linux-kernel - [PATCH] mm/vmalloc: Move resched point into alloc_vmap

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Message-ID: <20250917185906.1595454-1-urezki@gmail.com>
Date: Wed, 17 Sep 2025 20:59:06 +0200
From: "Uladzislau Rezki (Sony)" <urezki@...il.com>
To: linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>
Cc: Michal Hocko <mhocko@...nel.org>,
	Baoquan He <bhe@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Uladzislau Rezki <urezki@...il.com>
Subject: [PATCH] mm/vmalloc: Move resched point into alloc_vmap_area()

Currently vm_area_alloc_pages() contains two cond_resched() points.
However, the page allocator already has its own in slow path so an
extra resched is not optimal because it delays the loops.

The place where a CPU time can be stolen is a VA-space search in the
alloc_vmap_area(), especially if the space is really fragmented using
synthetic stress tests, after a fast path falls back to a slow one.

Move a single cond_resched() there, after dropping free_vmap_area_lock
in a slow path. This keeps fairness where it matters while removing
redundant yields from the page-allocation path.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
---
 mm/vmalloc.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 5edd536ba9d2..58204728123d 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2057,6 +2057,13 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
 		addr = __alloc_vmap_area(&free_vmap_area_root, &free_vmap_area_list,
 			size, align, vstart, vend);
 		spin_unlock(&free_vmap_area_lock);
+
+		/*
+		 * This is not a fast path. Check if yielding is
+		 * needed. This is the only one reschedule point
+		 * in vmalloc() path.
+		 */
+		cond_resched();
 	}
 
 	trace_alloc_vmap_area(addr, size, align, vstart, vend, IS_ERR_VALUE(addr));
@@ -3622,7 +3629,6 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
 							pages + nr_allocated);
 
 			nr_allocated += nr;
-			cond_resched();
 
 			/*
 			 * If zero or pages were obtained partly,
@@ -3664,7 +3670,6 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
 		for (i = 0; i < (1U << order); i++)
 			pages[nr_allocated + i] = page + i;
 
-		cond_resched();
 		nr_allocated += 1U << order;
 	}
 
-- 
2.47.3