <linux/rcupdate.h>
 #include <linux/preempt.h>		/* in_interrupt() */
-
+#include <linux/locallock.h>
 
 /* Number of nodes in fully populated tree of given height */
 static unsigned long height_to_maxnodes[RADIX_TREE_MAX_PATH + 1] __read_mostly;
@@ -68,6 +68,7 @@ struct radix_tree_preload {
 	struct radix_tree_node *nodes;
 };
 static DEFINE_PER_CPU(struct radix_tree_preload, radix_tree_preloads) = { 0, };
+static DEFINE_LOCAL_IRQ_LOCK(radix_tree_preloads_lock);
 
 static inline void *node_to_entry(void *ptr)
 {
@@ -290,14 +291,14 @@ radix_tree_node_alloc(struct radix_tree_root *root)
 		 * succeed in getting a node here (and never reach
 		 * kmem_cache_alloc)
 		 */
-		rtp = &get_cpu_var(radix_tree_preloads);
+		rtp = &get_locked_var(radix_tree_preloads_lock, radix_tree_preloads);
 		if (rtp->nr) {
 			ret = rtp->nodes;
 			rtp->nodes = ret->private_data;
 			ret->private_data = NULL;
 			rtp->nr--;
 		}
-		put_cpu_var(radix_tree_preloads);
+		put_locked_var(radix_tree_preloads_lock, radix_tree_preloads);
 		/*
 		 * Update the allocation stack trace as this is more useful
 		 * for debugging.
@@ -337,7 +338,6 @@ radix_tree_node_free(struct radix_tree_node *node)
 	call_rcu(&node->rcu_head, radix_tree_node_rcu_free);
 }
 
-#ifndef CONFIG_PREEMPT_RT_FULL
 /*
  * Load up this CPU's radix_tree_node buffer with sufficient objects to
  * ensure that the addition of a single element in the tree cannot fail.  On
@@ -359,14 +359,14 @@ static int __radix_tree_preload(gfp_t gfp_mask, int nr)
 	 */
 	gfp_mask &= ~__GFP_ACCOUNT;
 
-	preempt_disable();
+	local_lock(radix_tree_preloads_lock);
 	rtp = this_cpu_ptr(&radix_tree_preloads);
 	while (rtp->nr < nr) {
-		preempt_enable();
+		local_unlock(radix_tree_preloads_lock);
 		node = kmem_cache_alloc(radix_tree_node_cachep, gfp_mask);
 		if (node == NULL)
 			goto out;
-		preempt_disable();
+		local_lock(radix_tree_preloads_lock);
 		rtp = this_cpu_ptr(&radix_tree_preloads);
 		if (rtp->nr < nr) {
 			node->private_data = rtp->nodes;
@@ -408,7 +408,7 @@ int radix_tree_maybe_preload(gfp_t gfp_mask)
 	if (gfpflags_allow_blocking(gfp_mask))
 		return __radix_tree_preload(gfp_mask, RADIX_TREE_PRELOAD_SIZE);
 	/* Preloading doesn't help anything with this gfp mask, skip it */
-	preempt_disable();
+	local_lock(radix_tree_preloads_lock);
 	return 0;
 }
 EXPORT_SYMBOL(radix_tree_maybe_preload);
@@ -424,7 +424,7 @@ int radix_tree_maybe_preload_order(gfp_t gfp_mask, int order)
 
 	/* Preloading doesn't help anything with this gfp mask, skip it */
 	if (!gfpflags_allow_blocking(gfp_mask)) {
-		preempt_disable();
+		local_lock(radix_tree_preloads_lock);
 		return 0;
 	}
 
@@ -457,7 +457,11 @@ int radix_tree_maybe_preload_order(gfp_t gfp_mask, int order)
 
 	return __radix_tree_preload(gfp_mask, nr_nodes);
 }
-#endif
+
+void radix_tree_preload_end(void)
+{
+	local_unlock(radix_tree_preloads_lock);
+}
 
 /*
  * The maximum index which can be stored in a radix tree

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