Re: [PATCH v2 03/10] mm: don't pass "enum lru_list" to lru list addition functions

From: Yu Zhao
Date: Wed Feb 24 2021 - 03:41:10 EST

On Wed, Feb 24, 2021 at 04:06:45PM +0800, Alex Shi wrote:
>
>
> 在 2021/2/24 下午1:29, Yu Zhao 写道:
> > On Tue, Feb 23, 2021 at 02:50:11PM -0800, Andrew Morton wrote:
> >> On Tue, 26 Jan 2021 15:14:38 -0700 Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
> >>
> >>> On Tue, Jan 26, 2021 at 10:01:11PM +0000, Matthew Wilcox wrote:
> >>>> On Fri, Jan 22, 2021 at 03:05:53PM -0700, Yu Zhao wrote:
> >>>>> +++ b/mm/swap.c
> >>>>> @@ -231,7 +231,7 @@ static void pagevec_move_tail_fn(struct page *page, struct lruvec *lruvec)
> >>>>> if (!PageUnevictable(page)) {
> >>>>> del_page_from_lru_list(page, lruvec, page_lru(page));
> >>>>> ClearPageActive(page);
> >>>>> - add_page_to_lru_list_tail(page, lruvec, page_lru(page));
> >>>>> + add_page_to_lru_list_tail(page, lruvec);
> >>>>> __count_vm_events(PGROTATED, thp_nr_pages(page));
> >>>>> }
> >>>>
> >>>> Is it profitable to do ...
> >>>>
> >>>> - del_page_from_lru_list(page, lruvec, page_lru(page));
> >>>> + enum lru_list lru = page_lru(page);
> >>>> + del_page_from_lru_list(page, lruvec, lru);
> >>>> ClearPageActive(page);
> >>>> - add_page_to_lru_list_tail(page, lruvec, page_lru(page));
> >>>> + lru &= ~LRU_ACTIVE;
> >>>> + add_page_to_lru_list_tail(page, lruvec, lru);
> >>>
> >>> Ok, now we want to trade readability for size. Sure, I'll see how
> >>> much we could squeeze.
> >>
> >> As nothing has happened here and the code bloat issue remains, I'll
> >> hold this series out of 5.12-rc1.
> >
> > Sorry for the slow response. I was trying to ascertain why
> > page_lru(), a tiny helper, could bloat vmlinux by O(KB). It turned out
> > compound_head() included in Page{Active,Unevictable} is a nuisance in
> > our case. Testing PG_{active,unevictable} against
> > compound_head(page)->flags is really unnecessary because all lru
> > operations are eventually done on page->lru not
> > compound_head(page)->lru. With the following change, which sacrifices
> > the readability a bit, we gain 998 bytes with Clang but lose 227 bytes
> > with GCC, which IMO is a win. (We use Clang by default.)
> >
> >
> > diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> > index 355ea1ee32bd..ec0878a3cdfe 100644
> > --- a/include/linux/mm_inline.h
> > +++ b/include/linux/mm_inline.h
> > @@ -46,14 +46,12 @@ static __always_inline void __clear_page_lru_flags(struct page *page)
> > {
> > VM_BUG_ON_PAGE(!PageLRU(page), page);
> >
> > - __ClearPageLRU(page);
> > -
> > /* this shouldn't happen, so leave the flags to bad_page() */
> > - if (PageActive(page) && PageUnevictable(page))
> > + if ((page->flags & (BIT(PG_active) | BIT(PG_unevictable))) ==
> > + (BIT(PG_active) | BIT(PG_unevictable)))
> > return;
> >
> > - __ClearPageActive(page);
> > - __ClearPageUnevictable(page);
> > + page->flags &= ~(BIT(PG_lru) | BIT(PG_active) | BIT(PG_unevictable));
> > }
> >
> > /**
> > @@ -65,18 +63,12 @@ static __always_inline void __clear_page_lru_flags(struct page *page)
> > */
> > static __always_inline enum lru_list page_lru(struct page *page)
> > {
> > - enum lru_list lru;
> > + unsigned long flags = READ_ONCE(page->flags);
> >
> > VM_BUG_ON_PAGE(PageActive(page) && PageUnevictable(page), page);
> >
> > - if (PageUnevictable(page))
> > - return LRU_UNEVICTABLE;
> > -
> > - lru = page_is_file_lru(page) ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON;
> > - if (PageActive(page))
> > - lru += LRU_ACTIVE;
> > -
> > - return lru;
> > + return (flags & BIT(PG_unevictable)) ? LRU_UNEVICTABLE :
> > + (LRU_FILE * !(flags & BIT(PG_swapbacked)) + !!(flags & BIT(PG_active)));
>
> Currently each of page flags used different flags policy, does this mean above flags could be
> change to PF_ANY policy?

