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

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Message-ID: <cwn5sv6ggqrc6k6v55uwciuyohuwwtiknvlk6o3beqoocxtll6@yomkzc5u5mzz>
Date: Tue, 29 Oct 2024 10:18:14 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Breno Leitao <leitao@...ian.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>, 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()

* Breno Leitao <leitao@...ian.org> [241029 07:52]:
> On Fri, Oct 04, 2024 at 06:01:07PM +0100, Lorenzo Stoakes wrote:
> > On Fri, Oct 04, 2024 at 09:48:31AM -0700, Breno Leitao wrote:
> > > Performance analysis using branch annotation on a fleet of 200 hosts
> > > running web servers revealed that the 'likely' hint in
> > 
> > To be pedantic: *unlikely
> > 
> > > vms_gather_munmap_vmas() was 100% consistently incorrect. In all
> > > observed cases, the branch behavior contradicted the hint.
> > 
> > OK so this is probably because vm_mmap_pgoff() declares the userfaultfd
> > list head on the stack then passes it into do_mmap() and threads all the
> > way to this code... and yeah, so that would be 100%.
> > 
> > There are other code paths that aren't 100%, but the system call one is.
> > 
> > Nice spot!
> > 
> > >
> > > 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.
> > 
> > Yeah I'm generally not in favour of 'vibes' based likely()/unlikely(), I
> > think it should always be based on profiling.
> > 
> > It's understandable that there would be this expectation, and it may have
> > migrated from other code that already had this check in where perhaps it
> > wasn't always referencing a stack object, but yeah this is just wrong.
> > 
> > >
> > > Signed-off-by: Breno Leitao <leitao@...ian.org>
> > 
> > Liam will want a look too when he's back next week.
> 
> Liam, are you OK with this one? I suspect that Andrew is waiting for
> your review before merging it.

I'm fine with removing the unlikely.

This begs for a refactoring considering we are calling a function in
fs/userfaultfd on every vma on every munmap operation in this call path.

Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>

> 
> > 
> > Looks good to me though!
> > 
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > 
> > > ---
> > >  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
> > > --
> > > 2.43.5
> > >