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

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Message-ID: <ac29dce6-468a-4249-ba17-36be2ce8b1f8@suse.cz>
Date: Mon, 7 Oct 2024 13:50:06 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Breno Leitao <leitao@...ian.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: kernel-team@...a.com, "open list:VMA" <linux-mm@...ck.org>,
 open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: Remove misleading 'unlikely' hint in
 vms_gather_munmap_vmas()

On 10/4/24 6:48 PM, Breno Leitao wrote:
> 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>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  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