[PATCH v3 2/4] mm/vmap: preload a CPU with one object for split purpose

Uladzislau Rezki (Sony)
2019-05-27T09:39:04Z

Refactor the NE_FIT_TYPE split case when it comes to an
allocation of one extra object. We need it in order to
build a remaining space.

Introduce ne_fit_preload()/ne_fit_preload_end() functions
for preloading one extra vmap_area object to ensure that
we have it available when fit type is NE_FIT_TYPE.

The preload is done per CPU in non-atomic context thus with
GFP_KERNEL allocation masks. More permissive parameters can
be beneficial for systems which are suffer from high memory
pressure or low memory condition.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
mm/vmalloc.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 76 insertions(+), 3 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index ea1b65fac599..b553047aa05b 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -364,6 +364,13 @@ static LIST_HEAD(free_vmap_area_list);
*/
static struct rb_root free_vmap_area_root = RB_ROOT;

+/*
+ * Preload a CPU with one object for "no edge" split case. The
+ * aim is to get rid of allocations from the atomic context, thus
+ * to use more permissive allocation masks.
+ */
+static DEFINE_PER_CPU(struct vmap_area *, ne_fit_preload_node);
+
static __always_inline unsigned long
va_size(struct vmap_area *va)
{
@@ -950,9 +957,24 @@ adjust_va_to_fit_type(struct vmap_area *va,
* L V NVA V R
* |---|-------|---|
*/
- lva = kmem_cache_alloc(vmap_area_cachep, GFP_NOWAIT);
- if (unlikely(!lva))
- return -1;
+ lva = __this_cpu_xchg(ne_fit_preload_node, NULL);
+ if (unlikely(!lva)) {
+ /*
+ * For percpu allocator we do not do any pre-allocation
+ * and leave it as it is. The reason is it most likely
+ * never ends up with NE_FIT_TYPE splitting. In case of
+ * percpu allocations offsets and sizes are aligned to
+ * fixed align request, i.e. RE_FIT_TYPE and FL_FIT_TYPE
+ * are its main fitting cases.
+ *
+ * There are a few exceptions though, as an example it is
+ * a first allocation (early boot up) when we have "one"
+ * big free space that has to be split.
+ */
+ lva = kmem_cache_alloc(vmap_area_cachep, GFP_NOWAIT);
+ if (!lva)
+ return -1;
+ }

/*
* Build the remainder.
@@ -1023,6 +1045,48 @@ __alloc_vmap_area(unsigned long size, unsigned long align,
}

/*
+ * Preload this CPU with one extra vmap_area object to ensure
+ * that we have it available when fit type of free area is
+ * NE_FIT_TYPE.
+ *
+ * The preload is done in non-atomic context, thus it allows us
+ * to use more permissive allocation masks to be more stable under
+ * low memory condition and high memory pressure.
+ *
+ * If success it returns 1 with preemption disabled. In case
+ * of error 0 is returned with preemption not disabled. Note it
+ * has to be paired with ne_fit_preload_end().
+ */
+static int
+ne_fit_preload(int nid)
+{
+ preempt_disable();
+
+ if (!__this_cpu_read(ne_fit_preload_node)) {
+ struct vmap_area *node;
+
+ preempt_enable();
+ node = kmem_cache_alloc_node(vmap_area_cachep, GFP_KERNEL, nid);
+ if (node == NULL)
+ return 0;
+
+ preempt_disable();
+
+ if (__this_cpu_cmpxchg(ne_fit_preload_node, NULL, node))
+ kmem_cache_free(vmap_area_cachep, node);
+ }
+
+ return 1;
+}
+
+static void
+ne_fit_preload_end(int preloaded)
+{
+ if (preloaded)
+ preempt_enable();
+}
+
+/*
* Allocate a region of KVA of the specified size and alignment, within the
* vstart and vend.
*/
@@ -1034,6 +1098,7 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
struct vmap_area *va;
unsigned long addr;
int purged = 0;
+ int preloaded;

BUG_ON(!size);
BUG_ON(offset_in_page(size));
@@ -1056,6 +1121,12 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
kmemleak_scan_area(&va->rb_node, SIZE_MAX, gfp_mask & GFP_RECLAIM_MASK);

retry:
+ /*
+ * Even if it fails we do not really care about that.
+ * Just proceed as it is. "overflow" path will refill
+ * the cache we allocate from.
+ */
+ preloaded = ne_fit_preload(node);
spin_lock(&vmap_area_lock);

/*
@@ -1063,6 +1134,8 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
* returned. Therefore trigger the overflow path.
*/
addr = __alloc_vmap_area(size, align, vstart, vend);
+ ne_fit_preload_end(preloaded);
+
if (unlikely(addr == vend))
goto overflow;

--
2.11.0