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    SubjectRe: [PATCH 24/31] sched, numa, mm: Introduce last_nid in the pageframe
    On Thu, Oct 25, 2012 at 02:16:41PM +0200, Peter Zijlstra wrote:
    > Introduce a per-page last_nid field, fold this into the struct
    > page::flags field whenever possible.
    >

    This is used later in the series.

    > The unlikely/rare 32bit NUMA configs will likely grow the page-frame.
    >

    If someone files a bug report that lowmem pressure is higher on their
    32-bit NUMA machine I would be a bit surprised. Maybe abusers of NUMA
    care but screw it.

    > Completely dropping 32bit support for CONFIG_SCHED_NUMA would simplify
    > things, but it would also remove the warning if we grow enough 64bit
    > only page-flags to push the last-nid out.
    >
    > Suggested-by: Rik van Riel <riel@redhat.com>
    > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    > Cc: Linus Torvalds <torvalds@linux-foundation.org>
    > Cc: Andrew Morton <akpm@linux-foundation.org>
    > Cc: Rik van Riel <riel@redhat.com>
    > Cc: Andrea Arcangeli <aarcange@redhat.com>
    > Signed-off-by: Ingo Molnar <mingo@kernel.org>
    > ---
    > include/linux/mm.h | 90 ++++++++++++++++++++------------------
    > include/linux/mm_types.h | 5 ++
    > include/linux/mmzone.h | 14 -----
    > include/linux/page-flags-layout.h | 83 +++++++++++++++++++++++++++++++++++
    > mm/huge_memory.c | 1
    > mm/memory.c | 4 +
    > 6 files changed, 143 insertions(+), 54 deletions(-)
    > create mode 100644 include/linux/page-flags-layout.h
    >
    > Index: tip/include/linux/mm.h
    > ===================================================================
    > --- tip.orig/include/linux/mm.h
    > +++ tip/include/linux/mm.h
    > @@ -594,50 +594,11 @@ static inline pte_t maybe_mkwrite(pte_t
    > * sets it, so none of the operations on it need to be atomic.
    > */
    >
    > -
    > -/*
    > - * page->flags layout:
    > - *
    > - * There are three possibilities for how page->flags get
    > - * laid out. The first is for the normal case, without
    > - * sparsemem. The second is for sparsemem when there is
    > - * plenty of space for node and section. The last is when
    > - * we have run out of space and have to fall back to an
    > - * alternate (slower) way of determining the node.
    > - *
    > - * No sparsemem or sparsemem vmemmap: | NODE | ZONE | ... | FLAGS |
    > - * classic sparse with space for node:| SECTION | NODE | ZONE | ... | FLAGS |
    > - * classic sparse no space for node: | SECTION | ZONE | ... | FLAGS |
    > - */

    The move to page-flags-layout.h should have been a separate patch!
    Figuring out what you actually added to the layout is going to be a
    complete headache (headache was not the first word I was going to use).

    In other words I did not try very hard and I'll just be scanning for
    something obvious.

