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Date: Tue, 27 Nov 2012 14:22:27 +0100
From: Andrew Jones <drjones@...hat.com>
To: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Cc: riel@...hat.com, Peter Zijlstra <peterz@...radead.org>,
"H. Peter Anvin" <hpa@...or.com>, Avi Kivity <avi@...hat.com>,
Gleb Natapov <gleb@...hat.com>, Ingo Molnar <mingo@...hat.com>,
Marcelo Tosatti <mtosatti@...hat.com>,
Srikar <srikar@...ux.vnet.ibm.com>,
"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
KVM <kvm@...r.kernel.org>, Jiannan Ouyang <ouyang@...pitt.edu>,
Chegu Vinod <chegu_vinod@...com>,
"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
LKML <linux-kernel@...r.kernel.org>,
Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>
Subject: Re: [PATCH V3 RFC 2/2] kvm: Handle yield_to failure return code for
potential undercommit case
On Tue, Nov 27, 2012 at 03:57:25PM +0530, Raghavendra K T wrote:
> On 11/26/2012 07:13 PM, Andrew Jones wrote:
> >On Mon, Nov 26, 2012 at 05:38:04PM +0530, Raghavendra K T wrote:
> >>From: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
> >>
> >>yield_to returns -ESRCH, When source and target of yield_to
> >>run queue length is one. When we see three successive failures of
> >>yield_to we assume we are in potential undercommit case and abort
> >>from PLE handler.
> >>The assumption is backed by low probability of wrong decision
> >>for even worst case scenarios such as average runqueue length
> >>between 1 and 2.
> >>
> >>note that we do not update last boosted vcpu in failure cases.
> >>Thank Avi for raising question on aborting after first fail from
> yield_to.
> >>
> >>Reviewed-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> >>Signed-off-by: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
> >>---
> >> virt/kvm/kvm_main.c | 26 ++++++++++++++++----------
> >> 1 file changed, 16 insertions(+), 10 deletions(-)
> >>
> >>diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> >>index be70035..053f494 100644
> >>--- a/virt/kvm/kvm_main.c
> >>+++ b/virt/kvm/kvm_main.c
> >>@@ -1639,6 +1639,7 @@ bool kvm_vcpu_yield_to(struct kvm_vcpu *target)
> >> {
> >> struct pid *pid;
> >> struct task_struct *task = NULL;
> >>+ bool ret = false;
> >>
> >> rcu_read_lock();
> >> pid = rcu_dereference(target->pid);
> >>@@ -1646,17 +1647,15 @@ bool kvm_vcpu_yield_to(struct kvm_vcpu *target)
> >> task = get_pid_task(target->pid, PIDTYPE_PID);
> >> rcu_read_unlock();
> >> if (!task)
> >>- return false;
> >>+ return ret;
> >> if (task->flags & PF_VCPU) {
> >> put_task_struct(task);
> >>- return false;
> >>- }
> >>- if (yield_to(task, 1)) {
> >>- put_task_struct(task);
> >>- return true;
> >>+ return ret;
> >> }
> >>+ ret = yield_to(task, 1);
> >> put_task_struct(task);
> >>- return false;
> >>+
> >>+ return ret;
> >> }
> >> EXPORT_SYMBOL_GPL(kvm_vcpu_yield_to);
> >>
> >>@@ -1697,12 +1696,14 @@ bool
> kvm_vcpu_eligible_for_directed_yield(struct kvm_vcpu *vcpu)
> >> return eligible;
> >> }
> >> #endif
> >>+
> >> void kvm_vcpu_on_spin(struct kvm_vcpu *me)
> >> {
> >> struct kvm *kvm = me->kvm;
> >> struct kvm_vcpu *vcpu;
> >> int last_boosted_vcpu = me->kvm->last_boosted_vcpu;
> >> int yielded = 0;
> >>+ int try = 3;
> >> int pass;
> >> int i;
> >>
> >>@@ -1714,7 +1715,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
> >> * VCPU is holding the lock that we need and will release it.
> >> * We approximate round-robin by starting at the last boosted VCPU.
> >> */
> >>- for (pass = 0; pass < 2 && !yielded; pass++) {
> >>+ for (pass = 0; pass < 2 && !yielded && try; pass++) {
> >> kvm_for_each_vcpu(i, vcpu, kvm) {
> >> if (!pass && i <= last_boosted_vcpu) {
> >> i = last_boosted_vcpu;
> >>@@ -1727,10 +1728,15 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
> >> continue;
> >> if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
> >> continue;
> >>- if (kvm_vcpu_yield_to(vcpu)) {
> >>+
> >>+ yielded = kvm_vcpu_yield_to(vcpu);
> >>+ if (yielded > 0) {
> >> kvm->last_boosted_vcpu = i;
> >>- yielded = 1;
> >> break;
> >>+ } else if (yielded < 0) {
> >>+ try--;
> >>+ if (!try)
> >>+ break;
> >> }
> >> }
> >> }
> >>
>
> Drew, Thanks for reviewing this.
> >
> >The check done in patch 1/2 is done before the double_rq_lock, so it's
> >cheap. Now, this patch is to avoid doing too many get_pid_task calls.I
> >wonder if it would make more sense to change the vcpu state from tracking
> >the pid to tracking the task. If that was done, then I don't believe this
> >patch is necessary.
>
> We would need a logic not to break upon first failure of yield_to.
> (which happens otherwise with patch1 alone). Breaking upon first
> failure out of ple handler resulted in degradation in moderate
> overcommits due to false exits even when we have more than one task in
> other cpu run queues.
>
> But your suggestion triggered an idea to me, what would be the cost of
> iterating over all vcpus despite of yield_to failure?
>
> (Where we breakout of PLE handler only if we have successful yield
> i.e yielded > 0) with something like this:
>
> - for (pass = 0; pass < 2 && !yielded; pass++) {
> + for (pass = 0; pass < 2 && yielded <=0 ; pass++) {
> kvm_for_each_vcpu(i, vcpu, kvm) {
> if (!pass && i <= last_boosted_vcpu) {
> i = last_boosted_vcpu;
> @@ -1727,11 +1727,12 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
> continue;
> if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
> continue;
> - if (kvm_vcpu_yield_to(vcpu)) {
> +
> + yielded = kvm_vcpu_yield_to(vcpu);
> + if (yielded > 0) {
> kvm->last_boosted_vcpu = i;
> - yielded = 1;
> break;
>
OK, I had actually assumed that the first round of testing had been
implemented this way, but then the cost of get_pid_task() forced the
introduction of a try limit.
> Here is the result of the above patch w.r.t to base and current patch
> series.
>
> benchmark improvement w.r.t base improvement w.r.t current patch
> ebizzy_1x 131.22287 -9.76%
> ebizzy_4x -7.97198 -21.1%
>
> dbench_1x 25.67077 -25.55%
> dbench_4x -69.19086 -122.46%
>
>
> Current patches perform better. So this means iterating over vcpus
> has some overhead. Though we have IMO for bigger machine with large
> guests, this is
> significant..
>
> Let me know if this patch sounds good to you..
>
It does was it advertises - reduces the impact of the vcpu-on-spin
loop for undercommit scenarios, so I'm ok with it.
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