[PATCH v2 4/6] debugobjects: Don't start fill if there are remaining nodes locally

[Zhen Lei <thunder.leizhen@huawei.com>]

If the conditions for starting fill are met, it means that all cores that
call fill() later are blocked until the first core completes the fill
operation. But obviously, for a core that has free nodes locally, it does
not need to be blocked(see below for why). This is good in stress
situations.

1. In the case of no nesting, a core uses only one node at a time. As long
   as there is a local node, there is no need to use the free node in
   obj_pool.
2. In the case of nesting depth is one, nodes in obj_pool need to be used
   only when there is only one local node.
   #define ODEBUG_POOL_PERCPU_SIZE      64
   #define ODEBUG_BATCH_SIZE            16
   Assume that when nested, the probability of percpu_obj_pool having each
   number of nodes is the same. The probability of only one node is less
   than 1/17=6%. Assuming the probability of nesting is 5%, that's a
   pretty high estimate. Then the probability of using obj_pool is
   6% * 5% = 0.3%. In other words, a 333-core environment produces only
   one core to compete for obj_pool.
   #define ODEBUG_POOL_MIN_LEVEL        256
   #define ODEBUG_BATCH_SIZE            16
   But we can tolerate "256 / (16 + 1)" = 15 cores competing at the same
   time.
3. In the case of nesting depth more than one, the probability is lower
   and negligible.
   Nesting Depth=2: "2/17 * 5% * 5%" = 0.03%
   Nesting Depth=3: "3/17 * 5% * 5% * 5%" = 0.002%

However, to ensure sufficient reliability, obj_pool is not filled only
when there are more than two local nodes, reduce the probability of
problems to the impossible.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
---
 lib/debugobjects.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/lib/debugobjects.c b/lib/debugobjects.c
index 7a8ccc94cb037ba..4f64b5d4329c27d 100644
--- a/lib/debugobjects.c
+++ b/lib/debugobjects.c
@@ -131,6 +131,16 @@ static void fill_pool(void)
 	struct debug_obj *obj;
 	unsigned long flags;
 
+	/*
+	 * The upper-layer function uses only one node at a time. If there are
+	 * more than two local nodes, it means that even if nesting occurs, it
+	 * doesn't matter. The probability of nesting depth >= 2 is extremely
+	 * low, and the number of global free nodes guarded by
+	 * debug_objects_pool_min_level is adequate.
+	 */
+	if (likely(obj_cache) && this_cpu_read(percpu_obj_pool.obj_free) >= 2)
+		return;
+
 	if (likely(READ_ONCE(obj_pool_free) >= debug_objects_pool_min_level))
 		return;
 
-- 
2.34.1