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Message-ID: <112dcd31-70cf-9151-912c-da84fd6be7e9@linux.alibaba.com>
Date: Sun, 16 Aug 2020 21:53:37 +0800
From: Alex Shi <alex.shi@...ux.alibaba.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>,
Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm/pageblock: mitigation cmpxchg false sharing in
pageblock flags
在 2020/8/16 下午12:09, Matthew Wilcox 写道:
> On Sun, Aug 16, 2020 at 11:47:56AM +0800, Alex Shi wrote:
>> +++ b/mm/page_alloc.c
>> @@ -467,6 +467,8 @@ static inline int pfn_to_bitidx(struct page *page, unsigned long pfn)
>> return (pfn >> pageblock_order) * NR_PAGEBLOCK_BITS;
>> }
>>
>> +#define BITS_PER_CHAR 8
>
> include/linux/bits.h:#define BITS_PER_BYTE 8
Thank for reminder!
>
>> bitmap = get_pageblock_bitmap(page, pfn);
>> bitidx = pfn_to_bitidx(page, pfn);
>> - word_bitidx = bitidx / BITS_PER_LONG;
>> - bitidx &= (BITS_PER_LONG-1);
>> + word_bitidx = bitidx / BITS_PER_CHAR;
>> + bitidx &= (BITS_PER_CHAR-1);
>
> It's not a word any more. it's a byte.
>
Yes, will change this.
Thanks!
>From 963425639f56c1ba1c997653a4ff6885c81dc0ab Mon Sep 17 00:00:00 2001
From: Alex Shi <alex.shi@...ux.alibaba.com>
Date: Sat, 15 Aug 2020 22:54:17 +0800
Subject: [PATCH 1/2] mm/pageblock: mitigation cmpxchg false sharing in
pageblock flags
pageblock_flags is used as long, since every pageblock_flags is just 4
bits, 'long' size will include 8(32bit machine) or 16 pageblocks' flags,
that flag setting has to sync in cmpxchg with 7 or 15 other pageblock
flags. It would cause long waiting for sync.
If we could change the pageblock_flags variable as char, we could use
char size cmpxchg, which just sync up with 2 pageblock flags. it could
relief much false sharing in cmpxchg.
Signed-off-by: Alex Shi <alex.shi@...ux.alibaba.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Cc: linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org
---
include/linux/mmzone.h | 6 +++---
include/linux/pageblock-flags.h | 2 +-
mm/page_alloc.c | 38 +++++++++++++++++++-------------------
3 files changed, 23 insertions(+), 23 deletions(-)
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 0ed520954843..c92d6d24527d 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -438,7 +438,7 @@ struct zone {
* Flags for a pageblock_nr_pages block. See pageblock-flags.h.
* In SPARSEMEM, this map is stored in struct mem_section
*/
- unsigned long *pageblock_flags;
+ unsigned char *pageblock_flags;
#endif /* CONFIG_SPARSEMEM */
/* zone_start_pfn == zone_start_paddr >> PAGE_SHIFT */
@@ -1159,7 +1159,7 @@ struct mem_section_usage {
DECLARE_BITMAP(subsection_map, SUBSECTIONS_PER_SECTION);
#endif
/* See declaration of similar field in struct zone */
- unsigned long pageblock_flags[0];
+ unsigned char pageblock_flags[0];
};
void subsection_map_init(unsigned long pfn, unsigned long nr_pages);
@@ -1212,7 +1212,7 @@ struct mem_section {
extern struct mem_section mem_section[NR_SECTION_ROOTS][SECTIONS_PER_ROOT];
#endif
-static inline unsigned long *section_to_usemap(struct mem_section *ms)
+static inline unsigned char *section_to_usemap(struct mem_section *ms)
{
return ms->usage->pageblock_flags;
}
diff --git a/include/linux/pageblock-flags.h b/include/linux/pageblock-flags.h
index fff52ad370c1..d189441568eb 100644
--- a/include/linux/pageblock-flags.h
+++ b/include/linux/pageblock-flags.h
@@ -54,7 +54,7 @@ enum pageblock_bits {
/* Forward declaration */
struct page;
-unsigned long get_pfnblock_flags_mask(struct page *page,
+unsigned char get_pfnblock_flags_mask(struct page *page,
unsigned long pfn,
unsigned long mask);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 66d45e9cc358..eb5cca7e8683 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -447,7 +447,7 @@ static inline bool defer_init(int nid, unsigned long pfn, unsigned long end_pfn)
#endif
/* Return a pointer to the bitmap storing bits affecting a block of pages */
-static inline unsigned long *get_pageblock_bitmap(struct page *page,
+static inline unsigned char *get_pageblock_bitmap(struct page *page,
unsigned long pfn)
{
#ifdef CONFIG_SPARSEMEM
@@ -476,24 +476,24 @@ static inline int pfn_to_bitidx(struct page *page, unsigned long pfn)
* Return: pageblock_bits flags
*/
static __always_inline
-unsigned long __get_pfnblock_flags_mask(struct page *page,
+unsigned char __get_pfnblock_flags_mask(struct page *page,
unsigned long pfn,
unsigned long mask)
{
- unsigned long *bitmap;
- unsigned long bitidx, word_bitidx;
- unsigned long word;
+ unsigned char *bitmap;
+ unsigned long bitidx, byte_bitidx;
+ unsigned char byte;
bitmap = get_pageblock_bitmap(page, pfn);
bitidx = pfn_to_bitidx(page, pfn);
- word_bitidx = bitidx / BITS_PER_LONG;
- bitidx &= (BITS_PER_LONG-1);
+ byte_bitidx = bitidx / BITS_PER_BYTE;
+ bitidx &= (BITS_PER_BYTE-1);
- word = bitmap[word_bitidx];
- return (word >> bitidx) & mask;
+ byte = bitmap[byte_bitidx];
+ return (byte >> bitidx) & mask;
}
-unsigned long get_pfnblock_flags_mask(struct page *page, unsigned long pfn,
+unsigned char get_pfnblock_flags_mask(struct page *page, unsigned long pfn,
unsigned long mask)
{
return __get_pfnblock_flags_mask(page, pfn, mask);
@@ -515,29 +515,29 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
unsigned long pfn,
unsigned long mask)
{
- unsigned long *bitmap;
- unsigned long bitidx, word_bitidx;
- unsigned long old_word, word;
+ unsigned char *bitmap;
+ unsigned long bitidx, byte_bitidx;
+ unsigned char old_byte, byte;
BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
BUILD_BUG_ON(MIGRATE_TYPES > (1 << PB_migratetype_bits));
bitmap = get_pageblock_bitmap(page, pfn);
bitidx = pfn_to_bitidx(page, pfn);
- word_bitidx = bitidx / BITS_PER_LONG;
- bitidx &= (BITS_PER_LONG-1);
+ byte_bitidx = bitidx / BITS_PER_BYTE;
+ bitidx &= (BITS_PER_BYTE-1);
VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page);
mask <<= bitidx;
flags <<= bitidx;
- word = READ_ONCE(bitmap[word_bitidx]);
+ byte = READ_ONCE(bitmap[byte_bitidx]);
for (;;) {
- old_word = cmpxchg(&bitmap[word_bitidx], word, (word & ~mask) | flags);
- if (word == old_word)
+ old_byte = cmpxchg(&bitmap[byte_bitidx], byte, (byte & ~mask) | flags);
+ if (byte == old_byte)
break;
- word = old_word;
+ byte = old_byte;
}
}
--
1.8.3.1
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