Re: [PATCH] LoongArch: Make -mstrict-align be configurable
From: Jianmin Lv
Date: Mon Feb 06 2023 - 05:25:05 EST
On 2023/2/2 下午6:30, WANG Xuerui wrote:
On 2023/2/2 16:42, Huacai Chen wrote:
Introduce Kconfig option ARCH_STRICT_ALIGN to make -mstrict-align be
configurable.
Not all LoongArch cores support h/w unaligned access, we can use the
-mstrict-align build parameter to prevent unaligned accesses.
This option is disabled by default to optimise for performance, but you
can enabled it manually if you want to run kernel on systems without h/w
unaligned access support.
It's customary to accompany "performance-related" changes like this with
some benchmark numbers and concrete use cases where this would be
profitable. Especially given that arch/loongarch developer and user base
is relatively small, we probably don't want to allow customization of
such a low-level characteristic. In general kernel performance does not
vary much with compiler flags like this, so I'd really hope to see some
numbers here to convince people that this is *really* providing gains.
Also, defaulting to emitting unaligned accesses would mean those future,
likely embedded models (and AFAIK some existing models that haven't
reached GA yet) would lose support with the defconfig. Which means
downstream packagers that care about those use cases would have one more
non-default, non-generic option to carry within their Kconfig. We
probably don't want to repeat the history of other architectures (think
arch/arm or arch/mips) where there wasn't really generic builds and
board-specific tweaks proliferated.
Hi, Xuerui
I think the kernels produced with and without -mstrict-align have mainly
following differences:
- Diffirent size. I build two kernls (vmlinux), size of kernel with
-mstrict-align is 26533376 bytes and size of kernel without
-mstrict-align is 26123280 bytes.
- Diffirent performance. For example, in kernel function jhash(), the
assemble code slices with and without -mstrict-align are following:
without -mstrict-align:
900000000032736c <jhash>:
900000000032736c: 15bd5b6d lu12i.w $t1,
-136485(0xdeadb)
9000000000327370: 03bbbdad ori $t1, $t1, 0xeef
9000000000327374: 001019ad add.w $t1, $t1, $a2
9000000000327378: 001015ae add.w $t2, $t1, $a1
900000000032737c: 0280300c addi.w $t0, $zero,
12(0xc)
9000000000327380: 00150091 move $t5, $a0
9000000000327384: 001501d0 move $t4, $t2
9000000000327388: 001501c4 move $a0, $t2
900000000032738c: 6c009585 bgeu $t0, $a1,
148(0x94) # 9000000000327420 <jhash+0xb4>
9000000000327390: 02803012 addi.w $t6, $zero,
12(0xc)
9000000000327394: 24000a2f ldptr.w $t3, $t5, 8(0x8)
9000000000327398: 2400022d ldptr.w $t1, $t5, 0
900000000032739c: 2400062c ldptr.w $t0, $t5, 4(0x4)
90000000003273a0: 001011e4 add.w $a0, $t3, $a0
90000000003273a4: 001111af sub.w $t3, $t1, $a0
90000000003273a8: 001039ef add.w $t3, $t3, $t2
90000000003273ac: 004cf08e rotri.w $t2, $a0, 0x1c
90000000003273b0: 0010418c add.w $t0, $t0, $t4
...
with -mstrict-align:
90000000003310c0 <jhash>:
90000000003310c0: 15bd5b6f lu12i.w $t3,
-136485(0xdeadb)
90000000003310c4: 03bbbdef ori $t3, $t3, 0xeef
90000000003310c8: 001019ef add.w $t3, $t3, $a2
90000000003310cc: 001015e6 add.w $a2, $t3, $a1
90000000003310d0: 0280300d addi.w $t1, $zero, 12(0xc)
90000000003310d4: 0015008c move $t0, $a0
90000000003310d8: 001500d2 move $t6, $a2
90000000003310dc: 001500c4 move $a0, $a2
90000000003310e0: 6c0101a5 bgeu $t1, $a1,
256(0x100) # 90000000003311e0 <jhash+0x120>
90000000003310e4: 02803011 addi.w $t5, $zero, 12(0xc)
90000000003310e8: 2a002589 ld.bu $a5, $t0, 9(0x9)
90000000003310ec: 2a00218d ld.bu $t1, $t0, 8(0x8)
90000000003310f0: 2a002988 ld.bu $a4, $t0, 10(0xa)
90000000003310f4: 2a000587 ld.bu $a3, $t0, 1(0x1)
90000000003310f8: 2a002d8e ld.bu $t2, $t0, 11(0xb)
90000000003310fc: 2a00018b ld.bu $a7, $t0, 0
9000000000331100: 2a000994 ld.bu $t8, $t0, 2(0x2)
9000000000331104: 2a001593 ld.bu $t7, $t0, 5(0x5)
9000000000331108: 2a000d8f ld.bu $t3, $t0, 3(0x3)
900000000033110c: 00412129 slli.d $a5, $a5, 0x8
9000000000331110: 2a00118a ld.bu $a6, $t0, 4(0x4)
9000000000331114: 2a001990 ld.bu $t4, $t0, 6(0x6)
9000000000331118: 00153529 or $a5, $a5, $t1
...
