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From:   Guo Hui <guohui@uniontech.com>
To:     peterz@infradead.org
Cc:     jpoimboe@kernel.org, song@kernel.org, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        x86@kernel.org, hpa@zytor.com, daniel@iogearbox.net,
        will@kernel.org, longman@redhat.com, boqun.feng@gmail.com,
        wangxiaohua@uniontech.com, linux-kernel@vger.kernel.org,
        Guo Hui <guohui@uniontech.com>
Subject: [PATCH] x86/paravirt: useless assignment instructions cause Unixbench full core performance degradation
Date:   Thu, 23 Jun 2022 23:50:07 +0800
Message-Id: <20220623155007.3059-1-guohui@uniontech.com>
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Feedback-ID: bizesmtp:uniontech.com:qybgforeign:qybgforeign9
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The instructions assigned to the parameters of
the vcpu_is_preempted function in the X86 architecture
physical machine are redundant instructions, which cause the
multi-core performance of Unixbench to drop by
about 300 to 500 points. The C function is as follows:
static bool vcpu_is_preempted(long vcpu);

The parameter assignments in the function osq_lock
that call the function vcpu_is_preempted are as follows:
mov 0x14(%rax),%edi
sub $0x1,%edi

The above instructions are unnecessary
in the X86 Native operating environment,
causing high cache-misses and degrading performance.

This patch implements the replacement of the above instructions with
the nop instruction at system startup.
When the assignment C code changes,
the above assignment instructions may also change accordingly.
In order to flexibly replace the instructions generated by the C code,
this patch defines the macro ALTERNATIVE_C_CODE
from the ALTERNATIVE macro. Different from the first parameter
of the ALTERNATIVE macro, the ALTERNATIVE_C_CODE macro is
the first parameter is a C code statement and cannot contain
a return statement. To use the macro, for example,
replace the instructions generated by the
C code 'cpu = node->cpu - 1;' with the nop instruction:

OSQ_ALTERNATIVE_C_CODE(cpu = node->cpu - 1, "nop", 0);

The patch effect is as follows two machines,
Unixbench runs with full core score:

1. Machine configuration:
Intel(R) Xeon(R) Silver 4210 CPU @ 2.20GHz
CPU core: 40
Memory: 256G
OS Kernel: 5.19-rc3

Before using the patch:

System Benchmarks Index Values               BASELINE       RESULT    INDEX
Dhrystone 2 using register variables         116700.0  944608003.9  80943.3
Double-Precision Whetstone                       55.0     212045.8  38553.8
Execl Throughput                                 43.0      42772.3   9947.1
File Copy 1024 bufsize 2000 maxblocks          3960.0     462656.6   1168.3
File Copy 256 bufsize 500 maxblocks            1655.0     120043.6    725.3
File Copy 4096 bufsize 8000 maxblocks          5800.0    1544525.5   2663.0
Pipe Throughput                               12440.0   47277698.5  38004.6
Pipe-based Context Switching                   4000.0    1894556.8   4736.4
Process Creation                                126.0      86077.0   6831.5
Shell Scripts (1 concurrent)                     42.4      70236.3  16565.2
Shell Scripts (8 concurrent)                      6.0       8978.1  14963.4
System Call Overhead                          15000.0    4691260.0   3127.5
                                                                   ========
System Benchmarks Index Score                                        7980.9

After using the patch:

System Benchmarks Index Values               BASELINE       RESULT    INDEX
Dhrystone 2 using register variables         116700.0 2253984916.9 193143.5
Double-Precision Whetstone                       55.0     438940.3  79807.3
Execl Throughput                                 43.0      10720.3   2493.1
File Copy 1024 bufsize 2000 maxblocks          3960.0     312233.0    788.5
File Copy 256 bufsize 500 maxblocks            1655.0      80050.9    483.7
File Copy 4096 bufsize 8000 maxblocks          5800.0    1036101.7   1786.4
Pipe Throughput                               12440.0  117700315.3  94614.4
Pipe-based Context Switching                   4000.0    8421909.8  21054.8
Process Creation                                126.0      36742.0   2916.0
Shell Scripts (1 concurrent)                     42.4      52846.2  12463.7
Shell Scripts (8 concurrent)                      6.0       7058.1  11763.6
System Call Overhead                          15000.0    6791548.2   4527.7
                                                                   ========
System Benchmarks Index Score                                        8260.6

2. Machine configuration:
Hygon C86 7185 32-core Processor
CPU core: 128
Memory: 256G
OS Kernel: 5.19-rc3

Before using the patch:

System Benchmarks Index Values               BASELINE       RESULT    INDEX
Dhrystone 2 using register variables         116700.0 2256283941.6 193340.5
Double-Precision Whetstone                       55.0     439577.3  79923.2
Execl Throughput                                 43.0      10013.6   2328.7
File Copy 1024 bufsize 2000 maxblocks          3960.0     278121.5    702.3
File Copy 256 bufsize 500 maxblocks            1655.0      71835.5    434.1
File Copy 4096 bufsize 8000 maxblocks          5800.0     905654.2   1561.5
Pipe Throughput                               12440.0  117715166.2  94626.3
Pipe-based Context Switching                   4000.0    7731331.7  19328.3
Process Creation                                126.0      30157.8   2393.5
Shell Scripts (1 concurrent)                     42.4      48670.8  11479.0
Shell Scripts (8 concurrent)                      6.0       6595.6  10992.7
System Call Overhead                          15000.0    6766475.9   4511.0
                                                                   ========
System Benchmarks Index Score                                        7688.7

