Date:	Sun, 26 Jun 2016 14:10:57 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	panxinhui <xinhui@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...hat.com, dave@...olabs.net,
	will.deacon@....com, Waiman.Long@....com, benh@...nel.crashing.org
Subject: Re: [PATCH] locking/osq: Drop the overload of osq lock

On Sun, Jun 26, 2016 at 01:21:04PM +0800, panxinhui wrote:
> 
> > 在 2016年6月26日，03:20，Peter Zijlstra <peterz@...radead.org> 写道：
> > 
> > On Sun, Jun 26, 2016 at 01:27:56AM +0800, panxinhui wrote:
> >>>> Would that not have issues where the owner cpu is kept running but the
> >>>> spinner (ie. _this_ vcpu) gets preempted? I would think that in that
> >>>> case we too want to stop spinning.
> >>>> 
> >>> 
> >> do  you mean that the spinner detect itself had yield out during the
> >> big spin loop?
> >> 
> >> It is very possible to happen.  BUT if spinner(on this vcpu) yield
> >> out, the next spinner would break the spin loop.  AND if spinner
> >> detect itself yield out once, it’s very possible to get the osq lock
> >> soon as long as the ower vcpu is running.
> >> 
> >> SO I think we need just check the owner vcpu’s yield_count.
> > 
> > I had a quick look at KVM and it looks like it only has
> > kvm_cpu::preempted, which would suggest the interface boqun proposed.
> > 
> > We'll have to look at many of the other virt platforms as well to see
> > what they can do.
> > 
> > We could also provide _both_ interfaces and a platform can implement
> > whichever variant (or both) it can.
> > 
> the kvm code on ppc has implemented  yield_count.
> It let me feel a little relaxed. :)
> 
> looks like we could  introduce the interface like below.
> 
> bool vcpu_is_preempted(int cpu)
> {
> 	return arch_vcpu_is_preempted(cpu);
> }
> 
> #ifdef arch_vcpu_has_yield_count 
> bool vcpu_has_preemtped_once(int cpu, unsigned int yield_count)
>  {
> 	return arch_get_vcpu_yield_count() != yield_count;
> }
> 
> #else
> bool vcpu_has_preemtped_once(int cpu, unsigned int yield_count)
> {
> 	/*just let called know it is preepmpted*/
> 	return vcpu_is_preempted(cpu);
> }
> #endif
> 

Unfortunately, on PPC, we compile pseries code along with powernv code
in a kernel binary, therefore we need to wire the proper primitives at
runtime.

I've cooked the following patch, please note I haven't test this, this
is just a POC.

Regards,
Boqun

---------------->8
virt: Introduce vcpu preemption detection functions

Lock holder preemption is an serious problem in virtualized environment
for locking. Different archs and hypervisors employ different ways to
try to solve the problem in different locking mechanisms. And recently
Pan Xinhui found a significant performance drop in osq_lock(), which
could be solved by providing a mechanism to detect the preemption of
vcpu.

Therefore this patch introduces several vcpu preemption detection
primitives, which allows locking primtives to save the time of
unnecesarry spinning. These primitives act as an abstract layer between
locking functions and arch or hypervisor related code.

There are two sets of primitives:

1.	vcpu_preempt_count() and vcpu_has_preempted(), they must be used
	pairwisely in a same preempt disable critical section. And they
	could detect whether a vcpu preemption happens between them.

2.	vcpu_is_preempted(), used to check whether other cpu's vcpu is
	preempted.

This patch also implements those primitives on pseries and wire them up.

Signed-off-by: Boqun Feng <boqun.feng@...il.com>
---
 arch/powerpc/platforms/pseries/Kconfig |  1 +
 arch/powerpc/platforms/pseries/setup.c | 28 +++++++++++
 include/linux/vcpu_preempt.h           | 90 ++++++++++++++++++++++++++++++++++
 3 files changed, 119 insertions(+)
 create mode 100644 include/linux/vcpu_preempt.h

diff --git a/arch/powerpc/platforms/pseries/Kconfig b/arch/powerpc/platforms/pseries/Kconfig
index bec90fb30425..7d24c3e48878 100644
--- a/arch/powerpc/platforms/pseries/Kconfig
+++ b/arch/powerpc/platforms/pseries/Kconfig
@@ -21,6 +21,7 @@ config PPC_PSERIES
 	select HOTPLUG_CPU if SMP
 	select ARCH_RANDOM
 	select PPC_DOORBELL
+	select HAS_VCPU_PREEMPTION_DETECTION
 	default y
 
 config PPC_SPLPAR
diff --git a/arch/powerpc/platforms/pseries/setup.c b/arch/powerpc/platforms/pseries/setup.c
index 9883bc7ea007..5d4aed54e039 100644
--- a/arch/powerpc/platforms/pseries/setup.c
+++ b/arch/powerpc/platforms/pseries/setup.c
@@ -42,6 +42,7 @@
 #include <linux/of.h>
 #include <linux/of_pci.h>
 #include <linux/kexec.h>
+#include <linux/vcpu_preempt.h>
 