It seems that it's difficult for me to test the performance difference
in a real kernel path with unaligned-access code. So, I use a kernel
module (use simple test code) to show some difference on 3A5000 as
following:
c code:
preempt_disable();
start = ktime_get_ns();
for (i = 0; i < n; i++)
assign(p1[i], q1[i]);
end = ktime_get_ns();
preempt_enable();
printk("mstrict-align-test took: %lld nsec\n", end - start);
assemble code without -mstrict-align:
0: 260000ac ldptr.d $t0, $a1, 0
4: 2700008c stptr.d $t0, $a0, 0
8: 4c000020 jirl $zero, $ra, 0
assemble code with -mstrict-align:
0: 2a0000b3 ld.bu $t7, $a1, 0
4: 2a0004b2 ld.bu $t6, $a1, 1(0x1)
8: 2a0008b1 ld.bu $t5, $a1, 2(0x2)
c: 2a000cb0 ld.bu $t4, $a1, 3(0x3)
10: 2a0010af ld.bu $t3, $a1, 4(0x4)
14: 2a0014ae ld.bu $t2, $a1, 5(0x5)
18: 2a0018ad ld.bu $t1, $a1, 6(0x6)
1c: 2a001cac ld.bu $t0, $a1, 7(0x7)
20: 29000093 st.b $t7, $a0, 0
24: 29000492 st.b $t6, $a0, 1(0x1)
28: 29000891 st.b $t5, $a0, 2(0x2)
2c: 29000c90 st.b $t4, $a0, 3(0x3)
30: 2900108f st.b $t3, $a0, 4(0x4)
34: 2900148e st.b $t2, $a0, 5(0x5)
38: 2900188d st.b $t1, $a0, 6(0x6)
3c: 29001c8c st.b $t0, $a0, 7(0x7)
40: 4c000020 jirl $zero, $ra, 0
and test results (run 3 times) following:
the module without -mstrict-align testing:
[root@openEuler loongson]# insmod align-test.ko
[ 39.029931] mstrict-align-test took: 29603510 nsec
[root@openEuler loongson]# rmmod align-test.ko
[root@openEuler loongson]# insmod align-test.ko
[ 41.356007] mstrict-align-test took: 28816710 nsec
[root@openEuler loongson]# rmmod align-test.ko
[root@openEuler loongson]# insmod align-test.ko
[ 43.506624] mstrict-align-test took: 30030700 nsec
[root@openEuler loongson]# rmmod align-test.ko
the module with -mstrict-align testing:
root@openEuler ~]# insmod align-test.ko
[ 92.656477] mstrict-align-test took: 59629000 nsec
[root@openEuler ~]# rmmod align-test.ko
[root@openEuler ~]# insmod align-test.ko
[ 99.473011] mstrict-align-test took: 58972250 nsec
[root@openEuler ~]# rmmod align-test.ko
[root@openEuler ~]# insmod align-test.ko
[ 104.620103] mstrict-align-test took: 59419260 nsec
[root@openEuler ~]# rmmod align-test.ko
Thanks!
Jianmin
Signed-off-by: Huacai Chen <chenhuacai@xxxxxxxxxxx>
---
arch/loongarch/Kconfig | 10 ++++++++++
arch/loongarch/Makefile | 2 ++
2 files changed, 12 insertions(+)
diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
index 9cc8b84f7eb0..7470dcfb32f0 100644
--- a/arch/loongarch/Kconfig
+++ b/arch/loongarch/Kconfig
@@ -441,6 +441,16 @@ config ARCH_IOREMAP
protection support. However, you can enable LoongArch DMW-based
ioremap() for better performance.
+config ARCH_STRICT_ALIGN
+ bool "Enable -mstrict-align to prevent unaligned accesses"
+ help
+ Not all LoongArch cores support h/w unaligned access, we can use
+ -mstrict-align build parameter to prevent unaligned accesses.
+
+ This is disabled by default to optimise for performance, you can
+ enabled it manually if you want to run kernel on systems without
+ h/w unaligned access support.
+
config KEXEC
bool "Kexec system call"
select KEXEC_CORE
diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
index 4402387d2755..ccfb52700237 100644
--- a/arch/loongarch/Makefile
+++ b/arch/loongarch/Makefile
@@ -91,10 +91,12 @@ KBUILD_CPPFLAGS += -DVMLINUX_LOAD_ADDRESS=$(load-y)
# instead of .eh_frame so we don't discard them.
KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
+ifdef CONFIG_ARCH_STRICT_ALIGN
# Don't emit unaligned accesses.
# Not all LoongArch cores support unaligned access, and as kernel we
can't
# rely on others to provide emulation for these accesses.
KBUILD_CFLAGS += $(call cc-option,-mstrict-align)
+endif >
KBUILD_CFLAGS += -isystem $(shell $(CC) -print-file-name=include)