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Re: [Xen-devel] [PATCH v7 10/11] pvqspinlock, x86: Enable qspinlock PV support for KVM



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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