Re: [PATCH v7 4/6] powerpc: lib/locks.c: Add cpu yield/wake helper function
From: Boqun Feng <hidden>
Date: 2016-09-22 15:17:55
Also in:
lkml, virtualization
Hi Xinhui, On Mon, Sep 19, 2016 at 05:23:55AM -0400, Pan Xinhui wrote:
quoted hunk ↗ jump to hunk
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 <redacted> --- 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 #endifdiff --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 <asm/hvcall.h> #include <asm/smp.h> +/* + * 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)
{
--
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