Re: [PATCH v7 10/11] pvqspinlock, x86: Enable qspinlock PV support for KVM
From: Paolo Bonzini
Date: Thu Mar 20 2014 - 18:07:30 EST
Il 19/03/2014 21:14, Waiman Long ha scritto:
This patch adds the necessary KVM specific code to allow KVM to support
the sleeping and CPU kicking operations needed by the queue spinlock PV
code.
The remaining problem of this patch is that you cannot get the
generically optimal configuration: qspinlock on baremetal, PV qspinlock
on KVM/Xen, unfair qspinlock on VMware/Hyper-V. You really need to
disable unfair locks in kvm_spinlock_init.
Paolo
The XFS test had moderate spinlock contention of 1.6% whereas the
ext4 test had heavy spinlock contention of 15.4% as reported by perf.
A more interesting performance comparison is when there is
overcommit. With both PV guests (all 20 CPUs equally shared with
both guests - 200% overcommit) turned on and the "idle=poll" kernel
option to simulate a busy guest. The results of running the same AIM7
workloads are shown in the table below.
XFS Test:
kernel JPM Real Time Sys Time Usr Time
----- --- --------- -------- --------
PV ticketlock 597015 29.99 424.44 25.85
qspinlock 117493 153.20 1006.06 59.60
PV qspinlock 616438 29.20 397.57 23.31
unfair qspinlock 642398 28.02 396.42 25.42
unfair + PV qspinlock 633803 28.40 400.45 26.16
ext4 Test:
kernel JPM Real Time Sys Time Usr Time
----- --- --------- -------- --------
PV ticketlock 120984 148.78 2378.98 29.27
qspinlock 54995 327.30 5023.58 54.73
PV qspinlock 124215 144.91 2282.22 28.89
unfair qspinlock 467411 38.51 481.80 25.80
unfair + PV qspinlock 471080 38.21 482.40 25.09
The kernel build test (make -j 20) results are as follows:
kernel Real Time Sys Time Usr Time
----- --------- -------- --------
PV ticketlock 20m6.158s 41m7.167s 283m3.790s
qspinlock 26m41.294s 74m55.585s 346m31.981s
PV qspinlock 20m17.429s 41m19.434s 281m21.238s
unfair qspinlock 19m58.315s 40m18.011s 279m27.177s
unfair + PV qspinlock 20m0.030s 40m35.011s 278m50.522s
With no overcommit, the PV code doesn't really do anything. The unfair
lock does provide some performance advantage depending on the workload.
In an overcommited PV guest, however, there can be a big performance
benefit with PV and unfair lock. In term of spinlock contention,
the ordering of the 3 workloads are:
kernel build < AIM7 disk xfs < AIM7 disk ext4
With light spinlock contention, the PV ticketlock can be a bit
faster than PV qspinlock. On moderate to high spinlock contention,
PV qspinlock performs better. The unfair lock has the best performance
in all cases, especially with heavy spinlock contention.
Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
---
arch/x86/kernel/kvm.c | 82 +++++++++++++++++++++++++++++++++++++++++++++++++
kernel/Kconfig.locks | 2 +-
2 files changed, 83 insertions(+), 1 deletions(-)
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index f318e78..c28bc1b 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -568,6 +568,7 @@ static void kvm_kick_cpu(int cpu)
kvm_hypercall2(KVM_HC_KICK_CPU, flags, apicid);
}
+#ifndef CONFIG_QUEUE_SPINLOCK
enum kvm_contention_stat {
TAKEN_SLOW,
TAKEN_SLOW_PICKUP,
@@ -795,6 +796,82 @@ static void kvm_unlock_kick(struct arch_spinlock *lock, __ticket_t ticket)
}
}
}
+#else /* !CONFIG_QUEUE_SPINLOCK */
+
+#ifdef CONFIG_KVM_DEBUG_FS
+static struct dentry *d_spin_debug;
+static struct dentry *d_kvm_debug;
+static u32 kick_stats; /* CPU kick count */
+static u32 hibernate_stats; /* Hibernation count */
+
+static int __init kvm_spinlock_debugfs(void)
+{
+ d_kvm_debug = debugfs_create_dir("kvm-guest", NULL);
+ if (!d_kvm_debug) {
+ printk(KERN_WARNING
+ "Could not create 'kvm' debugfs directory\n");
+ return -ENOMEM;
+ }
+ d_spin_debug = debugfs_create_dir("spinlocks", d_kvm_debug);
+
+ debugfs_create_u32("kick_stats", 0644, d_spin_debug, &kick_stats);
+ debugfs_create_u32("hibernate_stats",
+ 0644, d_spin_debug, &hibernate_stats);
+ return 0;
+}
+
+static inline void inc_kick_stats(void)
+{
+ add_smp(&kick_stats, 1);
+}
+
+static inline void inc_hib_stats(void)
+{
+ add_smp(&hibernate_stats, 1);
+}
+
+fs_initcall(kvm_spinlock_debugfs);
+
+#else /* CONFIG_KVM_DEBUG_FS */
+static inline void inc_kick_stats(void)
+{
+}
+
+static inline void inc_hib_stats(void)
+{
+
+}
+#endif /* CONFIG_KVM_DEBUG_FS */
+
+static void kvm_kick_cpu_type(int cpu)
+{
+ kvm_kick_cpu(cpu);
+ inc_kick_stats();
+}
+
+/*
+ * Halt the current CPU & release it back to the host
+ */
+static void kvm_hibernate(void)
+{
+ unsigned long flags;
+
+ if (in_nmi())
+ return;
+
+ inc_hib_stats();
+ /*
+ * Make sure an interrupt handler can't upset things in a
+ * partially setup state.
+ */
+ local_irq_save(flags);
+ if (arch_irqs_disabled_flags(flags))
+ halt();
+ else
+ safe_halt();
+ local_irq_restore(flags);
+}
+#endif /* !CONFIG_QUEUE_SPINLOCK */
/*
* Setup pv_lock_ops to exploit KVM_FEATURE_PV_UNHALT if present.
@@ -807,8 +884,13 @@ void __init kvm_spinlock_init(void)
if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
return;
+#ifdef CONFIG_QUEUE_SPINLOCK
+ pv_lock_ops.kick_cpu = kvm_kick_cpu_type;
+ pv_lock_ops.hibernate = kvm_hibernate;
+#else
pv_lock_ops.lock_spinning = PV_CALLEE_SAVE(kvm_lock_spinning);
pv_lock_ops.unlock_kick = kvm_unlock_kick;
+#endif
}
static __init int kvm_spinlock_init_jump(void)
diff --git a/kernel/Kconfig.locks b/kernel/Kconfig.locks
index f185584..a70fdeb 100644
--- a/kernel/Kconfig.locks
+++ b/kernel/Kconfig.locks
@@ -229,4 +229,4 @@ config ARCH_USE_QUEUE_SPINLOCK
config QUEUE_SPINLOCK
def_bool y if ARCH_USE_QUEUE_SPINLOCK
- depends on SMP && !PARAVIRT_SPINLOCKS
+ depends on SMP && (!PARAVIRT_SPINLOCKS || !XEN)
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