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SubjectRe: [PATCH v2 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock
On Tue, Jun 04, 2019 at 07:21:13PM -0400, Alex Kogan wrote:

> Trying to resume this work, I am looking for concrete steps required
> to integrate CNA with the paravirt patching.
>
> Looking at alternative_instructions(), I wonder if I need to add
> another call, something like apply_numa() similar to apply_paravirt(),
> and do the patch work there. Or perhaps I should “just" initialize
> the pv_ops structure with the corresponding
> numa_queued_spinlock_slowpath() in paravirt.c?

Yeah, just initialize the pv_ops.lock.* thingies to contain the numa
variant before apply_paravirt() happens.

> Also, the paravirt code is under arch/x86, while CNA is generic (not
> x86-specific). Do you still want to see CNA-related patching residing
> under arch/x86?
>
> We still need a config option (something like NUMA_AWARE_SPINLOCKS) to
> enable CNA patching under this config only, correct?

There is the static_call() stuff that could be generic; I posted a new
version of that today (x86 only for now, but IIRC there's arm64 patches
for that around somewhere too).

https://lkml.kernel.org/r/20190605130753.327195108@infradead.org

Which would allow something a little like this:


diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
index bd5ac6cc37db..01feaf912bd7 100644
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -63,29 +63,7 @@ static inline bool vcpu_is_preempted(long cpu)
#endif

#ifdef CONFIG_PARAVIRT
-DECLARE_STATIC_KEY_TRUE(virt_spin_lock_key);
-
void native_pv_lock_init(void) __init;
-
-#define virt_spin_lock virt_spin_lock
-static inline bool virt_spin_lock(struct qspinlock *lock)
-{
- if (!static_branch_likely(&virt_spin_lock_key))
- 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;
-}
#else
static inline void native_pv_lock_init(void)
{
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 5169b8cc35bb..78be9e474e94 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -531,7 +531,7 @@ static void __init kvm_smp_prepare_cpus(unsigned int max_cpus)
{
native_smp_prepare_cpus(max_cpus);
if (kvm_para_has_hint(KVM_HINTS_REALTIME))
- static_branch_disable(&virt_spin_lock_key);
+ static_call_update(queued_spin_lock_slowpath, __queued_spin_lock_slowpath);
}

static void __init kvm_smp_prepare_boot_cpu(void)
diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index 98039d7fb998..ae6d15f84867 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -105,12 +105,10 @@ static unsigned paravirt_patch_jmp(void *insn_buff, const void *target,
}
#endif

-DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);
-
void __init native_pv_lock_init(void)
{
- if (!boot_cpu_has(X86_FEATURE_HYPERVISOR))
- static_branch_disable(&virt_spin_lock_key);
+ if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
+ static_call_update(queued_spin_lock_slowpath, __tas_spin_lock_slowpath);
}

unsigned paravirt_patch_default(u8 type, void *insn_buff,
diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 3776122c87cc..86808127b6e6 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -70,7 +70,7 @@ void xen_init_lock_cpu(int cpu)

if (!xen_pvspin) {
if (cpu == 0)
- static_branch_disable(&virt_spin_lock_key);
+ static_call_update(queued_spin_lock_slowpath, __queued_spin_lock_slowpath);
return;
}

diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
index fde943d180e0..8ca4dd9db931 100644
--- a/include/asm-generic/qspinlock.h
+++ b/include/asm-generic/qspinlock.h
@@ -65,7 +65,9 @@ static __always_inline int queued_spin_trylock(struct qspinlock *lock)
return likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL));
}

-extern void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
+extern void __queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
+
+DECLARE_STATIC_CALL(queued_spin_lock_slowpath, __queued_spin_lock_slowpath);

/**
* queued_spin_lock - acquire a queued spinlock
@@ -78,7 +80,7 @@ static __always_inline void queued_spin_lock(struct qspinlock *lock)
if (likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL)))
return;

- queued_spin_lock_slowpath(lock, val);
+ static_call(queued_spin_lock_slowpath, lock, val);
}

#ifndef queued_spin_unlock
@@ -95,13 +97,6 @@ static __always_inline void queued_spin_unlock(struct qspinlock *lock)
}
#endif

-#ifndef virt_spin_lock
-static __always_inline bool virt_spin_lock(struct qspinlock *lock)
-{
- return false;
-}
-#endif
-
/*
* Remapping spinlock architecture specific functions to the corresponding
* queued spinlock functions.
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 2473f10c6956..0e9e61637d56 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -290,6 +290,20 @@ static __always_inline u32 __pv_wait_head_or_lock(struct qspinlock *lock,

#endif /* _GEN_PV_LOCK_SLOWPATH */

+void __tas_spin_lock_slowpath(struct qspinlock *lock, u32 val)
+{
+ /*
+ * 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);
+}
+
/**
* queued_spin_lock_slowpath - acquire the queued spinlock
* @lock: Pointer to queued spinlock structure
@@ -311,7 +325,7 @@ static __always_inline u32 __pv_wait_head_or_lock(struct qspinlock *lock,
* contended : (*,x,y) +--> (*,0,0) ---> (*,0,1) -' :
* queue : ^--' :
*/
-void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
+void __queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
{
struct mcs_spinlock *prev, *next, *node;
u32 old, tail;
@@ -322,9 +336,6 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
if (pv_enabled())
goto pv_queue;

- if (virt_spin_lock(lock))
- return;
-
/*
* Wait for in-progress pending->locked hand-overs with a bounded
* number of spins so that we guarantee forward progress.
@@ -558,7 +569,9 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
*/
__this_cpu_dec(qnodes[0].mcs.count);
}
-EXPORT_SYMBOL(queued_spin_lock_slowpath);
+EXPORT_SYMBOL(__queued_spin_lock_slowpath);
+
+DEFINE_STATIC_CALL(queued_spin_lock_slowpath, __queued_spin_lock_slowpath);

/*
* Generate the paravirt code for queued_spin_unlock_slowpath().
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