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Message-ID: <20151021085500.GB15591@fergus.ozlabs.ibm.com>
Date: Wed, 21 Oct 2015 19:55:00 +1100
From: Paul Mackerras <paulus@...abs.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Daniel Wagner <daniel.wagner@...-carit.de>,
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 04:00:31PM +0200, Peter Zijlstra wrote:
> 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().
Do you mean it's buggy in calling finish_swait there, or it's buggy in
not immediately re-checking the condition? If the latter, then it's
OK because the sole caller of this function calls it in a loop and
checks the condition (all runnable vcpus in this vcore are idle) each
time around the loop.
> > 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)
This function could also be used in kvmppc_run_vcpu().
> {
> 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));
Wow, triple underscores, that must be an ultra-trendy function. :)
> +}
> +
> static int kvmppc_run_vcpu(struct kvm_run *kvm_run, struct kvm_vcpu *vcpu)
> {
> int n_ceded;
That all looks OK at a first glance, I'll give it a whirl.
Paul.
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