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

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

[Waiman Long]

 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

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

[Waiman 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

[PATCH v5 2/2] x86/kvm: Provide optimized version of vcpu_is_preempted() for x86-64

[Waiman Long]

It was found when running fio sequential write test with a XFS ramdisk
on a KVM guest 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.

To reduce this performance overhead, an optimized assembly version
of the the __raw_callee_save___kvm_vcpu_is_preempt() function is
provided for x86-64.

With this patch applied on a 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:

   I/O Type      w/o patch    with patch
   --------      ---------    ----------
   random read   8141.2 MB/s  8497.1 MB/s
   seq read      8229.4 MB/s  8304.2 MB/s
   random write  1675.5 MB/s  1701.5 MB/s
   seq write     1681.3 MB/s  1699.9 MB/s

There are some increases in the aggregated bandwidth because of
the patch.

The perf data 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

The assembly code was verified by using a test kernel module to
compare the output of C __kvm_vcpu_is_preempted() and that of assembly
__raw_callee_save___kvm_vcpu_is_preempt() to verify that they matched.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 arch/x86/kernel/asm-offsets_64.c |  9 +++++++++
 arch/x86/kernel/kvm.c            | 24 ++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/arch/x86/kernel/asm-offsets_64.c b/arch/x86/kernel/asm-offsets_64.c
index 210927e..99332f5 100644
--- a/arch/x86/kernel/asm-offsets_64.c
+++ b/arch/x86/kernel/asm-offsets_64.c
@@ -13,6 +13,10 @@
 #include <asm/syscalls_32.h>
 };
 
+#if defined(CONFIG_KVM_GUEST) && defined(CONFIG_PARAVIRT_SPINLOCKS)
+#include <asm/kvm_para.h>
+#endif
+
 int main(void)
 {
 #ifdef CONFIG_PARAVIRT
@@ -22,6 +26,11 @@ int main(void)
 	BLANK();
 #endif
 
+#if defined(CONFIG_KVM_GUEST) && defined(CONFIG_PARAVIRT_SPINLOCKS)
+	OFFSET(KVM_STEAL_TIME_preempted, kvm_steal_time, preempted);
+	BLANK();
+#endif
+
 #define ENTRY(entry) OFFSET(pt_regs_ ## entry, pt_regs, entry)
 	ENTRY(bx);
 	ENTRY(cx);
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 85ed343..14f65a5 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(long cpu)
 {
 	struct kvm_steal_time *src = &per_cpu(steal_time, cpu);
@@ -597,6 +598,29 @@ __visible bool __kvm_vcpu_is_preempted(long cpu)
 }
 PV_CALLEE_SAVE_REGS_THUNK(__kvm_vcpu_is_preempted);
 
+#else
+
+#include <asm/asm-offsets.h>
+
+extern bool __raw_callee_save___kvm_vcpu_is_preempted(long);
+
+/*
+ * Hand-optimize version for x86-64 to avoid 8 64-bit register saving and
+ * restoring to/from the stack.
+ */
+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:"
+"movq	__per_cpu_offset(,%rdi,8), %rax;"
+"cmpb	$0, " __stringify(KVM_STEAL_TIME_preempted) "+steal_time(%rax);"
+"setne	%al;"
+"ret;"
+".popsection");
+
+#endif
+
 /*
  * Setup pv_lock_ops to exploit KVM_FEATURE_PV_UNHALT if present.
  */
-- 
1.8.3.1

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

[Peter Zijlstra]

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>

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

[Paolo Bonzini]

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