Re: [PATCH v7 4/6] powerpc: lib/locks.c: Add cpu yield/wake helper function

[Boqun Feng]

Hi Xinhui,

On Mon, Sep 19, 2016 at 05:23:55AM -0400, Pan Xinhui wrote:
> Add two corresponding helper functions to support pv-qspinlock.
> 
> For normal use, __spin_yield_cpu will confer current vcpu slices to the
> target vcpu(say, a lock holder). If target vcpu is not specified or it
> is in running state, such conferging to lpar happens or not depends.
> 
> Because hcall itself will introduce latency and a little overhead. And
> we do NOT want to suffer any latency on some cases, e.g. in interrupt handler.
> The second parameter *confer* can indicate such case.
> 
> __spin_wake_cpu is simpiler, it will wake up one vcpu regardless of its
> current vcpu state.
> 
> Signed-off-by: Pan Xinhui 
> ---
>  arch/powerpc/include/asm/spinlock.h |  4 +++
>  arch/powerpc/lib/locks.c| 59 
> +
>  2 files changed, 63 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/spinlock.h 
> b/arch/powerpc/include/asm/spinlock.h
> index 6aef8dd..abb6b0f 100644
> --- a/arch/powerpc/include/asm/spinlock.h
> +++ b/arch/powerpc/include/asm/spinlock.h
> @@ -56,9 +56,13 @@
>  /* We only yield to the hypervisor if we are in shared processor mode */
>  #define SHARED_PROCESSOR (lppaca_shared_proc(local_paca->lppaca_ptr))
>  extern void __spin_yield(arch_spinlock_t *lock);
> +extern void __spin_yield_cpu(int cpu, int confer);
> +extern void __spin_wake_cpu(int cpu);
>  extern void __rw_yield(arch_rwlock_t *lock);
>  #else /* SPLPAR */
>  #define __spin_yield(x)  barrier()
> +#define __spin_yield_cpu(x,y) barrier()
> +#define __spin_wake_cpu(x) barrier()
>  #define __rw_yield(x)barrier()
>  #define SHARED_PROCESSOR 0
>  #endif
> diff --git a/arch/powerpc/lib/locks.c b/arch/powerpc/lib/locks.c
> index 6574626..892df7d 100644
> --- a/arch/powerpc/lib/locks.c
> +++ b/arch/powerpc/lib/locks.c
> @@ -23,6 +23,65 @@
>  #include 
>  #include 
>  
> +/*
> + * confer our slices to a specified cpu and return. If it is already running 
> or
> + * cpu is -1, then we will check confer. If confer is NULL, we will return
> + * otherwise we confer our slices to lpar.
> + */
> +void __spin_yield_cpu(int cpu, int confer)
> +{
> + unsigned int holder_cpu = cpu, yield_count;
> +

You don't need @holder_cpu at all, do you?

> + if (cpu == -1)
> + goto yield_to_lpar;
> +
> + BUG_ON(holder_cpu >= nr_cpu_ids);
> + yield_count = be32_to_cpu(lppaca_of(holder_cpu).yield_count);
> +
> + /* if cpu is running, confer slices to lpar conditionally*/
> + if ((yield_count & 1) == 0)
> + goto yield_to_lpar;
> +
> + plpar_hcall_norets(H_CONFER,
> + get_hard_smp_processor_id(holder_cpu), yield_count);
> + return;
> +
> +yield_to_lpar:
> + if (confer)
> + plpar_hcall_norets(H_CONFER, -1, 0);
> +}
> +EXPORT_SYMBOL_GPL(__spin_yield_cpu);
> +
> +void __spin_wake_cpu(int cpu)
> +{
> + unsigned int holder_cpu = cpu;
> +

Ditto.

Regards,
Boqun

> + BUG_ON(holder_cpu >= nr_cpu_ids);
> + /*
> +  * NOTE: we should always do this hcall regardless of
> +  * the yield_count of the holder_cpu.
> +  * as thers might be a case like below;
> +  * CPU  1   2
> +  *  yielded = true
> +  *  if (yielded)
> +  *  __spin_wake_cpu()
> +  *  __spin_yield_cpu()
> +  *
> +  * So we might lose a wake if we check the yield_count and
> +  * return directly if the holder_cpu is running.
> +  * IOW. do NOT code like below.
> +  *  yield_count = be32_to_cpu(lppaca_of(holder_cpu).yield_count);
> +  *  if ((yield_count & 1) == 0)
> +  *  return;
> +  *
> +  * a PROD hcall marks the target_cpu proded, which cause the next cede 
> or confer
> +  * called on the target_cpu invalid.
> +  */
> + plpar_hcall_norets(H_PROD,
> + get_hard_smp_processor_id(holder_cpu));
> +}
> +EXPORT_SYMBOL_GPL(__spin_wake_cpu);
> +
>  #ifndef CONFIG_QUEUED_SPINLOCKS
>  void __spin_yield(arch_spinlock_t *lock)
>  {
> -- 
> 2.4.11
> 


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[PATCH v7 4/6] powerpc: lib/locks.c: Add cpu yield/wake helper function

[Pan Xinhui]

Add two corresponding helper functions to support pv-qspinlock.

