[PATCH 1/2] mm: support fastpath if NUMA is enabled with numa off

[Janghyuck Kim]

Hi,

On Wed, Jun 16, 2021 at 07:10:06PM +0200, Vlastimil Babka wrote:
> On 6/16/21 10:37 AM, Janghyuck Kim wrote:
> > Architecture might support fake node when CONFIG_NUMA is enabled but any
> 
> I suppose you mean the dummy node, i.e. dummy_numa_init()?
> 
> Because fakenuma is something different and I think if someone defines fakenuma
> nodes they actually would want for the mempolicies to be honored as if there was
> a real NUMA setup.
> 

You are correct. I mean dummy node, which shows "Faking a node at ..."
message at boot time. So I called it fake node.

> > node settings were supported by ACPI or device tree. In this case,
> > getting memory policy during memory allocation path is meaningless.
> > 
> > Moreover, performance degradation was observed in the minor page fault
> > test, which is provided by (https://protect2.fireeye.com/v1/url?k=32536af8-6dc85232-3252e1b7-0cc47a31bee8-e52eadd28e1e9a6e&q=1&e=39db7dd8-7f21-41a4-b4a9-9ad395d36e23&u=https%3A%2F%2Flkml.org%2Flkml%2F2006%2F8%2F29%2F294).
> > Average faults/sec of enabling NUMA with fake node was 5~6 % worse than
> > disabling NUMA. To reduce this performance regression, fastpath is
> 
> So you have measured this overhead is all due to mempolicy evaluation?
> Interesting, sounds like a lot.
> 

It's early to conclude, but mempolicy evaluation seems to account for a
large portion of the total overhead. Since this patch, performance
regression has decreased from 5-6% to 2-3%. It is still unclear whether
the remainder is within the margin of error of the measurement results
or is affected by other NUMA-related codes.

> > introduced. fastpath can skip the memory policy checking if NUMA is
> > enabled but it uses fake node. If architecture doesn't support fake
> > node, fastpath affects nothing for memory allocation path.
> > 
> > Signed-off-by: Janghyuck Kim <janghyuck.kim at samsung.com>
> 
> Sounds like an interesting direction to improve CONFIG_NUMA built kernels on
> single-node systems, but why restrict it only to arm64 and not make it generic
> for all systems with a single node?
> We could also probably use a static key instead of this #define.
> That would even make it possible to switch in case memory hotplug onlines
> another node, etc.
> 

I'm participating in arm64 project now, so I'm not sure if other
architectures will accept this way. So I tried not to touch other
architecture. Of course, it can be changed in the generic way if agree.

> > ---
> >  mm/internal.h  | 4 ++++
> >  mm/mempolicy.c | 3 +++
> >  2 files changed, 7 insertions(+)
> > 
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 31ff935b2547..3b6c21814fbc 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -36,6 +36,10 @@ void page_writeback_init(void);
> >  
> >  vm_fault_t do_swap_page(struct vm_fault *vmf);
> >  
> > +#ifndef numa_off_fastpath
> > +#define numa_off_fastpath()	false
> > +#endif
> > +
> >  void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
> >  		unsigned long floor, unsigned long ceiling);
> >  
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index e32360e90274..21156671d941 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -2152,6 +2152,9 @@ struct page *alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
> >  	int preferred_nid;
> >  	nodemask_t *nmask;
> >  
> > +	if (numa_off_fastpath())
> > +		return __alloc_pages_nodemask(gfp, order, 0, NULL);
> > +
> >  	pol = get_vma_policy(vma, addr);
> >  
> >  	if (pol->mode == MPOL_INTERLEAVE) {
> > 
> 
>