Re: [PATCH] sched: introduce configurable delay before entering idle

[Wanpeng Li]

On Wed, 8 May 2019 at 02:57, Marcelo Tosatti <mtosatti@xxxxxxxxxx> 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?

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 <mtosatti@xxxxxxxxxx>
>
> ---
>  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);