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Date: Tue, 20 Oct 2015 16:00:31 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Daniel Wagner <daniel.wagner@...-carit.de>
Cc: linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
Marcelo Tosatti <mtosatti@...hat.com>,
Paolo Bonzini <pbonzini@...hat.com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Paul Gortmaker <paul.gortmaker@...driver.com>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 2/4] KVM: use simple waitqueue for vcpu->wq
On Tue, Oct 20, 2015 at 09:28:08AM +0200, Daniel Wagner wrote:
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 2280497..f534e15 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -2560,10 +2560,9 @@ static void kvmppc_vcore_blocked(struct kvmppc_vcore *vc)
> {
> struct kvm_vcpu *vcpu;
> int do_sleep = 1;
> + DECLARE_SWAITQUEUE(wait);
>
> - DEFINE_WAIT(wait);
> -
> - prepare_to_wait(&vc->wq, &wait, TASK_INTERRUPTIBLE);
> + prepare_to_swait(&vc->wq, &wait, TASK_INTERRUPTIBLE);
>
> /*
> * Check one last time for pending exceptions and ceded state after
> @@ -2577,7 +2576,7 @@ static void kvmppc_vcore_blocked(struct kvmppc_vcore *vc)
> }
>
> if (!do_sleep) {
> - finish_wait(&vc->wq, &wait);
> + finish_swait(&vc->wq, &wait);
> return;
> }
>
> @@ -2585,7 +2584,7 @@ static void kvmppc_vcore_blocked(struct kvmppc_vcore *vc)
> trace_kvmppc_vcore_blocked(vc, 0);
> spin_unlock(&vc->lock);
> schedule();
> - finish_wait(&vc->wq, &wait);
> + finish_swait(&vc->wq, &wait);
> spin_lock(&vc->lock);
> vc->vcore_state = VCORE_INACTIVE;
> trace_kvmppc_vcore_blocked(vc, 1);
This one looks buggy, one should _NOT_ assume that your blocking
condition is true after schedule().
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 8db1d93..45ab55f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2019,7 +2018,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
> }
>
> for (;;) {
> - prepare_to_wait(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
> + prepare_to_swait(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
>
> if (kvm_vcpu_check_block(vcpu) < 0)
> break;
> @@ -2028,7 +2027,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
> schedule();
> }
>
> - finish_wait(&vcpu->wq, &wait);
> + finish_swait(&vcpu->wq, &wait);
> cur = ktime_get();
>
> out:
Should we not take this opportunity to get rid of these open-coded wait
loops?
Does this work?
---
arch/powerpc/kvm/book3s_hv.c | 33 +++++++++++++++++----------------
virt/kvm/kvm_main.c | 13 ++-----------
2 files changed, 19 insertions(+), 27 deletions(-)
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 228049786888..b5b8bcad5105 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -2552,18 +2552,10 @@ static void kvmppc_wait_for_exec(struct kvmppc_vcore *vc,
finish_wait(&vcpu->arch.cpu_run, &wait);
}
-/*
- * All the vcpus in this vcore are idle, so wait for a decrementer
- * or external interrupt to one of the vcpus. vc->lock is held.
- */
-static void kvmppc_vcore_blocked(struct kvmppc_vcore *vc)
+static inline bool kvmppc_vcore_should_sleep(struct kvmppc_vcore *vc)
{
struct kvm_vcpu *vcpu;
- int do_sleep = 1;
-
- DEFINE_WAIT(wait);
-
- prepare_to_wait(&vc->wq, &wait, TASK_INTERRUPTIBLE);
+ bool sleep = true;
/*
* Check one last time for pending exceptions and ceded state after
@@ -2571,26 +2563,35 @@ static void kvmppc_vcore_blocked(struct kvmppc_vcore *vc)
*/
list_for_each_entry(vcpu, &vc->runnable_threads, arch.run_list) {
if (vcpu->arch.pending_exceptions || !vcpu->arch.ceded) {
- do_sleep = 0;
+ sleep = false;
break;
}
}
- if (!do_sleep) {
- finish_wait(&vc->wq, &wait);
- return;
- }
+ return sleep;
+}
+static inline void kvmppc_vcore_schedule(struct kvmppc_vcore *vc)
+{
vc->vcore_state = VCORE_SLEEPING;
trace_kvmppc_vcore_blocked(vc, 0);
spin_unlock(&vc->lock);
schedule();
- finish_wait(&vc->wq, &wait);
spin_lock(&vc->lock);
vc->vcore_state = VCORE_INACTIVE;
trace_kvmppc_vcore_blocked(vc, 1);
}
+/*
+ * All the vcpus in this vcore are idle, so wait for a decrementer
+ * or external interrupt to one of the vcpus. vc->lock is held.
+ */
+static void kvmppc_vcore_blocked(struct kvmppc_vcore *vc)
+{
+ ___wait_event(vc->wq, !kvmppc_vcore_should_sleep(vc), TASK_IDLE, 0, 0,
+ kvmppc_vcore_schedule(vc));
+}
+
static int kvmppc_run_vcpu(struct kvm_run *kvm_run, struct kvm_vcpu *vcpu)
{
int n_ceded;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 8db1d9361993..488f00d79059 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1996,7 +1996,6 @@ static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
void kvm_vcpu_block(struct kvm_vcpu *vcpu)
{
ktime_t start, cur;
- DEFINE_WAIT(wait);
bool waited = false;
u64 block_ns;
@@ -2018,17 +2017,9 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
} while (single_task_running() && ktime_before(cur, stop));
}
- for (;;) {
- prepare_to_wait(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
+ ___wait_event(vcpu->wq, kvm_cpu_check_block(vcpu) < 0, TASK_IDLE, 0, 0,
+ waited = true; schedule());
- if (kvm_vcpu_check_block(vcpu) < 0)
- break;
-
- waited = true;
- schedule();
- }
-
- finish_wait(&vcpu->wq, &wait);
cur = ktime_get();
out:
--
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