Re: [PATCH] mm: Remove misleading 'unlikely' hint in vms_gather_munmap_vmas()

[Breno Leitao <leitao@debian.org>]

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@debian.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.

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