linux-kernel - [PATCH] mm: Remove misleading 'unlikely' hint in vms_gather_munmap

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Message-ID: <20241004164832.218681-1-leitao@debian.org>
Date: Fri,  4 Oct 2024 09:48:31 -0700
From: Breno Leitao <leitao@...ian.org>
To: Andrew Morton <akpm@...ux-foundation.org>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: kernel-team@...a.com,
	linux-mm@...ck.org (open list:VMA),
	linux-kernel@...r.kernel.org (open list)
Subject: [PATCH] mm: Remove misleading 'unlikely' hint in vms_gather_munmap_vmas()

Performance analysis using branch annotation on a fleet of 200 hosts
running web servers revealed that the 'likely' hint in
vms_gather_munmap_vmas() was 100% consistently incorrect. In all
observed cases, the branch behavior contradicted the hint.

Remove the 'unlikely' qualifier from the condition checking 'vms->uf'.
By doing so, we allow the compiler to make optimization decisions based
on its own heuristics and profiling data, rather than relying on a
static hint that has proven to be inaccurate in real-world scenarios.

Signed-off-by: Breno Leitao <leitao@...ian.org>
---
 mm/vma.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vma.c b/mm/vma.c
index 4737afcb064c..9d4fe794dd07 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -1250,7 +1250,7 @@ int vms_gather_munmap_vmas(struct vma_munmap_struct *vms,
 		else if (is_data_mapping(next->vm_flags))
 			vms->data_vm += nrpages;

-		if (unlikely(vms->uf)) {
+		if (vms->uf) {
 			/*
 			 * If userfaultfd_unmap_prep returns an error the vmas
 			 * will remain split, but userland will get a
-- 
2.43.5