    > -#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
    > -#define SECTIONS_WIDTH SECTIONS_SHIFT
    > -#else
    > -#define SECTIONS_WIDTH 0
    > -#endif
    > -
    > -#define ZONES_WIDTH ZONES_SHIFT
    > -
    > -#if SECTIONS_WIDTH+ZONES_WIDTH+NODES_SHIFT <= BITS_PER_LONG - NR_PAGEFLAGS
    > -#define NODES_WIDTH NODES_SHIFT
    > -#else
    > -#ifdef CONFIG_SPARSEMEM_VMEMMAP
    > -#error "Vmemmap: No space for nodes field in page flags"
    > -#endif
    > -#define NODES_WIDTH 0
    > -#endif
    > -
    > -/* Page flags: | [SECTION] | [NODE] | ZONE | ... | FLAGS | */
    > +/* Page flags: | [SECTION] | [NODE] | ZONE | [LAST_NID] | ... | FLAGS | */
    > #define SECTIONS_PGOFF ((sizeof(unsigned long)*8) - SECTIONS_WIDTH)
    > #define NODES_PGOFF (SECTIONS_PGOFF - NODES_WIDTH)
    > #define ZONES_PGOFF (NODES_PGOFF - ZONES_WIDTH)
    > -
    > -/*
    > - * We are going to use the flags for the page to node mapping if its in
    > - * there. This includes the case where there is no node, so it is implicit.
    > - */
    > -#if !(NODES_WIDTH > 0 || NODES_SHIFT == 0)
    > -#define NODE_NOT_IN_PAGE_FLAGS
    > -#endif
    > +#define LAST_NID_PGOFF (ZONES_PGOFF - LAST_NID_WIDTH)
    >
    > /*
    > * Define the bit shifts to access each section. For non-existent
    > @@ -647,6 +608,7 @@ static inline pte_t maybe_mkwrite(pte_t
    > #define SECTIONS_PGSHIFT (SECTIONS_PGOFF * (SECTIONS_WIDTH != 0))
    > #define NODES_PGSHIFT (NODES_PGOFF * (NODES_WIDTH != 0))
    > #define ZONES_PGSHIFT (ZONES_PGOFF * (ZONES_WIDTH != 0))
    > +#define LAST_NID_PGSHIFT (LAST_NID_PGOFF * (LAST_NID_WIDTH != 0))
    >

    Why is LAST_NID_PGSHIFT != NODES_PGSHIFT?

    Oh, it more or less is but it's hidden. Screw that, get rid of
    LAST_NIFT_SHIFT WIDTH and express this entirely in terms of NODES_PFSHIFT
    and friends.

    > /* NODE:ZONE or SECTION:ZONE is used to ID a zone for the buddy allocator */
    > #ifdef NODE_NOT_IN_PAGE_FLAGS
    > @@ -668,6 +630,7 @@ static inline pte_t maybe_mkwrite(pte_t
    > #define ZONES_MASK ((1UL << ZONES_WIDTH) - 1)
    > #define NODES_MASK ((1UL << NODES_WIDTH) - 1)
    > #define SECTIONS_MASK ((1UL << SECTIONS_WIDTH) - 1)
    > +#define LAST_NID_MASK ((1UL << LAST_NID_WIDTH) - 1)
    > #define ZONEID_MASK ((1UL << ZONEID_SHIFT) - 1)
    >
    > static inline enum zone_type page_zonenum(const struct page *page)
    > @@ -706,6 +669,51 @@ static inline int page_to_nid(const stru
    > }
    > #endif
    >
    > +#ifdef CONFIG_SCHED_NUMA
    > +#ifdef LAST_NID_NOT_IN_PAGE_FLAGS
    > +static inline int page_xchg_last_nid(struct page *page, int nid)
    > +{
    > + return xchg(&page->_last_nid, nid);
    > +}
    > +
    > +static inline int page_last_nid(struct page *page)
    > +{
    > + return page->_last_nid;
    > +}
    > +#else
    > +static inline int page_xchg_last_nid(struct page *page, int nid)
    > +{
    > + unsigned long old_flags, flags;
    > + int last_nid;
    > +
    > + do {
    > + old_flags = flags = page->flags;
    > + last_nid = (flags >> LAST_NID_PGSHIFT) & LAST_NID_MASK;
    > +
    > + flags &= ~(LAST_NID_MASK << LAST_NID_PGSHIFT);
    > + flags |= (nid & LAST_NID_MASK) << LAST_NID_PGSHIFT;
    > + } while (unlikely(cmpxchg(&page->flags, old_flags, flags) != old_flags));
    > +

    This opens a very small window where this function messed up the flags
    and fixes them up very shortly afterwards. For most page flags it will
    not matter but potentially causes weirdness with the extended page
    flags or are we protected by something else?

