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SubjectRe: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a callee-save function
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On 10/02/2017 16:43, 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
>
> Making vcpu_is_preempted() a callee-save function has a relatively
> high cost on x86-64 primarily due to at least one more cacheline of
> data access from the saving and restoring of registers (8 of them)
> to and from stack as well as one more level of function call. As
> vcpu_is_preempted() is called within the spinlock, mutex and rwsem
> slowpaths, there isn't much to gain by making it callee-save. So it
> is now changed to a normal function call instead.
>
> With this patch applied on both bare-metal & KVM guest on a 2-socekt
> 16-core 32-thread system with 16 parallel jobs (8 on each socket), the
> aggregrate bandwidth of the fio test on an XFS ramdisk were as follows:
>
> Bare Metal KVM Guest
> I/O Type w/o patch with patch w/o patch with patch
> -------- --------- ---------- --------- ----------
> random read 8650.5 MB/s 8560.9 MB/s 7602.9 MB/s 8196.1 MB/s
> seq read 9104.8 MB/s 9397.2 MB/s 8293.7 MB/s 8566.9 MB/s
> random write 1623.8 MB/s 1626.7 MB/s 1590.6 MB/s 1700.7 MB/s
> seq write 1626.4 MB/s 1624.9 MB/s 1604.8 MB/s 1726.3 MB/s
>
> The perf data (on KVM guest) now became:
>
> 70.78% 0.58% fio [k] down_write
> 70.20% 0.01% fio [k] call_rwsem_down_write_failed
> 69.70% 1.17% fio [k] rwsem_down_write_failed
> 59.91% 55.42% fio [k] osq_lock
> 10.14% 10.14% fio [k] __kvm_vcpu_is_preempted
>
> On bare metal, the patch doesn't introduce any performance
> regression. On KVM guest, it produces noticeable performance
> improvement (up to 7%).
>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
> v1->v2:
> - Rerun the fio test on a different system on both bare-metal and a
> KVM guest. Both sockets were utilized in this test.
> - The commit log was updated with new performance numbers, but the
> patch wasn't changed.
> - Drop patch 2.
>
> arch/x86/include/asm/paravirt.h | 2 +-
> arch/x86/include/asm/paravirt_types.h | 2 +-
> arch/x86/kernel/kvm.c | 7 ++-----
> arch/x86/kernel/paravirt-spinlocks.c | 6 ++----
> arch/x86/xen/spinlock.c | 4 +---
> 5 files changed, 7 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
> index 864f57b..2515885 100644
> --- a/arch/x86/include/asm/paravirt.h
> +++ b/arch/x86/include/asm/paravirt.h
> @@ -676,7 +676,7 @@ static __always_inline void pv_kick(int cpu)
>
> static __always_inline bool pv_vcpu_is_preempted(int cpu)
> {
> - return PVOP_CALLEE1(bool, pv_lock_ops.vcpu_is_preempted, cpu);
> + return PVOP_CALL1(bool, pv_lock_ops.vcpu_is_preempted, cpu);
> }
>
> #endif /* SMP && PARAVIRT_SPINLOCKS */
> diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
> index bb2de45..88dc852 100644
> --- a/arch/x86/include/asm/paravirt_types.h
> +++ b/arch/x86/include/asm/paravirt_types.h
> @@ -309,7 +309,7 @@ struct pv_lock_ops {
> void (*wait)(u8 *ptr, u8 val);
> void (*kick)(int cpu);
>
> - struct paravirt_callee_save vcpu_is_preempted;
> + bool (*vcpu_is_preempted)(int cpu);
> };
>
> /* This contains all the paravirt structures: we get a convenient
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 099fcba..eb3753d 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -595,7 +595,6 @@ __visible bool __kvm_vcpu_is_preempted(int cpu)
>
> return !!src->preempted;
> }
> -PV_CALLEE_SAVE_REGS_THUNK(__kvm_vcpu_is_preempted);
>
> /*
> * Setup pv_lock_ops to exploit KVM_FEATURE_PV_UNHALT if present.
> @@ -614,10 +613,8 @@ void __init kvm_spinlock_init(void)
> pv_lock_ops.wait = kvm_wait;
> pv_lock_ops.kick = kvm_kick_cpu;
>
> - if (kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
> - pv_lock_ops.vcpu_is_preempted =
> - PV_CALLEE_SAVE(__kvm_vcpu_is_preempted);
> - }
> + if (kvm_para_has_feature(KVM_FEATURE_STEAL_TIME))
> + pv_lock_ops.vcpu_is_preempted = __kvm_vcpu_is_preempted;
> }
>
> #endif /* CONFIG_PARAVIRT_SPINLOCKS */
> diff --git a/arch/x86/kernel/paravirt-spinlocks.c b/arch/x86/kernel/paravirt-spinlocks.c
> index 6259327..da050bc 100644
> --- a/arch/x86/kernel/paravirt-spinlocks.c
> +++ b/arch/x86/kernel/paravirt-spinlocks.c
> @@ -24,12 +24,10 @@ __visible bool __native_vcpu_is_preempted(int cpu)
> {
> return false;
> }
> -PV_CALLEE_SAVE_REGS_THUNK(__native_vcpu_is_preempted);
>
> bool pv_is_native_vcpu_is_preempted(void)
> {
> - return pv_lock_ops.vcpu_is_preempted.func ==
> - __raw_callee_save___native_vcpu_is_preempted;
> + return pv_lock_ops.vcpu_is_preempted == __native_vcpu_is_preempted;
> }
>
> struct pv_lock_ops pv_lock_ops = {
> @@ -38,7 +36,7 @@ struct pv_lock_ops pv_lock_ops = {
> .queued_spin_unlock = PV_CALLEE_SAVE(__native_queued_spin_unlock),
> .wait = paravirt_nop,
> .kick = paravirt_nop,
> - .vcpu_is_preempted = PV_CALLEE_SAVE(__native_vcpu_is_preempted),
> + .vcpu_is_preempted = __native_vcpu_is_preempted,
> #endif /* SMP */
> };
> EXPORT_SYMBOL(pv_lock_ops);
> diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
> index 25a7c43..c85bb8f 100644
> --- a/arch/x86/xen/spinlock.c
> +++ b/arch/x86/xen/spinlock.c
> @@ -114,8 +114,6 @@ void xen_uninit_lock_cpu(int cpu)
> per_cpu(irq_name, cpu) = NULL;
> }
>
> -PV_CALLEE_SAVE_REGS_THUNK(xen_vcpu_stolen);
> -
> /*
> * Our init of PV spinlocks is split in two init functions due to us
> * using paravirt patching and jump labels patching and having to do
> @@ -138,7 +136,7 @@ void __init xen_init_spinlocks(void)
> pv_lock_ops.queued_spin_unlock = PV_CALLEE_SAVE(__pv_queued_spin_unlock);
> pv_lock_ops.wait = xen_qlock_wait;
> pv_lock_ops.kick = xen_qlock_kick;
> - pv_lock_ops.vcpu_is_preempted = PV_CALLEE_SAVE(xen_vcpu_stolen);
> + pv_lock_ops.vcpu_is_preempted = xen_vcpu_stolen;
> }
>
> static __init int xen_parse_nopvspin(char *arg)
>

Acked-by: Paolo Bonzini <pbonzini@redhat.com>

Thank you very much!

Paolo

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