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[PATCH] Don't touch single threaded PTEs which are on the right node

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  [PATCH] Don't touch single threaded PTEs which are on the right node Andi Kleen <andi@firstfloor.org> - 2016-10-12 18:20 +0200
    Re: [PATCH] Don't touch single threaded PTEs which are on the right  node Mel Gorman <mgorman@suse.de> - 2016-10-13 10:40 +0200
      Re: [PATCH] Don't touch single threaded PTEs which are on the right  node Andi Kleen <ak@linux.intel.com> - 2016-10-13 20:10 +0200
        Re: [PATCH] Don't touch single threaded PTEs which are on the right  node Mel Gorman <mgorman@suse.de> - 2016-10-13 20:20 +0200

#1499801 — [PATCH] Don't touch single threaded PTEs which are on the right node

From: Andi Kleen <ak@linux.intel.com>

We had some problems with pages getting unmapped in single threaded
affinitized processes. It was tracked down to NUMA scanning.

In this case it doesn't make any sense to unmap pages if the
process is single threaded and the page is already on the
node the process is running on.

Add a check for this case into the numa protection code,
and skip unmapping if true.

In theory the process could be migrated later, but we
will eventually rescan and unmap and migrate then.

In theory this could be made more fancy: remembering this
state per process or even whole mm. However that would
need extra tracking and be more complicated, and the
simple check seems to work fine so far.

v2: Only do it for private VMAs. Move most of check out of
loop.
Signed-off-by: Andi Kleen <ak@linux.intel.com>
---
 mm/mprotect.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/mprotect.c b/mm/mprotect.c
index a4830f0325fe..e9473e7e1468 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -68,11 +68,17 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 	pte_t *pte, oldpte;
 	spinlock_t *ptl;
 	unsigned long pages = 0;
+	int target_node = -1;
 
 	pte = lock_pte_protection(vma, pmd, addr, prot_numa, &ptl);
 	if (!pte)
 		return 0;
 
+	if (prot_numa &&
+	    !(vma->vm_flags & VM_SHARED) &&
+	    atomic_read(&vma->vm_mm->mm_users) == 1)
+	    target_node = cpu_to_node(raw_smp_processor_id());
+
 	arch_enter_lazy_mmu_mode();
 	do {
 		oldpte = *pte;
@@ -94,6 +100,13 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 				/* Avoid TLB flush if possible */
 				if (pte_protnone(oldpte))
 					continue;
+
+				/*
+				 * Don't mess with PTEs if page is already on the node
+				 * a single-threaded process is running on.
+				 */
+				if (target_node == page_to_nid(page))
+					continue;
 			}
 
 			ptent = ptep_modify_prot_start(mm, addr, pte);
-- 
2.5.5

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#1500093 — Re: [PATCH] Don't touch single threaded PTEs which are on the right node

On Wed, Oct 12, 2016 at 09:15:49AM -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@linux.intel.com>
> 
> We had some problems with pages getting unmapped in single threaded
> affinitized processes. It was tracked down to NUMA scanning.
> 
> In this case it doesn't make any sense to unmap pages if the
> process is single threaded and the page is already on the
> node the process is running on.
> 
> Add a check for this case into the numa protection code,
> and skip unmapping if true.
> 
> In theory the process could be migrated later, but we
> will eventually rescan and unmap and migrate then.
> 
> In theory this could be made more fancy: remembering this
> state per process or even whole mm. However that would
> need extra tracking and be more complicated, and the
> simple check seems to work fine so far.
> 
> v2: Only do it for private VMAs. Move most of check out of
> loop.
> Signed-off-by: Andi Kleen <ak@linux.intel.com>

Minor comments

> ---
>  mm/mprotect.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index a4830f0325fe..e9473e7e1468 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -68,11 +68,17 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>  	pte_t *pte, oldpte;
>  	spinlock_t *ptl;
>  	unsigned long pages = 0;
> +	int target_node = -1;
>  

Proper convention is to use NUMA_NO_NODE instead of -1 although it's not
always adhered to.

>  	pte = lock_pte_protection(vma, pmd, addr, prot_numa, &ptl);
>  	if (!pte)
>  		return 0;
>  
> +	if (prot_numa &&
> +	    !(vma->vm_flags & VM_SHARED) &&
> +	    atomic_read(&vma->vm_mm->mm_users) == 1)
> +	    target_node = cpu_to_node(raw_smp_processor_id());
> +

Use numa_node_id() instead of open-coding this. A short comment probably
would not hurt even if git blame should make it obvious.

>  	arch_enter_lazy_mmu_mode();
>  	do {
>  		oldpte = *pte;
> @@ -94,6 +100,13 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>  				/* Avoid TLB flush if possible */
>  				if (pte_protnone(oldpte))
>  					continue;
> +
> +				/*
> +				 * Don't mess with PTEs if page is already on the node
> +				 * a single-threaded process is running on.
> +				 */
> +				if (target_node == page_to_nid(page))
> +					continue;
>  			}
>  

Check target_node != NUMA_NODE && target_node == page_to_nid(page) to
avoid unnecessary page->flag masking and shifts?

The last one will be fairly marginal, the others are taste so whether
you spin a v3 with the corrections or not;

Acked-by: Mel Gorman <mgorman@suse.de>

Thanks.

-- 
Mel Gorman
SUSE Labs

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#1500482 — Re: [PATCH] Don't touch single threaded PTEs which are on the right node

> >  	do {
> >  		oldpte = *pte;
> > @@ -94,6 +100,13 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> >  				/* Avoid TLB flush if possible */
> >  				if (pte_protnone(oldpte))
> >  					continue;
> > +
> > +				/*
> > +				 * Don't mess with PTEs if page is already on the node
> > +				 * a single-threaded process is running on.
> > +				 */
> > +				if (target_node == page_to_nid(page))
> > +					continue;
> >  			}
> >  
> 
> Check target_node != NUMA_NODE && target_node == page_to_nid(page) to
> avoid unnecessary page->flag masking and shifts?

I didn't do this last change because I expect a potentially mispredicted
check is more expensive than some shifting/masking.

-Andi

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#1500488 — Re: [PATCH] Don't touch single threaded PTEs which are on the right node

On Thu, Oct 13, 2016 at 11:04:02AM -0700, Andi Kleen wrote:
> > >  	do {
> > >  		oldpte = *pte;
> > > @@ -94,6 +100,13 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> > >  				/* Avoid TLB flush if possible */
> > >  				if (pte_protnone(oldpte))
> > >  					continue;
> > > +
> > > +				/*
> > > +				 * Don't mess with PTEs if page is already on the node
> > > +				 * a single-threaded process is running on.
> > > +				 */
> > > +				if (target_node == page_to_nid(page))
> > > +					continue;
> > >  			}
> > >  
> > 
> > Check target_node != NUMA_NODE && target_node == page_to_nid(page) to
> > avoid unnecessary page->flag masking and shifts?
> 
> I didn't do this last change because I expect a potentially mispredicted
> check is more expensive than some shifting/masking.
> 

Ok, that's fair enough. For something that minor I expect it to be a
case of "you win some you lose some" depending on workload, CPU and
phase of the moon.

Thanks.

-- 
Mel Gorman
SUSE Labs

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