    > + return last_nid;
    > +}
    > +
    > +static inline int page_last_nid(struct page *page)
    > +{
    > + return (page->flags >> LAST_NID_PGSHIFT) & LAST_NID_MASK;
    > +}
    > +#endif /* LAST_NID_NOT_IN_PAGE_FLAGS */
    > +#else /* CONFIG_SCHED_NUMA */
    > +static inline int page_xchg_last_nid(struct page *page, int nid)
    > +{
    > + return page_to_nid(page);
    > +}
    > +
    > +static inline int page_last_nid(struct page *page)
    > +{
    > + return page_to_nid(page);
    > +}
    > +#endif /* CONFIG_SCHED_NUMA */
    > +
    > static inline struct zone *page_zone(const struct page *page)
    > {
    > return &NODE_DATA(page_to_nid(page))->node_zones[page_zonenum(page)];
    > Index: tip/include/linux/mm_types.h
    > ===================================================================
    > --- tip.orig/include/linux/mm_types.h
    > +++ tip/include/linux/mm_types.h
    > @@ -12,6 +12,7 @@
    > #include <linux/cpumask.h>
    > #include <linux/page-debug-flags.h>
    > #include <linux/uprobes.h>
    > +#include <linux/page-flags-layout.h>
    > #include <asm/page.h>
    > #include <asm/mmu.h>
    >
    > @@ -175,6 +176,10 @@ struct page {
    > */
    > void *shadow;
    > #endif
    > +
    > +#ifdef LAST_NID_NOT_IN_PAGE_FLAGS
    > + int _last_nid;
    > +#endif
    > }
    > /*
    > * The struct page can be forced to be double word aligned so that atomic ops
    > Index: tip/include/linux/mmzone.h
    > ===================================================================
    > --- tip.orig/include/linux/mmzone.h
    > +++ tip/include/linux/mmzone.h
    > @@ -15,7 +15,7 @@
    > #include <linux/seqlock.h>
    > #include <linux/nodemask.h>
    > #include <linux/pageblock-flags.h>
    > -#include <generated/bounds.h>
    > +#include <linux/page-flags-layout.h>
    > #include <linux/atomic.h>
    > #include <asm/page.h>
    >
    > @@ -317,16 +317,6 @@ enum zone_type {
    > * match the requested limits. See gfp_zone() in include/linux/gfp.h
    > */
    >
    > -#if MAX_NR_ZONES < 2
    > -#define ZONES_SHIFT 0
    > -#elif MAX_NR_ZONES <= 2
    > -#define ZONES_SHIFT 1
    > -#elif MAX_NR_ZONES <= 4
    > -#define ZONES_SHIFT 2
    > -#else
    > -#error ZONES_SHIFT -- too many zones configured adjust calculation
    > -#endif
    > -
    > struct zone {
    > /* Fields commonly accessed by the page allocator */
    >
    > @@ -1029,8 +1019,6 @@ static inline unsigned long early_pfn_to
    > * PA_SECTION_SHIFT physical address to/from section number
    > * PFN_SECTION_SHIFT pfn to/from section number
    > */
    > -#define SECTIONS_SHIFT (MAX_PHYSMEM_BITS - SECTION_SIZE_BITS)
    > -
    > #define PA_SECTION_SHIFT (SECTION_SIZE_BITS)
    > #define PFN_SECTION_SHIFT (SECTION_SIZE_BITS - PAGE_SHIFT)
    >
    > Index: tip/include/linux/page-flags-layout.h
    > ===================================================================
    > --- /dev/null
    > +++ tip/include/linux/page-flags-layout.h
    > @@ -0,0 +1,83 @@
    > +#ifndef _LINUX_PAGE_FLAGS_LAYOUT
    > +#define _LINUX_PAGE_FLAGS_LAYOUT
    > +
    > +#include <linux/numa.h>
    > +#include <generated/bounds.h>
    > +
    > +#if MAX_NR_ZONES < 2
    > +#define ZONES_SHIFT 0
    > +#elif MAX_NR_ZONES <= 2
    > +#define ZONES_SHIFT 1
    > +#elif MAX_NR_ZONES <= 4
    > +#define ZONES_SHIFT 2
    > +#else
    > +#error ZONES_SHIFT -- too many zones configured adjust calculation
    > +#endif
    > +
    > +#ifdef CONFIG_SPARSEMEM
