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

[Matthew Wilcox]

On Thu, Jun 17, 2021 at 08:55:44PM +0900, Janghyuck Kim wrote:
> On Wed, Jun 16, 2021 at 06:32:50PM +0100, Matthew Wilcox wrote:
> > On Wed, Jun 16, 2021 at 05:37:41PM +0900, Janghyuck Kim wrote:
> > > Architecture might support fake node when CONFIG_NUMA is enabled but any
> > > 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=c81407ae-978f3ea4-c8158ce1-0cc47a31384a-10187d5ead74c318&q=1&e=cbc91c9b-80e1-4ca0-b51a-9f79fad5b0c1&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
> > > 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.
> > 
> > This patch doesn't even apply to the current kernel, but putting that
> > aside, what's the expensive part of the current code?  That is,
> > comparing performance stats between this numa_off enabled and numa_off
> > disabled, where do you see taking a lot of time?
> > 
> 
> mempolicy related code that I skipped by this patch took a short time,
> taking only a few tens of nanoseconds that difficult to measure by
> sched_clock's degree of precision. But it can be affect the minor page
> fault test with large buffer size, because one page fault handling takes
> several ms. As I replied in previous mail, performance regression has
> been reduced from 5~6% to 2~3%.

I'm not proposing you use sched_clock.  Try perf.