Re: [PATCH v2 6/6] KVM: arm64: Support the VCPU preemption check

[Steven Price <steven.price@arm.com>]

On 26/12/2019 13:58, Zengruan Ye wrote:
> Support the vcpu_is_preempted() functionality under KVM/arm64. This will
> enhance lock performance on overcommitted hosts (more runnable VCPUs
> than physical CPUs in the system) as doing busy waits for preempted
> VCPUs will hurt system performance far worse than early yielding.
> 
> unix benchmark result:
>    host:  kernel 5.5.0-rc1, HiSilicon Kunpeng920, 8 CPUs
>    guest: kernel 5.5.0-rc1, 16 VCPUs
> 
>                 test-case                |    after-patch    |   before-patch
> ----------------------------------------+-------------------+------------------
>   Dhrystone 2 using register variables   | 334600751.0 lps   | 335319028.3 lps
>   Double-Precision Whetstone             |     32856.1 MWIPS |     32849.6 MWIPS
>   Execl Throughput                       |      3662.1 lps   |      2718.0 lps
>   File Copy 1024 bufsize 2000 maxblocks  |    432906.4 KBps  |    158011.8 KBps
>   File Copy 256 bufsize 500 maxblocks    |    116023.0 KBps  |     37664.0 KBps
>   File Copy 4096 bufsize 8000 maxblocks  |   1432769.8 KBps  |    441108.8 KBps
>   Pipe Throughput                        |   6405029.6 lps   |   6021457.6 lps
>   Pipe-based Context Switching           |    185872.7 lps   |    184255.3 lps
>   Process Creation                       |      4025.7 lps   |      3706.6 lps
>   Shell Scripts (1 concurrent)           |      6745.6 lpm   |      6436.1 lpm
>   Shell Scripts (8 concurrent)           |       998.7 lpm   |       931.1 lpm
>   System Call Overhead                   |   3913363.1 lps   |   3883287.8 lps
> ----------------------------------------+-------------------+------------------
>   System Benchmarks Index Score          |      1835.1       |      1327.6
> 
> Signed-off-by: Zengruan Ye <yezengruan@huawei.com>
> ---
>   arch/arm64/include/asm/paravirt.h |   3 +
>   arch/arm64/kernel/paravirt.c      | 117 ++++++++++++++++++++++++++++++
>   arch/arm64/kernel/setup.c         |   2 +
>   include/linux/cpuhotplug.h        |   1 +
>   4 files changed, 123 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/paravirt.h b/arch/arm64/include/asm/paravirt.h
> index 7b1c81b544bb..ca3a2c7881f3 100644
> --- a/arch/arm64/include/asm/paravirt.h
> +++ b/arch/arm64/include/asm/paravirt.h
> @@ -29,6 +29,8 @@ static inline u64 paravirt_steal_clock(int cpu)
>   
>   int __init pv_time_init(void);
>   
> +int __init pv_lock_init(void);
> +
>   __visible bool __native_vcpu_is_preempted(int cpu);
>   
>   static inline bool pv_vcpu_is_preempted(int cpu)
> @@ -39,6 +41,7 @@ static inline bool pv_vcpu_is_preempted(int cpu)
>   #else
>   
>   #define pv_time_init() do {} while (0)
> +#define pv_lock_init() do {} while (0)
>   
>   #endif // CONFIG_PARAVIRT
>   
> diff --git a/arch/arm64/kernel/paravirt.c b/arch/arm64/kernel/paravirt.c
> index d8f1ba8c22ce..bd2ad6a17a26 100644
> --- a/arch/arm64/kernel/paravirt.c
> +++ b/arch/arm64/kernel/paravirt.c
> @@ -22,6 +22,7 @@
>   #include <asm/paravirt.h>
>   #include <asm/pvclock-abi.h>
>   #include <asm/smp_plat.h>
> +#include <asm/pvlock-abi.h>
>   
>   struct static_key paravirt_steal_enabled;
>   struct static_key paravirt_steal_rq_enabled;
> @@ -35,6 +36,10 @@ struct pv_time_stolen_time_region {
>   	struct pvclock_vcpu_stolen_time *kaddr;
>   };
>   
> +struct pv_lock_state_region {
> +	struct pvlock_vcpu_state *kaddr;
> +};
> +
>   static DEFINE_PER_CPU(struct pv_time_stolen_time_region, stolen_time_region);
>   
>   static bool steal_acc = true;
> @@ -158,3 +163,115 @@ int __init pv_time_init(void)
>   
>   	return 0;
>   }
> +
> +static DEFINE_PER_CPU(struct pv_lock_state_region, lock_state_region);
> +
> +static bool kvm_vcpu_is_preempted(int cpu)
> +{
> +	struct pv_lock_state_region *reg;
> +	__le64 preempted_le;
> +
> +	reg = per_cpu_ptr(&lock_state_region, cpu);
> +	if (!reg->kaddr) {
> +		pr_warn_once("PV lock enabled but not configured for cpu %d\n",
> +			     cpu);
> +		return false;
> +	}
> +
> +	preempted_le = le64_to_cpu(READ_ONCE(reg->kaddr->preempted));
> +
> +	return !!(preempted_le & 1);

According to the documentation preempted != 0 means preempted, but here you are checking the LSB. You need to be consistent about the ABI.