    > +#include <asm/sparsemem.h>
    > +
    > +/*
    > + * SECTION_SHIFT #bits space required to store a section #
    > + */
    > +#define SECTIONS_SHIFT (MAX_PHYSMEM_BITS - SECTION_SIZE_BITS)
    > +#endif
    > +
    > +/*
    > + * page->flags layout:
    > + *
    > + * There are five possibilities for how page->flags get laid out. The first
    > + * (and second) is for the normal case, without sparsemem. The third is for
    > + * sparsemem when there is plenty of space for node and section. The last is
    > + * when we have run out of space and have to fall back to an alternate (slower)
    > + * way of determining the node.
    > + *
    > + * No sparsemem or sparsemem vmemmap: | NODE | ZONE | ... | FLAGS |
    > + * " plus space for last_nid:| NODE | ZONE | LAST_NID | ... | FLAGS |
    > + * classic sparse with space for node:| SECTION | NODE | ZONE | ... | FLAGS |
    > + * " plus space for last_nid:| SECTION | NODE | ZONE | LAST_NID | ... | FLAGS |
    > + * classic sparse no space for node: | SECTION | ZONE | ... | FLAGS |
    > + */
    > +#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
    > +
    > +#define SECTIONS_WIDTH SECTIONS_SHIFT
    > +#else
    > +#define SECTIONS_WIDTH 0
    > +#endif
    > +
    > +#define ZONES_WIDTH ZONES_SHIFT
    > +
    > +#if SECTIONS_WIDTH+ZONES_WIDTH+NODES_SHIFT <= BITS_PER_LONG - NR_PAGEFLAGS
    > +#define NODES_WIDTH NODES_SHIFT
    > +#else
    > +#ifdef CONFIG_SPARSEMEM_VMEMMAP
    > +#error "Vmemmap: No space for nodes field in page flags"
    > +#endif
    > +#define NODES_WIDTH 0
    > +#endif
    > +
    > +#ifdef CONFIG_SCHED_NUMA
    > +#define LAST_NID_SHIFT NODES_SHIFT
    > +#else
    > +#define LAST_NID_SHIFT 0
    > +#endif
    > +
    > +#if SECTIONS_WIDTH+ZONES_WIDTH+NODES_SHIFT+LAST_NID_SHIFT <= BITS_PER_LONG - NR_PAGEFLAGS
    > +#define LAST_NID_WIDTH LAST_NID_SHIFT
    > +#else
    > +#define LAST_NID_WIDTH 0
    > +#endif
    > +
    > +/*
    > + * We are going to use the flags for the page to node mapping if its in
    > + * there. This includes the case where there is no node, so it is implicit.
    > + */
    > +#if !(NODES_WIDTH > 0 || NODES_SHIFT == 0)
    > +#define NODE_NOT_IN_PAGE_FLAGS
    > +#endif
    > +
    > +#if defined(CONFIG_SCHED_NUMA) && LAST_NID_WIDTH == 0
    > +#define LAST_NID_NOT_IN_PAGE_FLAGS
    > +#endif
    > +
    > +#endif /* _LINUX_PAGE_FLAGS_LAYOUT */
    > Index: tip/mm/huge_memory.c
    > ===================================================================
    > --- tip.orig/mm/huge_memory.c
    > +++ tip/mm/huge_memory.c
    > @@ -1440,6 +1440,7 @@ static void __split_huge_page_refcount(s
    > page_tail->mapping = page->mapping;
    >
    > page_tail->index = page->index + i;
    > + page_xchg_last_nid(page, page_last_nid(page_tail));
    >
    > BUG_ON(!PageAnon(page_tail));
    > BUG_ON(!PageUptodate(page_tail));
    > Index: tip/mm/memory.c
    > ===================================================================
    > --- tip.orig/mm/memory.c
    > +++ tip/mm/memory.c
    > @@ -68,6 +68,10 @@
    >
    > #include "internal.h"
    >
    > +#ifdef LAST_NID_NOT_IN_PAGE_FLAGS
    > +#warning Unfortunate NUMA config, growing page-frame for last_nid.
    > +#endif
    > +
    > #ifndef CONFIG_NEED_MULTIPLE_NODES
    > /* use the per-pgdat data instead for discontigmem - mbligh */
    > unsigned long max_mapnr;
    >
    >

    --
    Mel Gorman
    SUSE Labs


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