Re: [RFC PATCH v2 2/4] mm/__free_one_page: skip merge for order-0 page unless compaction failed

["Figo.zhang" <figo1802@gmail.com>]

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2018-03-20 18:59 GMT-07:00 Aaron Lu <aaron.lu@intel.com>:

> On Tue, Mar 20, 2018 at 03:58:51PM -0700, Figo.zhang wrote:
> > 2018-03-20 1:54 GMT-07:00 Aaron Lu <aaron.lu@intel.com>:
> >
> > > Running will-it-scale/page_fault1 process mode workload on a 2 sockets
> > > Intel Skylake server showed severe lock contention of zone->lock, as
> > > high as about 80%(42% on allocation path and 35% on free path) CPU
> > > cycles are burnt spinning. With perf, the most time consuming part
> inside
> > > that lock on free path is cache missing on page structures, mostly on
> > > the to-be-freed page's buddy due to merging.
> > >
> > > One way to avoid this overhead is not do any merging at all for order-0
> > > pages. With this approach, the lock contention for zone->lock on free
> > > path dropped to 1.1% but allocation side still has as high as 42% lock
> > > contention. In the meantime, the dropped lock contention on free side
> > > doesn't translate to performance increase, instead, it's consumed by
> > > increased lock contention of the per node lru_lock(rose from 5% to 37%)
> > > and the final performance slightly dropped about 1%.
> > >
> > > Though performance dropped a little, it almost eliminated zone lock
> > > contention on free path and it is the foundation for the next patch
> > > that eliminates zone lock contention for allocation path.
> > >
> > > A new document file called "struct_page_filed" is added to explain
> > > the newly reused field in "struct page".
> > >
> > > Suggested-by: Dave Hansen <dave.hansen@intel.com>
> > > Signed-off-by: Aaron Lu <aaron.lu@intel.com>
> > > ---
> > >  Documentation/vm/struct_page_field |  5 +++
> > >  include/linux/mm_types.h           |  1 +
> > >  mm/compaction.c                    | 13 +++++-
> > >  mm/internal.h                      | 27 ++++++++++++
> > >  mm/page_alloc.c                    | 89 ++++++++++++++++++++++++++++++
> > > +++-----
> > >  5 files changed, 122 insertions(+), 13 deletions(-)
> > >  create mode 100644 Documentation/vm/struct_page_field
> > >
> > > diff --git a/Documentation/vm/struct_page_field
> b/Documentation/vm/struct_
> > > page_field
> > > new file mode 100644
> > > index 000000000000..1ab6c19ccc7a
> > > --- /dev/null
> > > +++ b/Documentation/vm/struct_page_field
> > > @@ -0,0 +1,5 @@
> > > +buddy_merge_skipped:
> > > +Used to indicate this page skipped merging when added to buddy. This
> > > +field only makes sense if the page is in Buddy and is order zero.
> > > +It's a bug if any higher order pages in Buddy has this field set.
> > > +Shares space with index.
> > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > > index fd1af6b9591d..7edc4e102a8e 100644
> > > --- a/include/linux/mm_types.h
> > > +++ b/include/linux/mm_types.h
> > > @@ -91,6 +91,7 @@ struct page {
> > >                 pgoff_t index;          /* Our offset within mapping.
> */
> > >                 void *freelist;         /* sl[aou]b first free object
> */
> > >                 /* page_deferred_list().prev    -- second tail page */
> > > +               bool buddy_merge_skipped; /* skipped merging when
> added to
> > > buddy */
> > >         };
> > >
> > >         union {
> > > diff --git a/mm/compaction.c b/mm/compaction.c
> > > index 2c8999d027ab..fb9031fdca41 100644
> > > --- a/mm/compaction.c
> > > +++ b/mm/compaction.c
> > > @@ -776,8 +776,19 @@ isolate_migratepages_block(struct compact_control
> > > *cc, unsigned long low_pfn,
> > >                  * potential isolation targets.
> > >                  */
> > >                 if (PageBuddy(page)) {
> > > -                       unsigned long freepage_order =
> > > page_order_unsafe(page);
> > > +                       unsigned long freepage_order;
> > >
> > > +                       /*
> > > +                        * If this is a merge_skipped page, do merge
> now
> > > +                        * since high-order pages are needed. zone lock
> > > +                        * isn't taken for the merge_skipped check so
> the
> > > +                        * check could be wrong but the worst case is
> we
> > > +                        * lose a merge opportunity.
> > > +                        */
> > > +                       if (page_merge_was_skipped(page))
> > > +                               try_to_merge_page(page);
> > > +
> > > +                       freepage_order = page_order_unsafe(page);
> > >                         /*
> > >                          * Without lock, we cannot be sure that what we
> > > got is
> > >                          * a valid page order. Consider only values in
> the
> > >
> >
> > when the system memory is very very low and try a lot of failures and
> then
>
> >If the system memory is very very low, it doesn't appear there is a need
> >to do compaction since compaction needs to have enough order 0 pages to
> >make a high order one.
>
>
suppose that in free_one_page() will try to merge to high order anytime ,
but now in your patch,
those merge has postponed when system in low memory status, it is very easy
let system trigger
low memory state and get poor performance.

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