Waiman Long
2022-Jun-27 03:02 UTC
[PATCH v2] x86/paravirt: useless assignment instructions cause Unixbench full core performance degradation
On 6/26/22 22:13, Guo Hui wrote:> The instructions assigned to the vcpu_is_preempted function parameter > in the X86 architecture physical machine are redundant instructions, > causing the multi-core performance of Unixbench to drop by about 4% to 5%. > The C function is as follows: > static bool vcpu_is_preempted(long vcpu); > > The parameter 'vcpu' in the function osq_lock > that calls the function vcpu_is_preempted is assigned as follows: > > The C code is in the function node_cpu: > cpu = node->cpu - 1; > > The instructions corresponding to the C code are: > 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 uses static_key to not execute this instruction > in the Native runtime environment. > > The code is as follows: > > DEFINE_STATIC_KEY_FALSE(preemted_key); > > static inline int node_cpu(struct optimistic_spin_node *node) > { > int cpu = 0; > > if (!static_branch_unlikely(&preemted_key)) > cpu = node->cpu - 1; > > return cpu; > } > > 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 948326591.2 81261.9 > Double-Precision Whetstone 55.0 211986.3 38543.0 > Execl Throughput 43.0 43453.2 10105.4 > File Copy 1024 bufsize 2000 maxblocks 3960.0 438936.2 1108.4 > File Copy 256 bufsize 500 maxblocks 1655.0 118197.4 714.2 > File Copy 4096 bufsize 8000 maxblocks 5800.0 1534674.7 2646.0 > Pipe Throughput 12440.0 46482107.6 37365.0 > Pipe-based Context Switching 4000.0 1915094.2 4787.7 > Process Creation 126.0 85442.2 6781.1 > Shell Scripts (1 concurrent) 42.4 69400.7 16368.1 > Shell Scripts (8 concurrent) 6.0 8877.2 14795.3 > System Call Overhead 15000.0 4714906.1 3143.3 > =======> System Benchmarks Index Score 7923.3 > > After using the patch: > System Benchmarks Index Values BASELINE RESULT INDEX > Dhrystone 2 using register variables 116700.0 947032915.5 81151.1 > Double-Precision Whetstone 55.0 211971.2 38540.2 > Execl Throughput 43.0 45054.8 10477.9 > File Copy 1024 bufsize 2000 maxblocks 3960.0 515024.9 1300.6 > File Copy 256 bufsize 500 maxblocks 1655.0 146354.6 884.3 > File Copy 4096 bufsize 8000 maxblocks 5800.0 1679995.9 2896.5 > Pipe Throughput 12440.0 46466394.2 37352.4 > Pipe-based Context Switching 4000.0 1898221.4 4745.6 > Process Creation 126.0 85653.1 6797.9 > Shell Scripts (1 concurrent) 42.4 69437.3 16376.7 > Shell Scripts (8 concurrent) 6.0 8898.9 14831.4 > System Call Overhead 15000.0 4658746.7 3105.8 > =======> System Benchmarks Index Score 8248.8 > > 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 2256644068.3 193371.4 > Double-Precision Whetstone 55.0 438969.9 79812.7 > Execl Throughput 43.0 10108.6 2350.8 > File Copy 1024 bufsize 2000 maxblocks 3960.0 275892.8 696.7 > File Copy 256 bufsize 500 maxblocks 1655.0 72082.7 435.5 > File Copy 4096 bufsize 8000 maxblocks 5800.0 925043.4 1594.9 > Pipe Throughput 12440.0 118905512.5 95583.2 > Pipe-based Context Switching 4000.0 7820945.7 19552.4 > Process Creation 126.0 31233.3 2478.8 > Shell Scripts (1 concurrent) 42.4 49042.8 11566.7 > Shell Scripts (8 concurrent) 6.0 6656.0 11093.3 > System Call Overhead 15000.0 6816047.5 4544.0 > =======> System Benchmarks Index Score 7756.6 > > After using the patch: > System Benchmarks Index Values BASELINE RESULT INDEX > Dhrystone 2 using register variables 116700.0 2252272929.4 192996.8 > Double-Precision Whetstone 55.0 451847.2 82154.0 > Execl Throughput 43.0 10595.1 2464.0 > File Copy 1024 bufsize 2000 maxblocks 3960.0 301279.3 760.8 > File Copy 256 bufsize 500 maxblocks 1655.0 79291.3 479.1 > File Copy 4096 bufsize 8000 maxblocks 5800.0 1039755.2 1792.7 > Pipe Throughput 12440.0 118701468.1 95419.2 > Pipe-based Context Switching 4000.0 8073453.3 20183.6 > Process Creation 126.0 33440.9 2654.0 > Shell Scripts (1 concurrent) 42.4 52722.6 12434.6 > Shell Scripts (8 concurrent) 6.0 7050.4 11750.6 > System Call Overhead 15000.0 6834371.5 4556.2 > =======> System Benchmarks Index Score 8157.8 > > Signed-off-by: Guo Hui <guohui at uniontech.com> > --- > arch/x86/kernel/paravirt-spinlocks.c | 4 ++++ > kernel/locking/osq_lock.c | 9 ++++++++- > 2 files changed, 12 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kernel/paravirt-spinlocks.c b/arch/x86/kernel/paravirt-spinlocks.c > index 9e1ea99ad..7a55f8407 100644 > --- a/arch/x86/kernel/paravirt-spinlocks.c > +++ b/arch/x86/kernel/paravirt-spinlocks.c > @@ -33,6 +33,8 @@ bool pv_is_native_vcpu_is_preempted(void) > __raw_callee_save___native_vcpu_is_preempted; > } > > +DECLARE_STATIC_KEY_FALSE(preemted_key); > + > void __init paravirt_set_cap(void) > { > if (!pv_is_native_spin_unlock()) > @@ -40,4 +42,6 @@ void __init paravirt_set_cap(void) > > if (!pv_is_native_vcpu_is_preempted()) > setup_force_cpu_cap(X86_FEATURE_VCPUPREEMPT); > + else > + static_branch_enable(&preemted_key); > } > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c > index d5610ad52..a8798e701 100644 > --- a/kernel/locking/osq_lock.c > +++ b/kernel/locking/osq_lock.c > @@ -22,9 +22,16 @@ static inline int encode_cpu(int cpu_nr) > return cpu_nr + 1; > } > > +DEFINE_STATIC_KEY_FALSE(preemted_key); > + > static inline int node_cpu(struct optimistic_spin_node *node) > { > - return node->cpu - 1; > + int cpu = 0; > + > + if (!static_branch_unlikely(&preemted_key)) > + cpu = node->cpu - 1; > + > + return cpu; > } > > static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)This looks much better now. However vcpu_is_preempted() is not just used by x86, it is also used in arm64, powerpc and s390. If you want to default disable node_cpu(), you have to either make the necessary changes in all those places or you have to change it a way that won't affect other arches while disabling it for the x86 case. Cheers, Longman