Re: [PATCH] ARM: mm: Speed up page list initialization during boot

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Thu, Dec 31, 2015 at 05:05:54AM -0800, Chirantan Ekbote wrote:
> On Mon, Dec 28, 2015 at 2:15 AM, Jungseung Lee <js07.lee@samsung.com> wrote:
> > Hi,
> >
> >>During boot, we populate the page lists by using the page freeing
> >>mechanism on every individual page.  Unfortunately, this is very
> >>inefficient because the memory manager spends a lot of time coalescing
> >>pairs of adjacent free pages into bigger blocks.
> >>
> >>Rather than adding a single order 0 page at a time, we can take
> >>advantage of the fact that we know that all the pages are available and
> >>free up big blocks of pages at a time instead.
> >>
> >>Signed-off-by: Chirantan Ekbote <chirantan at chromium.org>
> >>---
> >> arch/arm/mm/init.c  | 19 +++++++++++++++++--  include/linux/gfp.h |  1
> >>+
> >> mm/internal.h       |  1 -
> >> 3 files changed, 18 insertions(+), 3 deletions(-)
> >>
> >>diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c index
> >>97c293e..c7fc2d8 100644
> >>--- a/arch/arm/mm/init.c
> >>+++ b/arch/arm/mm/init.c
> >>@@ -22,6 +22,7 @@
> >> #include <linux/memblock.h>
> >> #include <linux/dma-contiguous.h>
> >> #include <linux/sizes.h>
> >>+#include <linux/bitops.h>
> >>
> >> #include <asm/mach-types.h>
> >> #include <asm/memblock.h>
> >>@@ -469,8 +470,22 @@ static void __init free_unused_memmap(struct
> >>meminfo
> > *mi)
> >> #ifdef CONFIG_HIGHMEM
> >> static inline void free_area_high(unsigned long pfn, unsigned long
> >>end)  {
> >>-      for (; pfn < end; pfn++)
> >>-              free_highmem_page(pfn_to_page(pfn));
> >>+      while (pfn < end) {
> >>+              struct page *page = pfn_to_page(pfn);
> >>+              unsigned long order = min(__ffs(pfn), MAX_ORDER - 1);
> >>+              unsigned long nr_pages = 1 << order;
> >>+              unsigned long rem = end - pfn;
> >>+
> >>+              if (nr_pages > rem) {
> >>+                      order = __fls(rem);
> >>+                      nr_pages = 1 << order;
> >>+              }
> >>+
> >>+              __free_pages_bootmem(page, order);
> >>+              totalram_pages += nr_pages;
> >>+              totalhigh_pages += nr_pages;
> >>+              pfn += nr_pages;
> >>+      }
> >> }
> >> #endif
> >>
> >>diff --git a/include/linux/gfp.h b/include/linux/gfp.h index
> >>39b81dc..a63d666 100644
> >>--- a/include/linux/gfp.h
> >>+++ b/include/linux/gfp.h
> >>@@ -367,6 +367,7 @@ void *alloc_pages_exact_nid(int nid, size_t size,
> >>gfp_t
> > gfp_mask);
> >> #define __get_dma_pages(gfp_mask, order) \
> >>               __get_free_pages((gfp_mask) | GFP_DMA, (order))
> >>
> >>+extern void __free_pages_bootmem(struct page *page, unsigned int
> >>+order);
> >> extern void __free_pages(struct page *page, unsigned int order);
> >>extern void free_pages(unsigned long addr, unsigned int order);  extern
> >>void free_hot_cold_page(struct page *page, int cold); diff --git
> >>a/mm/internal.h b/mm/internal.h index 29e1e76..d2b8738 100644
> >>--- a/mm/internal.h
> >>+++ b/mm/internal.h
> >>@@ -93,7 +93,6 @@ extern pmd_t *mm_find_pmd(struct mm_struct *mm,
> >>unsigned
> > long address);
> >> /*
> >>  * in mm/page_alloc.c
> >>  */
> >>-extern void __free_pages_bootmem(struct page *page, unsigned int
> >>order);  extern void prep_compound_page(struct page *page, unsigned
> >>long order);  #ifdef CONFIG_MEMORY_FAILURE  extern bool
> >>is_free_buddy_page(struct page *page);
> >>--
> >>1.9.1.423.g4596e3a
> >
> > This patch really could save boot time.
> > Is there any reason this patch is not merged to mainline kernel?
> >
> 
> Well it was ignored when I originally posted it so I assumed mainline
> developers weren't really interested.  I can re-spin and send a new
> version if there's interest in getting it merged now.

Not getting a reply can be for many reasons: people may be too busy
and there may be too much other mail.  I generally have a major problem
with email in that it's all too easy for stuff to get buried and
forgotten.  Remember, some of us get a lot of emails a day, and mails
which should get a reply do get dropped simply because there isn't
enough time to read them and properly write replies to every message
that needs a response.

So, it's good practice to resend after a week or so if you think your
message has been missed; it may well have been missed and buried under
a thousand or more other messages by that time.

In any case, it would be nice for such "speed up" changes to be
quantified with some kind of measurement.  How much does it speed the
boot process up, and in what circumstances?

Thanks.

-- 
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