That's a good question. Semantically, no because
PG_{active,unevictable} only apply to head pages. But practically,
I think the answer is yes, and the only place that needs to
explicitly call compound_head() is gather_stats() in
fs/proc/task_mmu.c, IIRC.

>
> Thanks
> Alex
>
> > }
> >
> > static __always_inline void add_page_to_lru_list(struct page *page,
> >
> >
> > I'll post this as a separate patch. Below the bloat-o-meter collected
> > on top of c03c21ba6f4e.
> >
> > $ ./scripts/bloat-o-meter ../vmlinux.clang.orig ../vmlinux.clang
> > add/remove: 0/1 grow/shrink: 7/10 up/down: 191/-1189 (-998)
> > Function old new delta
> > lru_lazyfree_fn 848 893 +45
> > lru_deactivate_file_fn 1037 1075 +38
> > perf_trace_mm_lru_insertion 515 548 +33
> > check_move_unevictable_pages 983 1006 +23
> > __activate_page 706 729 +23
> > trace_event_raw_event_mm_lru_insertion 476 497 +21
> > lru_deactivate_fn 691 699 +8
> > __bpf_trace_mm_lru_insertion 13 11 -2
> > __traceiter_mm_lru_insertion 67 62 -5
> > move_pages_to_lru 964 881 -83
> > __pagevec_lru_add_fn 665 581 -84
> > isolate_lru_page 524 419 -105
> > __munlock_pagevec 1609 1481 -128
> > isolate_migratepages_block 3370 3237 -133
> > __page_cache_release 556 413 -143
> > lruvec_lru_size 151 - -151
> > release_pages 1025 866 -159
> > pagevec_move_tail_fn 805 609 -196
> > Total: Before=19502982, After=19501984, chg -0.01%
> >
> > $ ./scripts/bloat-o-meter ../vmlinux.gcc.orig ../vmlinux.gcc
> > add/remove: 0/1 grow/shrink: 9/9 up/down: 1010/-783 (227)
> > Function old new delta
> > shrink_lruvec 1690 1950 +260
> > lru_deactivate_file_fn 961 1128 +167
> > isolate_migratepages_block 3286 3427 +141
> > check_move_unevictable_pages 1042 1170 +128
> > lru_lazyfree_fn 709 822 +113
> > lru_deactivate_fn 665 724 +59
> > __activate_page 703 760 +57
> > trace_event_raw_event_mm_lru_insertion 432 478 +46
> > perf_trace_mm_lru_insertion 464 503 +39
> > __bpf_trace_mm_lru_insertion 13 11 -2
> > __traceiter_mm_lru_insertion 66 57 -9
> > isolate_lru_page 472 405 -67
> > __munlock_pagevec 1282 1212 -70
> > __pagevec_lru_add 976 893 -83
> > __page_cache_release 508 418 -90
> > release_pages 978 887 -91
> > move_pages_to_lru 954 853 -101
> > lruvec_lru_size 131 - -131
> > pagevec_move_tail_fn 770 631 -139
> > Total: Before=19237248, After=19237475, chg +0.00%
> >