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@xxxxxxxxxxxxxxxxx>
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@xxxxxxxxx>
Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
---
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