After using the patch:

System Benchmarks Index Values               BASELINE       RESULT    INDEX
Dhrystone 2 using register variables         116700.0 2253984916.9 193143.5
Double-Precision Whetstone                       55.0     438940.3  79807.3
Execl Throughput                                 43.0      10720.3   2493.1
File Copy 1024 bufsize 2000 maxblocks          3960.0     312233.0    788.5
File Copy 256 bufsize 500 maxblocks            1655.0      80050.9    483.7
File Copy 4096 bufsize 8000 maxblocks          5800.0    1036101.7   1786.4
Pipe Throughput                               12440.0  117700315.3  94614.4
Pipe-based Context Switching                   4000.0    8421909.8  21054.8
Process Creation                                126.0      36742.0   2916.0
Shell Scripts (1 concurrent)                     42.4      52846.2  12463.7
Shell Scripts (8 concurrent)                      6.0       7058.1  11763.6
System Call Overhead                          15000.0    6791548.2   4527.7
                                                                   ========
System Benchmarks Index Score                                        8260.6

Signed-off-by: Guo Hui <guohui@uniontech.com>
---
 arch/x86/include/asm/alternative.h | 15 +++++++++++++++
 arch/x86/kernel/alternative.c      | 11 +++++++++++
 include/linux/osq_lock.h           |  7 +++++++
 kernel/locking/osq_lock.c          |  6 +++++-
 4 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 9b10c8c76..5979ebe89 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -167,6 +167,21 @@ static inline int alternatives_text_reserved(void *start, void *end)
 	ALTINSTR_REPLACEMENT(newinstr, 1)				\
 	".popsection\n"
 
+/* alternative c code primitive: */
+#define ALTERNATIVE_C_CODE(oldinstr, newinstr, feature)			\
+do {									\
+	asm volatile("661:\n\t");					\
+	oldinstr;							\
+	asm volatile("\n662:\n"						\
+		alt_end_marker ":\n"					\
+		".pushsection .altinstructions,\"a\"\n"			\
+		ALTINSTR_ENTRY(feature, 1)				\
+		".popsection\n"						\
+		".pushsection .altinstr_replacement, \"ax\"\n"		\
+		ALTINSTR_REPLACEMENT(newinstr, 1)			\
+		".popsection\n");					\
+} while (0)
+
 #define ALTERNATIVE_2(oldinstr, newinstr1, feature1, newinstr2, feature2)\
 	OLDINSTR_2(oldinstr, 1, 2)					\
 	".pushsection .altinstructions,\"a\"\n"				\
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index e257f6c80..cf77be884 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -251,6 +251,8 @@ static void __init_or_module noinline optimize_nops(u8 *instr, size_t len)
 	}
 }
 
+extern bool pv_is_native_vcpu_is_preempted(void);
+
 /*
  * Replace instructions with better alternatives for this CPU type. This runs
  * before SMP is initialized to avoid SMP problems with self modifying code.
@@ -285,6 +287,15 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 
 		instr = (u8 *)&a->instr_offset + a->instr_offset;
 		replacement = (u8 *)&a->repl_offset + a->repl_offset;
+
+		if (*replacement == 0x90 && a->replacementlen == 1) {
+#if defined(CONFIG_PARAVIRT_SPINLOCKS)
+			if (pv_is_native_vcpu_is_preempted())
+				add_nops(instr, a->instrlen);
+#endif
+			continue;
+		}
+
 		BUG_ON(a->instrlen > sizeof(insn_buff));
 		BUG_ON(feature >= (NCAPINTS + NBUGINTS) * 32);
 
diff --git a/include/linux/osq_lock.h b/include/linux/osq_lock.h
index 5581dbd3b..ee960e3aa 100644
--- a/include/linux/osq_lock.h
+++ b/include/linux/osq_lock.h
@@ -38,4 +38,11 @@ static inline bool osq_is_locked(struct optimistic_spin_queue *lock)
 	return atomic_read(&lock->tail) != OSQ_UNLOCKED_VAL;
 }
 
+#ifdef ALTERNATIVE_C_CODE
+#define OSQ_ALTERNATIVE_C_CODE(oldinstr, newinstr, feature) \
+		ALTERNATIVE_C_CODE(oldinstr, newinstr, feature)
+#else
+#define OSQ_ALTERNATIVE_C_CODE(oldinstr, newinstr, feature) oldinstr
+#endif
+
 #endif
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index d5610ad52..bbe7d640c 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -24,7 +24,11 @@ static inline int encode_cpu(int cpu_nr)
 
 static inline int node_cpu(struct optimistic_spin_node *node)
 {
-	return node->cpu - 1;
+	int cpu = 0;
+
+	OSQ_ALTERNATIVE_C_CODE(cpu = node->cpu - 1, "nop", 0);
+
+	return cpu;
 }
 
 static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
-- 
2.20.1