[PATCH v2 3/3] mm/slub: simplify get_partial_node()

[<xiongwei.song@windriver.com>]

From: Xiongwei Song <xiongwei.song@windriver.com>

The break conditions for filling cpu partial can be more readable and
simple.

If slub_get_cpu_partial() returns 0, we can confirm that we don't need
to fill cpu partial, then we should break from the loop. On the other
hand, we also should break from the loop if we have added enough cpu
partial slabs.

Meanwhile, the logic above gets rid of the #ifdef and also fixes a weird
corner case that if we set cpu_partial_slabs to 0 from sysfs, we still
allocate at least one here.

Signed-off-by: Xiongwei Song <xiongwei.song@windriver.com>
---

The measurement below is to compare the performance effects when checking
if we need to break from the filling cpu partial loop with the following
either-or condition:

Condition 1:
When the count of added cpu slabs is greater than cpu_partial_slabs/2:
(partial_slabs > slub_get_cpu_partial(s) / 2)

Condition 2:
When the count of added cpu slabs is greater than or equal to
cpu_partial_slabs/2:
(partial_slabs >= slub_get_cpu_partial(s) / 2)

The change of breaking condition can effect how many cpu partial slabs
would be put on the cpu partial list.

Run the test with a "Intel(R) Core(TM) i7-10700 CPU @ 2.90GHz" cpu with
16 cores. The OS is Ubuntu 22.04.

hackbench-process-pipes
                  6.9-rc2(with ">")      6.9.0-rc2(with ">=")
Amean     1       0.0373 (   0.00%)      0.0356 *   4.60%*
Amean     4       0.0984 (   0.00%)      0.1014 *  -3.05%*
Amean     7       0.1803 (   0.00%)      0.1851 *  -2.69%*
Amean     12      0.2947 (   0.00%)      0.3141 *  -6.59%*
Amean     21      0.4577 (   0.00%)      0.4927 *  -7.65%*
Amean     30      0.6326 (   0.00%)      0.6649 *  -5.10%*
Amean     48      0.9396 (   0.00%)      0.9884 *  -5.20%*
Amean     64      1.2321 (   0.00%)      1.3004 *  -5.54%*

hackbench-process-sockets
                  6.9-rc2(with ">")      6.9.0-rc2(with ">=")
Amean     1       0.0609 (   0.00%)      0.0623 *  -2.35%*
Amean     4       0.2107 (   0.00%)      0.2140 *  -1.56%*
Amean     7       0.3754 (   0.00%)      0.3966 *  -5.63%*
Amean     12      0.6456 (   0.00%)      0.6734 *  -4.32%*
Amean     21      1.1440 (   0.00%)      1.1769 *  -2.87%*
Amean     30      1.6629 (   0.00%)      1.7031 *  -2.42%*
Amean     48      2.7321 (   0.00%)      2.7897 *  -2.11%*
Amean     64      3.7397 (   0.00%)      3.7640 *  -0.65%*

It seems there is a bit performance penalty when using ">=" to break up
the loop. Hence, we should still use ">" here.
---
 mm/slub.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 590cc953895d..6beff3b1e22c 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2619,13 +2619,10 @@ static struct slab *get_partial_node(struct kmem_cache *s,
 			stat(s, CPU_PARTIAL_NODE);
 			partial_slabs++;
 		}
-#ifdef CONFIG_SLUB_CPU_PARTIAL
-		if (partial_slabs > s->cpu_partial_slabs / 2)
-			break;
-#else
-		break;
-#endif
 
+		if ((slub_get_cpu_partial(s) == 0) ||
+		    (partial_slabs > slub_get_cpu_partial(s) / 2))
+			break;
 	}
 	spin_unlock_irqrestore(&n->list_lock, flags);
 	return partial;
-- 
2.34.1