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Message-ID: <e57ab7f6-09e0-9189-8b08-4d68be76f518@redhat.com>
Date: Sun, 26 Jun 2022 23:02:14 -0400
From: Waiman Long <longman@...hat.com>
To: Guo Hui <guohui@...ontech.com>, peterz@...radead.org
Cc: jgross@...e.com, srivatsa@...il.mit.edu, amakhalov@...are.com,
pv-drivers@...are.com, tglx@...utronix.de, mingo@...hat.com,
bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
hpa@...or.com, will@...nel.org, boqun.feng@...il.com,
virtualization@...ts.linux-foundation.org,
wangxiaohua@...ontech.com, linux-kernel@...r.kernel.org
Subject: Re: [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@...ontech.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
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