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Date: Wed, 28 Nov 2012 20:00:54 -0600
From: Andrew Theurer <habanero@...ux.vnet.ibm.com>
To: Chegu Vinod <chegu_vinod@...com>
Cc: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
Andrew Jones <drjones@...hat.com>,
Marcelo Tosatti <mtosatti@...hat.com>,
Gleb Natapov <gleb@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
Avi Kivity <avi@...hat.com>, Rik van Riel <riel@...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>,
LKML <linux-kernel@...r.kernel.org>,
Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>
Subject: Re: [PATCH V3 RFC 1/2] sched: Bail out of yield_to when source and
target runqueue has one task
On Wed, 2012-11-28 at 14:26 -0800, Chegu Vinod wrote:
> On 11/27/2012 6:23 AM, Chegu Vinod wrote:
>
> > On 11/27/2012 2:30 AM, Raghavendra K T wrote:
> > > On 11/26/2012 07:05 PM, Andrew Jones wrote:
> > > > On Mon, Nov 26, 2012 at 05:37:54PM +0530, Raghavendra K T
> > > > wrote:
> > > > > From: Peter Zijlstra <peterz@...radead.org>
> > > > >
> > > > > In case of undercomitted scenarios, especially in large
> > > > > guests
> > > > > yield_to overhead is significantly high. when run queue length
> > > > > of
> > > > > source and target is one, take an opportunity to bail out and
> > > > > return
> > > > > -ESRCH. This return condition can be further exploited to
> > > > > quickly come
> > > > > out of PLE handler.
> > > > >
> > > > > (History: Raghavendra initially worked on break out of kvm ple
> > > > > handler upon
> > > > > seeing source runqueue length = 1, but it had to export rq
> > > > > length).
> > > > > Peter came up with the elegant idea of return -ESRCH in
> > > > > scheduler core.
> > > > >
> > > > > Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> > > > > Raghavendra, Checking the rq length of target vcpu condition
> > > > > added.(thanks Avi)
> > > > > Reviewed-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> > > > > Signed-off-by: Raghavendra K T
> > > > > <raghavendra.kt@...ux.vnet.ibm.com>
> > > > > ---
> > > > >
> > > > > kernel/sched/core.c | 25 +++++++++++++++++++------
> > > > > 1 file changed, 19 insertions(+), 6 deletions(-)
> > > > >
> > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > > > index 2d8927f..fc219a5 100644
> > > > > --- a/kernel/sched/core.c
> > > > > +++ b/kernel/sched/core.c
> > > > > @@ -4289,7 +4289,10 @@ EXPORT_SYMBOL(yield);
> > > > > * It's the caller's job to ensure that the target task
> > > > > struct
> > > > > * can't go away on us before we can do any checks.
> > > > > *
> > > > > - * Returns true if we indeed boosted the target task.
> > > > > + * Returns:
> > > > > + * true (>0) if we indeed boosted the target task.
> > > > > + * false (0) if we failed to boost the target.
> > > > > + * -ESRCH if there's no task to yield to.
> > > > > */
> > > > > bool __sched yield_to(struct task_struct *p, bool preempt)
> > > > > {
> > > > > @@ -4303,6 +4306,15 @@ bool __sched yield_to(struct
> > > > > task_struct *p, bool preempt)
> > > > >
> > > > > again:
> > > > > p_rq = task_rq(p);
> > > > > + /*
> > > > > + * If we're the only runnable task on the rq and target
> > > > > rq also
> > > > > + * has only one task, there's absolutely no point in
> > > > > yielding.
> > > > > + */
> > > > > + if (rq->nr_running == 1 && p_rq->nr_running == 1) {
> > > > > + yielded = -ESRCH;
> > > > > + goto out_irq;
> > > > > + }
> > > > > +
> > > > > double_rq_lock(rq, p_rq);
> > > > > while (task_rq(p) != p_rq) {
> > > > > double_rq_unlock(rq, p_rq);
> > > > > @@ -4310,13 +4322,13 @@ again:
> > > > > }
> > > > >
> > > > > if (!curr->sched_class->yield_to_task)
> > > > > - goto out;
> > > > > + goto out_unlock;
> > > > >
> > > > > if (curr->sched_class != p->sched_class)
> > > > > - goto out;
> > > > > + goto out_unlock;
> > > > >
> > > > > if (task_running(p_rq, p) || p->state)
> > > > > - goto out;
> > > > > + goto out_unlock;
> > > > >
> > > > > yielded = curr->sched_class->yield_to_task(rq, p,
> > > > > preempt);
> > > > > if (yielded) {
> > > > > @@ -4329,11 +4341,12 @@ again:
> > > > > resched_task(p_rq->curr);
> > > > > }
> > > > >
> > > > > -out:
> > > > > +out_unlock:
> > > > > double_rq_unlock(rq, p_rq);
> > > > > +out_irq:
> > > > > local_irq_restore(flags);
> > > > >
> > > > > - if (yielded)
> > > > > + if (yielded > 0)
> > > > > schedule();
> > > > >
> > > > > return yielded;
> > > > >
> > > >
> > > > Acked-by: Andrew Jones <drjones@...hat.com>
> > > >
> > >
> > > Thank you Drew.
