On 02/10/2017 11:19 AM, Peter Zijlstra wrote:
> On Fri, Feb 10, 2017 at 10:43:09AM -0500, Waiman Long wrote:
>> It was found when running fio sequential write test with a XFS ramdisk
>> on a VM running on a 2-socket x86-64 system, the %CPU times as reported
>> by perf were as follows:
>>
>> 69.75% 0.59% fio [k] down_write
>> 69.15% 0.01% fio [k] call_rwsem_down_write_failed
>> 67.12% 1.12% fio [k] rwsem_down_write_failed
>> 63.48% 52.77% fio [k] osq_lock
>> 9.46% 7.88% fio [k] __raw_callee_save___kvm_vcpu_is_preempt
>> 3.93% 3.93% fio [k] __kvm_vcpu_is_preempted
>>
> Thinking about this again, wouldn't something like the below also work?
>
>
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 099fcba4981d..6aa33702c15c 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -589,6 +589,7 @@ static void kvm_wait(u8 *ptr, u8 val)
> local_irq_restore(flags);
> }
>
> +#ifdef CONFIG_X86_32
> __visible bool __kvm_vcpu_is_preempted(int cpu)
> {
> struct kvm_steal_time *src = &per_cpu(steal_time, cpu);
> @@ -597,6 +598,31 @@ __visible bool __kvm_vcpu_is_preempted(int cpu)
> }
> PV_CALLEE_SAVE_REGS_THUNK(__kvm_vcpu_is_preempted);
>
> +#else
> +
> +extern bool __raw_callee_save___kvm_vcpu_is_preempted(int);
> +
> +asm(
> +".pushsection .text;"
> +".global __raw_callee_save___kvm_vcpu_is_preempted;"
> +".type __raw_callee_save___kvm_vcpu_is_preempted, @function;"
> +"__raw_callee_save___kvm_vcpu_is_preempted:"
> +FRAME_BEGIN
> +"push %rdi;"
> +"push %rdx;"
> +"movslq %edi, %rdi;"
> +"movq $steal_time+16, %rax;"
> +"movq __per_cpu_offset(,%rdi,8), %rdx;"
> +"cmpb $0, (%rdx,%rax);"
> +"setne %al;"
> +"pop %rdx;"
> +"pop %rdi;"
> +FRAME_END
> +"ret;"
> +".popsection");
> +
> +#endif
> +
> /*
> * Setup pv_lock_ops to exploit KVM_FEATURE_PV_UNHALT if present.
> */
That should work for now. I have done something similar for
__pv_queued_spin_unlock. However, this has the problem of creating a
dependency on the exact layout of the steal_time structure. Maybe the
constant 16 can be passed in as a parameter offsetof(struct
kvm_steal_time, preempted) to the asm call.
Cheers,
Longman
