Get rid of preempt_disable() and preempt_enable() when the
preload is done for splitting purpose. The reason is that
calling spin_lock() with disabled preemtion is forbidden in
CONFIG_PREEMPT_RT kernel.
Therefore, we do not guarantee that a CPU is preloaded, instead
we minimize the case when it is not with this change.
For example i run the special test case that follows the preload
pattern and path. 20 "unbind" threads run it and each does
1000000 allocations. Only 3.5 times among 1000000 a CPU was
not preloaded. So it can happen but the number is negligible.
V1 -> V2:
- move __this_cpu_cmpxchg check when spin_lock is taken,
as proposed by Andrew Morton
- add more explanation in regard of preloading
- adjust and move some comments
Fixes: 82dd23e84be3 ("mm/vmalloc.c: preload a CPU with one object for split purpose")
Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
---
mm/vmalloc.c | 50 +++++++++++++++++++++++++++++++++-----------------
1 file changed, 33 insertions(+), 17 deletions(-)
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index e92ff5f7dd8b..f48cd0711478 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -969,6 +969,19 @@ adjust_va_to_fit_type(struct vmap_area *va,
* 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.
+ *
+ * Also we can hit this path in case of regular "vmap"
+ * allocations, if "this" current CPU was not preloaded.
+ * See the comment in alloc_vmap_area() why. If so, then
+ * GFP_NOWAIT is used instead to get an extra object for
+ * split purpose. That is rare and most time does not
+ * occur.
+ *
+ * What happens if an allocation gets failed. Basically,
+ * an "overflow" path is triggered to purge lazily freed
+ * areas to free some memory, then, the "retry" path is
+ * triggered to repeat one more time. See more details
+ * in alloc_vmap_area() function.
*/
lva = kmem_cache_alloc(vmap_area_cachep, GFP_NOWAIT);
if (!lva)
@@ -1078,31 +1091,34 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
retry:
/*
- * 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.
+ * Preload this CPU with one extra vmap_area object. It is used
+ * when fit type of free area is NE_FIT_TYPE. Please note, it
+ * does not guarantee that an allocation occurs on a CPU that
+ * is preloaded, instead we minimize the case when it is not.
+ * It can happen because of migration, because there is a race
+ * until the below spinlock is taken.
*
* 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.
+ * low memory condition and high memory pressure. In rare case,
+ * if not preloaded, GFP_NOWAIT is used.
*
- * 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.
+ * Set "pva" to NULL here, because of "retry" path.
*/
- preempt_disable();
- if (!__this_cpu_read(ne_fit_preload_node)) {
- preempt_enable();
- pva = kmem_cache_alloc_node(vmap_area_cachep, GFP_KERNEL, node);
- preempt_disable();
+ pva = NULL;
- if (__this_cpu_cmpxchg(ne_fit_preload_node, NULL, pva)) {
- if (pva)
- kmem_cache_free(vmap_area_cachep, pva);
- }
- }
+ if (!this_cpu_read(ne_fit_preload_node))
+ /*
+ * Even if it fails we do not really care about that.
+ * Just proceed as it is. If needed "overflow" path
+ * will refill the cache we allocate from.
+ */
+ pva = kmem_cache_alloc_node(vmap_area_cachep, GFP_KERNEL, node);
spin_lock(&vmap_area_lock);
- preempt_enable();
+
+ if (pva && __this_cpu_cmpxchg(ne_fit_preload_node, NULL, pva))
+ kmem_cache_free(vmap_area_cachep, pva);
/*
* If an allocation fails, the "vend" address is
--
2.20.1