> +}
> +
> +static int pvlock_vcpu_state_dying_cpu(unsigned int cpu)
> +{
> +	struct pv_lock_state_region *reg;
> +
> +	reg = this_cpu_ptr(&lock_state_region);
> +	if (!reg->kaddr)
> +		return 0;
> +
> +	memunmap(reg->kaddr);
> +	memset(reg, 0, sizeof(*reg));
> +
> +	return 0;
> +}
> +
> +static int init_pvlock_vcpu_state(unsigned int cpu)
> +{
> +	struct pv_lock_state_region *reg;
> +	struct arm_smccc_res res;
> +
> +	reg = this_cpu_ptr(&lock_state_region);
> +
> +	arm_smccc_1_1_invoke(ARM_SMCCC_HV_PV_LOCK_PREEMPTED, &res);
> +
> +	if (res.a0 == SMCCC_RET_NOT_SUPPORTED) {
> +		pr_warn("Failed to init PV lock data structure\n");
> +		return -EINVAL;
> +	}
> +
> +	reg->kaddr = memremap(res.a0,
> +			      sizeof(struct pvlock_vcpu_state),
> +			      MEMREMAP_WB);
> +
> +	if (!reg->kaddr) {
> +		pr_warn("Failed to map PV lock data structure\n");
> +		return -ENOMEM;
> +	}
> +
> +	return 0;
> +}
> +
> +static int kvm_arm_init_pvlock(void)
> +{
> +	int ret;
> +
> +	ret = cpuhp_setup_state(CPUHP_AP_ARM_KVM_PVLOCK_STARTING,
> +				"hypervisor/arm/pvlock:starting",
> +				init_pvlock_vcpu_state,
> +				pvlock_vcpu_state_dying_cpu);
> +	if (ret < 0) {
> +		pr_warn("PV-lock init failed\n");
> +		return ret;
> +	}
> +
> +	return 0;
> +}
> +
> +static bool has_kvm_pvlock(void)
> +{
> +	struct arm_smccc_res res;
> +
> +	/* To detect the presence of PV lock support we require SMCCC 1.1+ */
> +	if (psci_ops.smccc_version < SMCCC_VERSION_1_1)
> +		return false;
> +
> +	arm_smccc_1_1_invoke(ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
> +			     ARM_SMCCC_HV_PV_LOCK_FEATURES, &res);

As mentioned previously we could do with something more robust to check that the hypervisor is actually KVM before assuming that vendor specific IDs are valid.

Steve

> +
> +	if (res.a0 != SMCCC_RET_SUCCESS)
> +		return false;
> +
> +	return true;
> +}
> +
> +int __init pv_lock_init(void)
> +{
> +	int ret;
> +
> +	if (is_hyp_mode_available())
> +		return 0;
> +
> +	if (!has_kvm_pvlock())
> +		return 0;
> +
> +	ret = kvm_arm_init_pvlock();
> +	if (ret)
> +		return ret;
> +
> +	pv_ops.lock.vcpu_is_preempted = kvm_vcpu_is_preempted;
> +	pr_info("using PV-lock preempted\n");
> +
> +	return 0;
> +}
> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> index 56f664561754..aa3a8b9e710f 100644
> --- a/arch/arm64/kernel/setup.c
> +++ b/arch/arm64/kernel/setup.c
> @@ -341,6 +341,8 @@ void __init setup_arch(char **cmdline_p)
>   	smp_init_cpus();
>   	smp_build_mpidr_hash();
>   
> +	pv_lock_init();
> +
>   	/* Init percpu seeds for random tags after cpus are set up. */
>   	kasan_init_tags();
>   
> diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
> index e51ee772b9f5..f72ff95ab63a 100644
> --- a/include/linux/cpuhotplug.h
> +++ b/include/linux/cpuhotplug.h
> @@ -138,6 +138,7 @@ enum cpuhp_state {
>   	CPUHP_AP_DUMMY_TIMER_STARTING,
>   	CPUHP_AP_ARM_XEN_STARTING,
>   	CPUHP_AP_ARM_KVMPV_STARTING,
> +	CPUHP_AP_ARM_KVM_PVLOCK_STARTING,
>   	CPUHP_AP_ARM_CORESIGHT_STARTING,
>   	CPUHP_AP_ARM64_ISNDEP_STARTING,
>   	CPUHP_AP_SMPCFD_DYING,
>