linux-kernel - [PATCH v2] mm/mmap: move vma operations to mm_struct out of the critical section of file mapping lock

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Date:   Tue, 11 Jul 2023 13:20:20 -0400
From:   Yu Ma <yu.ma@...el.com>
To:     yu.ma@...el.com, Liam.Howlett@...cle.com, kirill@...temov.name,
        akpm@...ux-foundation.org
Cc:     dan.j.williams@...el.com, dave.hansen@...el.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        lipeng.zhu@...el.com, pan.deng@...el.com, shakeelb@...gle.com,
        tianyou.li@...el.com, tim.c.chen@...el.com,
        tim.c.chen@...ux.intel.com
Subject: [PATCH v2] mm/mmap: move vma operations to mm_struct out of the critical section of file mapping lock

UnixBench/Execl represents a class of workload where bash scripts are
spawned frequently to do some short jobs. When running multiple parallel
tasks, hot osq_lock is observed from do_mmap and exit_mmap. Both of them
come from load_elf_binary through the call chain
"execl->do_execveat_common->bprm_execve->load_elf_binary". In do_mmap,it will
call mmap_region to create vma node, initialize it and insert it to vma
maintain structure in mm_struct and i_mmap tree of the mapping file, then
increase map_count to record the number of vma nodes used. The hot osq_lock
is to protect operations on file’s i_mmap tree. For the mm_struct member
change like vma insertion and map_count update, they do not affect i_mmap
tree. Move those operations out of the lock's critical section, to reduce
hold time on the lock.

With this change, on Intel Sapphire Rapids 112C/224T platform, based on
v6.0-rc6, the 160 parallel score improves by 12%. The patch has no
obvious performance gain on v6.4-rc4 due to regression of this benchmark
from this commit f1a7941243c102a44e8847e3b94ff4ff3ec56f25 (mm: convert
mm's rss stats into percpu_counter). Related discussion and conclusion
can be referred at the mail thread initiated by 0day as below:
Link: https://lore.kernel.org/linux-mm/a4aa2e13-7187-600b-c628-7e8fb108def0@intel.com/

Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>
Signed-off-by: Yu Ma <yu.ma@...el.com>
---
v1 -> v2:
- Update vma_link() to reduce the hold time on file mapping lock as well.
Based on v6.4-rc7, vma_link() is only called by insert_vm_struct ()
and copy_vma(), which are both protected by mmap_lock.
---
 mm/mmap.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index d600404580b2..6f42ca2ab84a 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -444,14 +444,11 @@ static int vma_link(struct mm_struct *mm, struct vm_area_struct *vma)
 	if (vma_iter_prealloc(&vmi))
 		return -ENOMEM;

+	vma_iter_store(&vmi, vma);
+
 	if (vma->vm_file) {
 		mapping = vma->vm_file->f_mapping;
 		i_mmap_lock_write(mapping);
-	}
-
-	vma_iter_store(&vmi, vma);
-
-	if (mapping) {
 		__vma_link_file(vma, mapping);
 		i_mmap_unlock_write(mapping);
 	}
@@ -2708,12 +2705,10 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 	if (vma_iter_prealloc(&vmi))
 		goto close_and_free_vma;

-	if (vma->vm_file)
-		i_mmap_lock_write(vma->vm_file->f_mapping);
-
 	vma_iter_store(&vmi, vma);
 	mm->map_count++;
 	if (vma->vm_file) {
+		i_mmap_lock_write(vma->vm_file->f_mapping);
 		if (vma->vm_flags & VM_SHARED)
 			mapping_allow_writable(vma->vm_file->f_mapping);

-- 
2.39.3