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[PATCH v5 0/2] x86/kvm: Reduce vcpu_is_preempted() overhead

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  [PATCH v5 0/2] x86/kvm: Reduce vcpu_is_preempted() overhead Waiman Long <longman@redhat.com> - 2017-02-20 19:40 +0100
    [PATCH v5 1/2] x86/paravirt: Change vcp_is_preempted() arg type to long Waiman Long <longman@redhat.com> - 2017-02-20 19:40 +0100
    Re: [PATCH v5 0/2] x86/kvm: Reduce vcpu_is_preempted() overhead Peter Zijlstra <peterz@infradead.org> - 2017-02-20 21:00 +0100
      Re: [PATCH v5 0/2] x86/kvm: Reduce vcpu_is_preempted() overhead Paolo Bonzini <pbonzini@redhat.com> - 2017-02-21 12:50 +0100

#1584817 — [PATCH v5 0/2] x86/kvm: Reduce vcpu_is_preempted() overhead

 v4->v5:
  - As suggested by PeterZ, use the asm-offsets header file generation
    mechanism to get the offset of the preempted field in
    kvm_steal_time instead of hardcoding it.

 v3->v4:
  - Fix x86-32 build error.

 v2->v3:
  - Provide an optimized __raw_callee_save___kvm_vcpu_is_preempted()
    in assembly as suggested by PeterZ.
  - Add a new patch to change vcpu_is_preempted() argument type to long
    to ease the writing of the assembly code.

 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.

As it was found that the overhead of callee-save vcpu_is_preempted()
can have some impact on system performance on a VM guest, especially
of x86-64 guest, this patch set intends to reduce this performance
overhead by replacing the C __kvm_vcpu_is_preempted() function by
an optimized version of __raw_callee_save___kvm_vcpu_is_preempted()
written in assembly.

Waiman Long (2):
  x86/paravirt: Change vcp_is_preempted() arg type to long
  x86/kvm: Provide optimized version of vcpu_is_preempted() for x86-64

 arch/x86/include/asm/paravirt.h      |  2 +-
 arch/x86/include/asm/qspinlock.h     |  2 +-
 arch/x86/kernel/asm-offsets_64.c     |  9 +++++++++
 arch/x86/kernel/kvm.c                | 26 +++++++++++++++++++++++++-
 arch/x86/kernel/paravirt-spinlocks.c |  2 +-
 5 files changed, 37 insertions(+), 4 deletions(-)

-- 
1.8.3.1

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#1584818 — [PATCH v5 1/2] x86/paravirt: Change vcp_is_preempted() arg type to long

The cpu argument in the function prototype of vcpu_is_preempted()
is changed from int to long. That makes it easier to provide a better
optimized assembly version of that function.

For Xen, vcpu_is_preempted(long) calls xen_vcpu_stolen(int), the
downcast from long to int is not a problem as vCPU number won't exceed
32 bits.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 arch/x86/include/asm/paravirt.h      | 2 +-
 arch/x86/include/asm/qspinlock.h     | 2 +-
 arch/x86/kernel/kvm.c                | 2 +-
 arch/x86/kernel/paravirt-spinlocks.c | 2 +-
 4 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 1eea6ca..f75fbfe 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -673,7 +673,7 @@ static __always_inline void pv_kick(int cpu)
 	PVOP_VCALL1(pv_lock_ops.kick, cpu);
 }
 
-static __always_inline bool pv_vcpu_is_preempted(int cpu)
+static __always_inline bool pv_vcpu_is_preempted(long cpu)
 {
 	return PVOP_CALLEE1(bool, pv_lock_ops.vcpu_is_preempted, cpu);
 }
diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
index c343ab5..48a706f 100644
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -34,7 +34,7 @@ static inline void queued_spin_unlock(struct qspinlock *lock)
 }
 
 #define vcpu_is_preempted vcpu_is_preempted
-static inline bool vcpu_is_preempted(int cpu)
+static inline bool vcpu_is_preempted(long cpu)
 {
 	return pv_vcpu_is_preempted(cpu);
 }
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 099fcba..85ed343 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -589,7 +589,7 @@ static void kvm_wait(u8 *ptr, u8 val)
 	local_irq_restore(flags);
 }
 
-__visible bool __kvm_vcpu_is_preempted(int cpu)
+__visible bool __kvm_vcpu_is_preempted(long cpu)
 {
 	struct kvm_steal_time *src = &per_cpu(steal_time, cpu);
 
diff --git a/arch/x86/kernel/paravirt-spinlocks.c b/arch/x86/kernel/paravirt-spinlocks.c
index 6259327..8f2d1c9 100644
--- a/arch/x86/kernel/paravirt-spinlocks.c
+++ b/arch/x86/kernel/paravirt-spinlocks.c
@@ -20,7 +20,7 @@ bool pv_is_native_spin_unlock(void)
 		__raw_callee_save___native_queued_spin_unlock;
 }
 
-__visible bool __native_vcpu_is_preempted(int cpu)
+__visible bool __native_vcpu_is_preempted(long cpu)
 {
 	return false;
 }
-- 
1.8.3.1

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#1584866

On Mon, Feb 20, 2017 at 01:36:02PM -0500, Waiman Long wrote:
> Waiman Long (2):
>   x86/paravirt: Change vcp_is_preempted() arg type to long
>   x86/kvm: Provide optimized version of vcpu_is_preempted() for x86-64
> 
>  arch/x86/include/asm/paravirt.h      |  2 +-
>  arch/x86/include/asm/qspinlock.h     |  2 +-
>  arch/x86/kernel/asm-offsets_64.c     |  9 +++++++++
>  arch/x86/kernel/kvm.c                | 26 +++++++++++++++++++++++++-
>  arch/x86/kernel/paravirt-spinlocks.c |  2 +-
>  5 files changed, 37 insertions(+), 4 deletions(-)

I'm assuming this will go through the KVM tree, if people want me to
take it through the tip tree, please let me know.

Acked-by: Peter Zijlstra (Intel) <peterz@infraded.org>

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#1585247

On 20/02/2017 20:54, Peter Zijlstra wrote:
> On Mon, Feb 20, 2017 at 01:36:02PM -0500, Waiman Long wrote:
>> Waiman Long (2):
>>   x86/paravirt: Change vcp_is_preempted() arg type to long
>>   x86/kvm: Provide optimized version of vcpu_is_preempted() for x86-64
>>
>>  arch/x86/include/asm/paravirt.h      |  2 +-
>>  arch/x86/include/asm/qspinlock.h     |  2 +-
>>  arch/x86/kernel/asm-offsets_64.c     |  9 +++++++++
>>  arch/x86/kernel/kvm.c                | 26 +++++++++++++++++++++++++-
>>  arch/x86/kernel/paravirt-spinlocks.c |  2 +-
>>  5 files changed, 37 insertions(+), 4 deletions(-)
> 
> I'm assuming this will go through the KVM tree, if people want me to
> take it through the tip tree, please let me know.
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@infraded.org>

Applied, thanks.

Paolo

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