[PATCH] slub: limit count of partial slabs scanned to gather statistics

[Konstantin Khlebnikov]

To get exact count of free and used objects slub have to scan list of
partial slabs. This may take at long time. Scanning holds spinlock and
blocks allocations which move partial slabs to per-cpu lists and back.

Example found in the wild:

# cat /sys/kernel/slab/dentry/partial
14478538 N0=7329569 N1=7148969
# time cat /sys/kernel/slab/dentry/objects
286225471 N0=136967768 N1=149257703

real	0m1.722s
user	0m0.001s
sys	0m1.721s

The same problem in slab was addressed in commit f728b0a5d72a ("mm, slab:
faster active and free stats") by adding more kmem cache statistics.
For slub same approach requires atomic op on fast path when object frees.

Let's simply limit count of scanned slabs and print warning.
Limit set in /sys/module/slub/parameters/max_partial_to_count.
Default is 10000 which should be enough for most sane cases.

Return linear approximation if list of partials is longer than limit.
Nobody should notice difference.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
---
 mm/slub.c |   15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index 9bf44955c4f1..86a366f7acb6 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2407,16 +2407,29 @@ static inline unsigned long node_nr_objs(struct kmem_cache_node *n)
 #endif /* CONFIG_SLUB_DEBUG */
 
 #if defined(CONFIG_SLUB_DEBUG) || defined(CONFIG_SYSFS)
+
+static unsigned long max_partial_to_count __read_mostly = 10000;
+module_param(max_partial_to_count, ulong, 0644);
+
 static unsigned long count_partial(struct kmem_cache_node *n,
 					int (*get_count)(struct page *))
 {
+	unsigned long counted = 0;
 	unsigned long flags;
 	unsigned long x = 0;
 	struct page *page;
 
 	spin_lock_irqsave(&n->list_lock, flags);
-	list_for_each_entry(page, &n->partial, slab_list)
+	list_for_each_entry(page, &n->partial, slab_list) {
 		x += get_count(page);
+
+		if (++counted > max_partial_to_count) {
+			pr_warn_once("SLUB: too much partial slabs to count all objects, increase max_partial_to_count.\n");
+			/* Approximate total count of objects */
+			x = mult_frac(x, n->nr_partial, counted);
+			break;
+		}
+	}
 	spin_unlock_irqrestore(&n->list_lock, flags);
 	return x;
 }