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Subject: Re: [PATCH 1/2] mm/pageblock: mitigation cmpxchg false sharing in
 pageblock flags
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <1597549677-7480-1-git-send-email-alex.shi@linux.alibaba.com>
 <20200816040905.GF17456@casper.infradead.org>
From:   Alex Shi <alex.shi@linux.alibaba.com>
Message-ID: <112dcd31-70cf-9151-912c-da84fd6be7e9@linux.alibaba.com>
Date:   Sun, 16 Aug 2020 21:53:37 +0800
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?? 2020/8/16 ????12:09, Matthew Wilcox д??:
> 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@linux.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@linux.alibaba.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.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