Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested

[David Woodhouse]

On Wed, 2018-07-11 at 09:49 -0700, Paul E. McKenney wrote:
> And here is an updated v4.15 patch with Marius's Reported-by and David's
> fix to my lost exclamation point.

Thanks. Are you sending the original version of that to Linus? It'd be
useful to have the commit ID so that we can watch for it landing, and
chase this one up to Greg.

As discussed on IRC, this patch reduces synchronize_sched() latency for
us from ~4600s to ~160ms, which is nice.

However, it isn't going to be sufficient in the NO_HZ_FULL case. For
that you want a patch like the one below, which happily reduces the
latency in our (!NO_HZ_FULL) case still further to ~40ms.

Adding kvm list for better review...

From: David Woodhouse <dwmw@xxxxxxxxxxxx>
Subject: [PATCH] kvm/x86: Inform RCU of quiescent state when entering guest mode

RCU can spend long periods of time waiting for a CPU which is actually in
KVM guest mode, entirely pointlessly. Treat it like the idle and userspace
modes, and don't wait for it.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
 arch/x86/kvm/x86.c      |  2 ++
 include/linux/rcutree.h |  2 ++
 kernel/rcu/tree.c       | 16 ++++++++++++++++
 3 files changed, 20 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0046aa70205a..b0c82f70afa7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7458,7 +7458,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 		vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
 	}
 
+	rcu_kvm_enter();
 	kvm_x86_ops->run(vcpu);
+	rcu_kvm_exit();
 
 	/*
 	 * Do this here before restoring debug registers on the host.  And
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 914655848ef6..6d07af5a50fc 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -82,6 +82,8 @@ void cond_synchronize_sched(unsigned long oldstate);
 
 void rcu_idle_enter(void);
 void rcu_idle_exit(void);
+void rcu_kvm_enter(void);
+void rcu_kvm_exit(void);
 void rcu_irq_enter(void);
 void rcu_irq_exit(void);
 void rcu_irq_enter_irqson(void);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index aa7cade1b9f3..df7893273939 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1019,6 +1019,22 @@ void rcu_irq_enter_irqson(void)
 	local_irq_restore(flags);
 }
 
+/*
+ * These are currently identical to the _idle_ versions but let's
+ * explicitly have separate copies to keep Paul honest in future.
+ */
+void rcu_kvm_enter(void)
+{
+	rcu_idle_enter();
+}
+EXPORT_SYMBOL_GPL(rcu_kvm_enter);
+
+void rcu_kvm_exit(void)
+{
+	rcu_idle_exit();
+}
+EXPORT_SYMBOL_GPL(rcu_kvm_exit);
+
 /**
  * rcu_is_watching - see if RCU thinks that the current CPU is idle
  *
-- 
2.17.1


-- 
dwmw2