[PATCH 26/29] mm: disable rcu safe vma freeing for single threaded user space

[Michel Lespinasse]

Performance tuning: as single threaded userspace does not use
speculative page faults, it does not require rcu safe vma freeing.
Turn this off to avoid the related (small) extra overheads.

For multi threaded userspace, we often see a performance benefit from
the rcu safe vma freeing - even in tests that do not have any frequent
concurrent page faults ! This is because rcu safe vma freeing prevents
recently released vmas from being immediately reused in a new thread.

Signed-off-by: Michel Lespinasse <michel@xxxxxxxxxxxxxx>
---
 kernel/fork.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 7c22bf2b1f9d..18659d802d24 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -381,10 +381,12 @@ static void __vm_area_free(struct rcu_head *head)
 void vm_area_free(struct vm_area_struct *vma)
 {
 #ifdef CONFIG_SPECULATIVE_PAGE_FAULT
-	call_rcu(&vma->vm_rcu, __vm_area_free);
-#else
-	kmem_cache_free(vm_area_cachep, vma);
+	if (atomic_read(&vma->vm_mm->mm_users) > 1) {
+		call_rcu(&vma->vm_rcu, __vm_area_free);
+		return;
+	}
 #endif
+	kmem_cache_free(vm_area_cachep, vma);
 }
 
 static void account_kernel_stack(struct task_struct *tsk, int account)
-- 
2.20.1