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SubjectRe: [PATCH 3/4] paravirt: add virt_spin_lock pvops function
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On 09/05/2017 10:18 AM, Juergen Gross wrote:
> On 05/09/17 16:10, Waiman Long wrote:
>> On 09/05/2017 09:24 AM, Juergen Gross wrote:
>>> There are cases where a guest tries to switch spinlocks to bare metal
>>> behavior (e.g. by setting "xen_nopvspin" boot parameter). Today this
>>> has the downside of falling back to unfair test and set scheme for
>>> qspinlocks due to virt_spin_lock() detecting the virtualized
>>> environment.
>>>
>>> Make virt_spin_lock() a paravirt operation in order to enable users
>>> to select an explicit behavior like bare metal.
>>>
>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>> ---
>>> arch/x86/include/asm/paravirt.h | 5 ++++
>>> arch/x86/include/asm/paravirt_types.h | 1 +
>>> arch/x86/include/asm/qspinlock.h | 48 ++++++++++++++++++++++++-----------
>>> arch/x86/kernel/paravirt-spinlocks.c | 14 ++++++++++
>>> arch/x86/kernel/smpboot.c | 2 ++
>>> 5 files changed, 55 insertions(+), 15 deletions(-)
>>>
>>> diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
>>> index c25dd22f7c70..d9e954fb37df 100644
>>> --- a/arch/x86/include/asm/paravirt.h
>>> +++ b/arch/x86/include/asm/paravirt.h
>>> @@ -725,6 +725,11 @@ static __always_inline bool pv_vcpu_is_preempted(long cpu)
>>> return PVOP_CALLEE1(bool, pv_lock_ops.vcpu_is_preempted, cpu);
>>> }
>>>
>>> +static __always_inline bool pv_virt_spin_lock(struct qspinlock *lock)
>>> +{
>>> + return PVOP_CALLEE1(bool, pv_lock_ops.virt_spin_lock, lock);
>>> +}
>>> +
>>> #endif /* SMP && PARAVIRT_SPINLOCKS */
>>>
>>> #ifdef CONFIG_X86_32
>>> diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
>>> index 19efefc0e27e..928f5e7953a7 100644
>>> --- a/arch/x86/include/asm/paravirt_types.h
>>> +++ b/arch/x86/include/asm/paravirt_types.h
>>> @@ -319,6 +319,7 @@ struct pv_lock_ops {
>>> void (*kick)(int cpu);
>>>
>>> struct paravirt_callee_save vcpu_is_preempted;
>>> + struct paravirt_callee_save virt_spin_lock;
>>> } __no_randomize_layout;
>>>
>>> /* This contains all the paravirt structures: we get a convenient
>>> diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
>>> index 48a706f641f2..fbd98896385c 100644
>>> --- a/arch/x86/include/asm/qspinlock.h
>>> +++ b/arch/x86/include/asm/qspinlock.h
>>> @@ -17,6 +17,25 @@ static inline void native_queued_spin_unlock(struct qspinlock *lock)
>>> smp_store_release((u8 *)lock, 0);
>>> }
>>>
>>> +static inline bool native_virt_spin_lock(struct qspinlock *lock)
>>> +{
>>> + if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
>>> + return false;
>>> +
>>> + /*
>>> + * On hypervisors without PARAVIRT_SPINLOCKS support we fall
>>> + * back to a Test-and-Set spinlock, because fair locks have
>>> + * horrible lock 'holder' preemption issues.
>>> + */
>>> +
>>> + do {
>>> + while (atomic_read(&lock->val) != 0)
>>> + cpu_relax();
>>> + } while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0);
>>> +
>>> + return true;
>>> +}
>>> +
>>> #ifdef CONFIG_PARAVIRT_SPINLOCKS
>>> extern void native_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
>>> extern void __pv_init_lock_hash(void);
>>> @@ -38,33 +57,32 @@ static inline bool vcpu_is_preempted(long cpu)
>>> {
>>> return pv_vcpu_is_preempted(cpu);
>>> }
>>> +
>>> +void native_pv_lock_init(void) __init;
>>> #else
>>> static inline void queued_spin_unlock(struct qspinlock *lock)
>>> {
>>> native_queued_spin_unlock(lock);
>>> }
>>> +
>>> +static inline void native_pv_lock_init(void)
>>> +{
>>> +}
>>> #endif
>>>
>>> #ifdef CONFIG_PARAVIRT
>>> #define virt_spin_lock virt_spin_lock
>>> +#ifdef CONFIG_PARAVIRT_SPINLOCKS
>>> static inline bool virt_spin_lock(struct qspinlock *lock)
>>> {
>>> - if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
>>> - return false;
>> Have you consider just add one more jump label here to skip
>> virt_spin_lock when KVM or Xen want to do so?
> Why? Did you look at patch 4? This is the way to do it...

I asked this because of my performance concern as stated later in the email.

>>> -
>>> - /*
>>> - * On hypervisors without PARAVIRT_SPINLOCKS support we fall
>>> - * back to a Test-and-Set spinlock, because fair locks have
>>> - * horrible lock 'holder' preemption issues.
>>> - */
>>> -
>>> - do {
>>> - while (atomic_read(&lock->val) != 0)
>>> - cpu_relax();
>>> - } while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0);
>>> -
>>> - return true;
>>> + return pv_virt_spin_lock(lock);
>>> +}
>>> +#else
>>> +static inline bool virt_spin_lock(struct qspinlock *lock)
>>> +{
>>> + return native_virt_spin_lock(lock);
>>> }
>>> +#endif /* CONFIG_PARAVIRT_SPINLOCKS */
>>> #endif /* CONFIG_PARAVIRT */
>>>
>>> #include <asm-generic/qspinlock.h>
>>> diff --git a/arch/x86/kernel/paravirt-spinlocks.c b/arch/x86/kernel/paravirt-spinlocks.c
>>> index 26e4bd92f309..1be187ef8a38 100644
>>> --- a/arch/x86/kernel/paravirt-spinlocks.c
>>> +++ b/arch/x86/kernel/paravirt-spinlocks.c
>>> @@ -20,6 +20,12 @@ bool pv_is_native_spin_unlock(void)
>>> __raw_callee_save___native_queued_spin_unlock;
>>> }
>>>
>>> +__visible bool __native_virt_spin_lock(struct qspinlock *lock)
>>> +{
>>> + return native_virt_spin_lock(lock);
>>> +}
>>> +PV_CALLEE_SAVE_REGS_THUNK(__native_virt_spin_lock);
>> I have some concern about the overhead of register saving/restoring have
>> on spin lock performance in case the kernel is under a non-KVM/Xen
>> hypervisor.
> We are on the slow path already.

That is true, but I still still believe there will be performance impact
on lock contention behavior where the slowpath will be used.

Cheers,
Longman

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