On Mon, 2019-05-13 at 07:31 -0400, Konrad Rzeszutek Wilk wrote:
> On May 13, 2019 5:20:37 AM EDT, Wanpeng Li <kernel...@gmail.com> wrote:
> >
> > On Wed, 8 May 2019 at 02:57, Marcelo Tosatti <mtosa...@redhat.com>
> > wrote:
> > >
> > >
> > >
> > > Certain workloads perform poorly on KVM compared to baremetal
> > > due to baremetal's ability to perform mwait on NEED_RESCHED
> > > bit of task flags (therefore skipping the IPI).
> >
> > KVM supports expose mwait to the guest, if it can solve this?
> >
>
>
> There is a bit of problem with that. The host will see 100% CPU utilization
> even if the guest is idle and taking long naps..
>
> Which depending on your dashboard can look like the machine is on fire.
This can also be fixed. I have a patch that kind of expose proper information
about the *real* utilization here if that would be help.
>
> CCing Ankur and Boris
>
> >
> > Regards,
> > Wanpeng Li
> >
> > >
> > >
> > > This patch introduces a configurable busy-wait delay before entering
> > the
> > >
> > > architecture delay routine, allowing wakeup IPIs to be skipped
> > > (if the IPI happens in that window).
> > >
> > > The real-life workload which this patch improves performance
> > > is SAP HANA (by 5-10%) (for which case setting idle_spin to 30
> > > is sufficient).
> > >
> > > This patch improves the attached server.py and client.py example
> > > as follows:
> > >
> > > Host: 31.814230202231556
> > > Guest: 38.17718765199993 (83 %)
> > > Guest, idle_spin=50us: 33.317709898000004 (95 %)
> > > Guest, idle_spin=220us: 32.27826551499999 (98 %)
> > >
> > > Signed-off-by: Marcelo Tosatti <mtosa...@redhat.com>
> > >
> > > ---
> > > kernel/sched/idle.c | 86
> > ++++++++++++++++++++++++++++++++++++++++++
> > >
> > > 1 file changed, 86 insertions(+)
> > >
> > > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> > > index f5516bae0c1b..bca7656a7ea0 100644
> > > --- a/kernel/sched/idle.c
> > > +++ b/kernel/sched/idle.c
> > > @@ -216,6 +216,29 @@ static void cpuidle_idle_call(void)
> > > rcu_idle_exit();
> > > }
> > >
> > > +static unsigned int spin_before_idle_us;
> > >
> > > +static void do_spin_before_idle(void)
> > > +{
> > > + ktime_t now, end_spin;
> > > +
> > > + now = ktime_get();
> > > + end_spin = ktime_add_ns(now, spin_before_idle_us*1000);
> > > +
> > > + rcu_idle_enter();
> > > + local_irq_enable();
> > > + stop_critical_timings();
> > > +
> > > + do {
> > > + cpu_relax();
> > > + now = ktime_get();
> > > + } while (!tif_need_resched() && ktime_before(now, end_spin));
> > > +
> > > + start_critical_timings();
> > > + rcu_idle_exit();
> > > + local_irq_disable();
> > > +}
> > > +
> > > /*
> > > * Generic idle loop implementation
> > > *
> > > @@ -259,6 +282,8 @@ static void do_idle(void)
> > > tick_nohz_idle_restart_tick();
> > > cpu_idle_poll();
> > > } else {
> > > + if (spin_before_idle_us)
> > > + do_spin_before_idle();
> > > cpuidle_idle_call();
> > > }
> > > arch_cpu_idle_exit();
> > > @@ -465,3 +490,64 @@ const struct sched_class idle_sched_class = {
> > > .switched_to = switched_to_idle,
> > > .update_curr = update_curr_idle,
> > > };
> > > +
> > > +
> > > +static ssize_t store_idle_spin(struct kobject *kobj,
> > > + struct kobj_attribute *attr,
> > > + const char *buf, size_t count)
> > > +{
> > > + unsigned int val;
> > > +
> > > + if (kstrtouint(buf, 10, &val) < 0)
> > > + return -EINVAL;
> > > +
> > > + if (val > USEC_PER_SEC)
> > > + return -EINVAL;
> > > +
> > > + spin_before_idle_us = val;
> > > + return count;
> > > +}
> > > +
> > > +static ssize_t show_idle_spin(struct kobject *kobj,
> > > + struct kobj_attribute *attr,
> > > + char *buf)
> > > +{
> > > + ssize_t ret;
> > > +
> > > + ret = sprintf(buf, "%d\n", spin_before_idle_us);
> > > +
> > > + return ret;
> > > +}
> > > +
> > > +static struct kobj_attribute idle_spin_attr =
> > > + __ATTR(idle_spin, 0644, show_idle_spin, store_idle_spin);
> > > +
> > > +static struct attribute *sched_attrs[] = {
> > > + &idle_spin_attr.attr,
> > > + NULL,
> > > +};
> > > +
> > > +static const struct attribute_group sched_attr_group = {
> > > + .attrs = sched_attrs,
> > > +};
> > > +
> > > +static struct kobject *sched_kobj;
> > > +
> > > +static int __init sched_sysfs_init(void)
> > > +{
> > > + int error;
> > > +
> > > + sched_kobj = kobject_create_and_add("sched", kernel_kobj);
> > > + if (!sched_kobj)
> > > + return -ENOMEM;
> > > +
> > > + error = sysfs_create_group(sched_kobj, &sched_attr_group);
> > > + if (error)
> > > + goto err;
> > > + return 0;
> > > +
> > > +err:
> > > + kobject_put(sched_kobj);
> > > + return error;
> > > +}
> > > +postcore_initcall(sched_sysfs_init);
>
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