2018-03-20 18:59 GMT-07:00 Aaron Lu <aaron.lu@xxxxxxxxx>:
>If the system memory is very very low, it doesn't appear there is a needOn Tue, Mar 20, 2018 at 03:58:51PM -0700, Figo.zhang wrote:
> 2018-03-20 1:54 GMT-07:00 Aaron Lu <aaron.lu@xxxxxxxxx>:
>
> > 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@xxxxxxxxx>
> > Signed-off-by: Aaron Lu <aaron.lu@xxxxxxxxx>
> > ---
> > 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
>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.
