+ mm-vmalloc-avoid-false-sharing-with-drain_vmap_work.patch added to mm-new branch

+ mm-vmalloc-avoid-false-sharing-with-drain_vmap_work.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm/vmalloc: avoid false sharing with drain_vmap_work
has been added to the -mm mm-new branch.  Its filename is
     mm-vmalloc-avoid-false-sharing-with-drain_vmap_work.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-vmalloc-avoid-false-sharing-with-drain_vmap_work.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: "=?UTF-8?B?Sm9uYXNaaG91LW9j?=" <jonaszhou-oc@zhaoxin.com>
Subject: mm/vmalloc: avoid false sharing with drain_vmap_work
Date: Tue, 25 Aug 2026 18:46:59 +0800

free_vmap_area_noflush() queues drain_vmap_work after the number of lazily
freed pages exceeds lazy_max_pages().  Until the worker purges those
pages, concurrent frees keep calling schedule_work().  Even if the work is
already pending, queue_work_on() performs a locked test_and_set_bit() on
the pending bit in the work item.

On the tested x86-64 build, drain_vmap_work and vmap_nodes occupy the same
64-byte cache line.  The work item starts at offset 0 and the vmap_nodes
pointer at offset 32.  The latter is read by vmap allocation and free
paths, so updates to the work item invalidate a cache line read by all
CPUs.

Put drain_vmap_work in the cacheline-aligned data section.  Tests were run
on Linux 7.2.

On a two-socket Intel Xeon Silver 4208 system using 16 workers, the
runtimes of vmalloc.fix_align, vmalloc.fix_size, and
vmalloc.no_block_alloc decreased by 12.61%, 5.78%, and 6.87%,
respectively.  HITM samples for the affected cache line and total HITM
samples decreased by 96.55% and 13.36%, respectively.

Link: https://lore.kernel.org/20260825104659.100134-1-jonaszhou-oc@zhaoxin.com
Signed-off-by: JonasZhou <jonaszhou-oc@zhaoxin.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: <jianhuizzzzz@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/vmalloc.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/mm/vmalloc.c~mm-vmalloc-avoid-false-sharing-with-drain_vmap_work
+++ a/mm/vmalloc.c
@@ -1089,7 +1089,12 @@ RB_DECLARE_CALLBACKS_MAX(static, free_vm
 static void reclaim_and_purge_vmap_areas(void);
 static BLOCKING_NOTIFIER_HEAD(vmap_notify_list);
 static void drain_vmap_area_work(struct work_struct *work);
-static DECLARE_WORK(drain_vmap_work, drain_vmap_area_work);
+/*
+ * Keep the work item, whose pending bit is updated by freeing CPUs,
+ * away from vmap metadata read by allocation and free paths.
+ */
+static __cacheline_aligned_in_smp
+DECLARE_WORK(drain_vmap_work, drain_vmap_area_work);
 
 static __cacheline_aligned_in_smp atomic_long_t vmap_lazy_nr;
 
_

Patches currently in -mm which might be from jonaszhou-oc@zhaoxin.com are

mm-vmalloc-avoid-false-sharing-with-drain_vmap_work.patch