 #include <asm/mmu.h>
 #include <asm/processor.h>
@@ -501,6 +502,31 @@ static void __init find_and_init_phbs(void)
 	of_pci_check_probe_only();
 }
 
+struct vcpu_preempt_ops vcpu_preempt_ops = DEFAULT_VCPU_PREEMPT_OPS;
+EXPORT_SYMBOL(vcpu_preempt_ops);
+
+static long pseries_vcpu_preempt_count(void)
+{
+	return be32_to_cpu(get_lppaca()->yield_count);
+}
+
+static bool pseries_vcpu_is_preempted(int cpu)
+{
+	return !!(be32_to_cpu(lppaca_of(cpu).yield_count) & 1);
+}
+
+static bool pseries_vcpu_has_preempted(long vpc)
+{
+	return pseries_vcpu_preempt_count() != vpc;
+}
+
+static void __init pseries_setup_vcpu_preempt_ops(void)
+{
+	vcpu_preempt_ops.preempt_count = pseries_vcpu_preempt_count;
+	vcpu_preempt_ops.is_preempted = pseries_vcpu_is_preempted;
+	vcpu_preempt_ops.has_preempted = pseries_vcpu_has_preempted;
+}
+
 static void __init pSeries_setup_arch(void)
 {
 	set_arch_panic_timeout(10, ARCH_PANIC_TIMEOUT);
@@ -549,6 +575,8 @@ static void __init pSeries_setup_arch(void)
 				"%ld\n", rc);
 		}
 	}
+
+	pseries_setup_vcpu_preempt_ops();
 }
 
 static int __init pSeries_init_panel(void)
diff --git a/include/linux/vcpu_preempt.h b/include/linux/vcpu_preempt.h
new file mode 100644
index 000000000000..4a88414bb83f
--- /dev/null
+++ b/include/linux/vcpu_preempt.h
@@ -0,0 +1,90 @@
+/*
+ * Primitives for checking the vcpu preemption from the guest.
+ */
+
+static long __vcpu_preempt_count(void)
+{
+	return 0;
+}
+
+static bool __vcpu_has_preempted(long vpc)
+{
+	return false;
+}
+
+static bool __vcpu_is_preempted(int cpu)
+{
+	return false;
+}
+
+struct vcpu_preempt_ops {
+	/*
+	 * Get the current vcpu's "preempt count", which is going to use for
+	 * checking whether the current vcpu has ever been preempted
+	 */
+	long (*preempt_count)(void);
+
+	/*
+	 * Return whether a vcpu is preempted
+	 */
+	bool (*is_preempted)(int cpu);
+
+	/*
+	 * Given a "vcpu preempt count", Return whether a vcpu preemption ever
+	 * happened after the .preempt_count() was called.
+	 */
+	bool (*has_preempted)(long vpc);
+};
+
+struct vcpu_preempt_ops vcpu_preempt_ops;
+
+/* Default boilerplate */
+#define DEFAULT_VCPU_PREEMPT_OPS			\
+	{						\
+		.preempt_count = __vcpu_preempt_count,	\
+		.is_preempted = __vcpu_is_preempted,	\
+		.has_preempted = __vcpu_has_preempted	\
+	}
+
+#ifdef CONFIG_HAS_VCPU_PREEMPTION_DETECTION
+/*
+ * vcpu_preempt_count: Get the current cpu's "vcpu preempt count"(vpc).
+ *
+ * The vpc is used for checking whether the current vcpu has ever been
+ * preempted via vcpu_has_preempted().
+ *
+ * This function and vcpu_has_preepmted() should be called in the same
+ * preemption disabled critical section.
+ */
+static long vcpu_preempt_count(void)
+{
+	return vcpu_preempt_ops.preempt_count();
+}
+
+/*
+ * vcpu_is_preempted: Check whether @cpu's vcpu is preempted.
+ */
+static bool vcpu_is_preempted(int cpu)
+{
+	return vcpu_preempt_ops.is_preempted(cpu);
+}
+
+/*
+ * vcpu_has_preepmted: Check whether the current cpu's vcpu has ever been
+ * preempted.
+ *
+ * The checked duration is between the vcpu_preempt_count() which returns @vpc
+ * is called and this function called.
+ *
+ * This function and corresponding vcpu_preempt_count() should be in the same
+ * preemption disabled cirtial section.
+ */
+static bool vcpu_has_preempted(long vpc)
+{
+	return vcpu_preempt_ops.has_preempt(vpc);
+}
+#else /* CONFIG_HAS_VCPU_PREEMPTION_DETECTION */
+#define vcpu_preempt_count() __vcpu_preempt_count()
+#define vcpu_is_preempted(cpu) __vcpu_is_preempted(cpu)
+#define vcpu_has_preempted(vpc) __vcpu_has_preempted(vpc)
+#endif /* CONFIG_HAS_VCPU_PREEPMTION_DETECTION */
-- 
2.9.0

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