For normal use, __spin_yield_cpu will confer current vcpu slices to the
target vcpu(say, a lock holder). If target vcpu is not specified or it
is in running state, such conferging to lpar happens or not depends.

Because hcall itself will introduce latency and a little overhead. And
we do NOT want to suffer any latency on some cases, e.g. in interrupt handler.
The second parameter *confer* can indicate such case.

__spin_wake_cpu is simpiler, it will wake up one vcpu regardless of its
current vcpu state.

Signed-off-by: Pan Xinhui 
---
 arch/powerpc/include/asm/spinlock.h |  4 +++
 arch/powerpc/lib/locks.c| 59 +
 2 files changed, 63 insertions(+)

diff --git a/arch/powerpc/include/asm/spinlock.h 
b/arch/powerpc/include/asm/spinlock.h
index 6aef8dd..abb6b0f 100644
--- a/arch/powerpc/include/asm/spinlock.h
+++ b/arch/powerpc/include/asm/spinlock.h
@@ -56,9 +56,13 @@
 /* We only yield to the hypervisor if we are in shared processor mode */
 #define SHARED_PROCESSOR (lppaca_shared_proc(local_paca->lppaca_ptr))
 extern void __spin_yield(arch_spinlock_t *lock);
+extern void __spin_yield_cpu(int cpu, int confer);
+extern void __spin_wake_cpu(int cpu);
 extern void __rw_yield(arch_rwlock_t *lock);
 #else /* SPLPAR */
 #define __spin_yield(x)barrier()
+#define __spin_yield_cpu(x,y) barrier()
+#define __spin_wake_cpu(x) barrier()
 #define __rw_yield(x)  barrier()
 #define SHARED_PROCESSOR   0
 #endif
diff --git a/arch/powerpc/lib/locks.c b/arch/powerpc/lib/locks.c
index 6574626..892df7d 100644
--- a/arch/powerpc/lib/locks.c
+++ b/arch/powerpc/lib/locks.c
@@ -23,6 +23,65 @@
 #include 
 #include 
 
+/*
+ * confer our slices to a specified cpu and return. If it is already running or
+ * cpu is -1, then we will check confer. If confer is NULL, we will return
+ * otherwise we confer our slices to lpar.
+ */
+void __spin_yield_cpu(int cpu, int confer)
+{
+   unsigned int holder_cpu = cpu, yield_count;
+
+   if (cpu == -1)
+   goto yield_to_lpar;
+
+   BUG_ON(holder_cpu >= nr_cpu_ids);
+   yield_count = be32_to_cpu(lppaca_of(holder_cpu).yield_count);
+
+   /* if cpu is running, confer slices to lpar conditionally*/
+   if ((yield_count & 1) == 0)
+   goto yield_to_lpar;
+
+   plpar_hcall_norets(H_CONFER,
+   get_hard_smp_processor_id(holder_cpu), yield_count);
+   return;
+
+yield_to_lpar:
+   if (confer)
+   plpar_hcall_norets(H_CONFER, -1, 0);
+}
+EXPORT_SYMBOL_GPL(__spin_yield_cpu);
+
+void __spin_wake_cpu(int cpu)
+{
+   unsigned int holder_cpu = cpu;
+
+   BUG_ON(holder_cpu >= nr_cpu_ids);
+   /*
+* NOTE: we should always do this hcall regardless of
+* the yield_count of the holder_cpu.
+* as thers might be a case like below;
+* CPU  1   2
+*  yielded = true
+*  if (yielded)
+*  __spin_wake_cpu()
+*  __spin_yield_cpu()
+*
+* So we might lose a wake if we check the yield_count and
+* return directly if the holder_cpu is running.
+* IOW. do NOT code like below.
+*  yield_count = be32_to_cpu(lppaca_of(holder_cpu).yield_count);
+*  if ((yield_count & 1) == 0)
+*  return;
+*
+* a PROD hcall marks the target_cpu proded, which cause the next cede 
or confer
+* called on the target_cpu invalid.
+*/
+   plpar_hcall_norets(H_PROD,
+   get_hard_smp_processor_id(holder_cpu));
+}
+EXPORT_SYMBOL_GPL(__spin_wake_cpu);
+
 #ifndef CONFIG_QUEUED_SPINLOCKS
 void __spin_yield(arch_spinlock_t *lock)
 {
-- 
2.4.11

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