[PATCH v2] slub: keep empty main sheaf as spare in __pcs_replace_empty_main()

[Hao Li <haolee.swjtu@gmail.com>]

From: Hao Li <haolee.swjtu@gmail.com>

When __pcs_replace_empty_main() fails to obtain a full sheaf directly
from the barn, it may either:

  - Refill an empty sheaf obtained via barn_get_empty_sheaf(), or
  - Allocate a brand new full sheaf via alloc_full_sheaf().

After reacquiring the per-CPU lock, if pcs->main is still empty and
pcs->spare is NULL, the current code donates the empty main sheaf to
the barn via barn_put_empty_sheaf() and installs the full sheaf as
pcs->main, leaving pcs->spare unpopulated.

Instead, keep the existing empty main sheaf locally as the spare:

  pcs->spare = pcs->main;
  pcs->main = full;

This populates pcs->spare earlier, which can reduce future barn traffic.

Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Hao Li <haolee.swjtu@gmail.com>
---

The Gmail account(haoli.tcs) I used to send v1 of the patch has been
restricted from sending emails for unknown reasons, so I'm sending v2
from this address instead. Thanks.

 mm/slub.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/slub.c b/mm/slub.c
index a0b905c2a557..a3e73ebb0cc8 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5077,6 +5077,11 @@ __pcs_replace_empty_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs,
 	 */
 
 	if (pcs->main->size == 0) {
+		if (!pcs->spare) {
+			pcs->spare = pcs->main;
+			pcs->main = full;
+			return pcs;
+		}
 		barn_put_empty_sheaf(barn, pcs->main);
 		pcs->main = full;
 		return pcs;
-- 
2.50.1