> > >
> > > Marcelo Gleb.. Please let me know if you have comments / concerns
> > > on the patches..
> > >
> > > Andrew, Vinod, IMO, the patch set looks good for undercommit
> > > scenarios
> > > especially for large guests where we do have overhead of vcpu
> > > iteration
> > > of ple handler..
> > >
> > > .
> > >
> > Thanks Raghu. Will try to get this latest patch set evaluated and
> > get back to you.
> >
> >
> Hi Raghu,
>
> Here is some preliminary data with your latest set of PLE patches (&
> also with Andrew's throttled yield_to() change).
>
> Ran a single guest on a 80 core Westmere platform. [Note: Host and
> Guest had the latest kernel from kvm.git and also using the latest
> qemu from qemu.git as of yesterday morning].
>
> The guest was running a AIM7 high_systime workload. (Note:
> high_systime is a kernel intensive micro-benchmark but in this case it
> was run just as a workload in the guest to trigger spinlock etc.
> contention in the guest OS and hence PLE (i.e. this is not a real
> benchmark run). 'have run this workload with a constant # (i.e. 2000)
> users with 100 jobs per user. The numbers below represent the # of
> jobs per minute (JPM) - higher the better) .
>
> 40VCPU 60VCPU 80VCPU
>
> a) 3.7.0-rc6+ w/ ple_gap=0 ~102K ~88K ~81K
>
> b) 3.7.0-rc6+ ~53K ~25K ~18-20K
> c) 3.7.0-rc6+ w/ PLE patches ~100K ~81K ~48K-69K <- lot of variation from run to run.
>
> d) 3.7.0-rc6+ w/ throttled ~101K ~87K ~78K
> yield_to() change
>
FYI here's the latest throttled yield_to() patch (the one Vinod tested).
Signed-off-by: Andrew Theurer <habanero@...ux.vnet.ibm.com>
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index ecc5543..61d12ea 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -192,6 +192,7 @@ struct kvm_vcpu {
int mode;
unsigned long requests;
unsigned long guest_debug;
+ unsigned long last_yield_to;
struct mutex mutex;
struct kvm_run *run;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index be70035..987a339 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -49,6 +49,7 @@
#include <linux/slab.h>
#include <linux/sort.h>
#include <linux/bsearch.h>
+#include <linux/jiffies.h>
#include <asm/processor.h>
#include <asm/io.h>
@@ -222,6 +223,7 @@ int kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
vcpu->kvm = kvm;
vcpu->vcpu_id = id;
vcpu->pid = NULL;
+ vcpu->last_yield_to = 0;
init_waitqueue_head(&vcpu->wq);
kvm_async_pf_vcpu_init(vcpu);
@@ -1708,29 +1710,38 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
kvm_vcpu_set_in_spin_loop(me, true);
/*
+ * A yield_to() can be quite expensive, so we try to limit
+ * its use to just 1 per jiffie.
+ */
+ if (me->last_yield_to == jiffies)
+ yield();
+ else {
+ /*
* We boost the priority of a VCPU that is runnable but not
* currently running, because it got preempted by something
* else and called schedule in __vcpu_run. Hopefully that
* 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++) {
- kvm_for_each_vcpu(i, vcpu, kvm) {
- if (!pass && i <= last_boosted_vcpu) {
- i = last_boosted_vcpu;
- continue;
- } else if (pass && i > last_boosted_vcpu)
- break;
- if (vcpu == me)
- continue;
- if (waitqueue_active(&vcpu->wq))
- continue;
- if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
- continue;
- if (kvm_vcpu_yield_to(vcpu)) {
- kvm->last_boosted_vcpu = i;
- yielded = 1;
- break;
+ for (pass = 0; pass < 2 && !yielded; pass++) {
+ kvm_for_each_vcpu(i, vcpu, kvm) {
+ if (!pass && i <= last_boosted_vcpu) {
+ i = last_boosted_vcpu;
+ continue;
+ } else if (pass && i > last_boosted_vcpu)
+ break;
+ if (vcpu == me)
+ continue;
+ if (waitqueue_active(&vcpu->wq))
+ continue;
+ if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
+ continue;
+ if (kvm_vcpu_yield_to(vcpu)) {
+ kvm->last_boosted_vcpu = i;
+ me->last_yield_to = jiffies;
+ yielded = 1;
+ break;
+ }
}
}
}
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