linux-kernel - Re: [PATCH v2 1/2] mm/pageblock: mitigation cmpxchg false sharing in pageblock flags

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Date:   Sun, 30 Aug 2020 18:14:33 +0800
From:   Alex Shi <alex.shi@...ux.alibaba.com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>
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 v2 1/2] mm/pageblock: mitigation cmpxchg false sharing in
 pageblock flags



在 2020/8/19 下午3:55, Anshuman Khandual 写道:
> 
> 
> On 08/19/2020 11:17 AM, Alex Shi wrote:
>> 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.
> 
> Do you have numbers demonstrating claimed performance improvement
> after this change ?
> 

the performance data show in another email.

LKP reported the arm6 has a bug on this patchset, since it has no cmpxchgb
solution, so maybe let's fallback to cmpxchg on it.

>From db3d97ba8cc5e206b440bd40a92ef6955ad86bc0 Mon Sep 17 00:00:00 2001
From: Alex Shi <alex.shi@...ux.alibaba.com>
Date: Tue, 18 Aug 2020 15:51:18 +0800
Subject: [PATCH v2 3/3] armv6: fix armv6 build issue

Arm v6 can not simulate cmpxchg1 func, so we have to use cmpxchg4 on it.

   arm-linux-gnueabi-ld: mm/page_alloc.o: in function `set_pfnblock_flags_mask':
   (.text+0x32b4): undefined reference to `__bad_cmpxchg'
   arm-linux-gnueabi-ld: (.text+0x32e0): undefined reference to `__bad_cmpxchg'
   arm-linux-gnueabi-ld: drivers/net/ethernet/aquantia/atlantic/hw_atl/hw_atl_b0.o: in function `hw_atl_b0_get_mac_temp':
   hw_atl_b0.c:(.text+0x30fc): undefined reference to `__bad_udelay'

Reported-by: kernel test robot <lkp@...el.com>
Signed-off-by: Alex Shi <alex.shi@...ux.alibaba.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: Russell King <linux@...linux.org.uk>
Cc: linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org
Cc: linux-arm-kernel@...ts.infradead.org
---
 mm/page_alloc.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7da09d66233b..c09146a8946c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -517,7 +517,11 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
 {
 	unsigned char *bitmap;
 	unsigned long bitidx, byte_bitidx;
+#ifdef	CONFIG_CPU_V6
+	unsigned long old_byte, byte;
+#else
 	unsigned char old_byte, byte;
+#endif
 
 	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != BITS_PER_BYTE);
 	BUILD_BUG_ON(MIGRATE_TYPES > (1 << PB_migratetype_bits));
@@ -532,9 +536,18 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
 	mask <<= bitidx;
 	flags <<= bitidx;
 
+#ifdef	CONFIG_CPU_V6
+	byte = (unsigned long)READ_ONCE(bitmap[byte_bitidx]);
+#else
 	byte = READ_ONCE(bitmap[byte_bitidx]);
+#endif
 	for (;;) {
+#ifdef	CONFIG_CPU_V6
+		/* arm v6 has no cmpxchgb function, so still false sharing long word */
+		old_byte = cmpxchg((unsigned long*)&bitmap[byte_bitidx], byte, (byte & ~mask) | flags);
+#else
 		old_byte = cmpxchg(&bitmap[byte_bitidx], byte, (byte & ~mask) | flags);
+#endif
 		if (byte == old_byte)
 			break;
 		byte = old_byte;
-- 
1.8.3.1