linux-kernel - Re: [PATCH 16/16] KVM-GST: adjust scheduler cpu power

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Date:	Mon, 24 Jan 2011 19:32:00 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Glauber Costa <glommer@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	aliguori@...ibm.com, Rik van Riel <riel@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Avi Kivity <avi@...hat.com>
Subject: Re: [PATCH 16/16] KVM-GST: adjust scheduler cpu power

On Mon, 2011-01-24 at 13:06 -0500, Glauber Costa wrote:
> This is a first proposal for using steal time information
> to influence the scheduler. There are a lot of optimizations
> and fine grained adjustments to be done, but it is working reasonably
> so far for me (mostly)
> 
> With this patch (and some host pinnings to demonstrate the situation),
> two vcpus with very different steal time (Say 80 % vs 1 %) will not get
> an even distribution of processes. This is a situation that can naturally
> arise, specially in overcommited scenarios. Previosly, the guest scheduler
> would wrongly think that all cpus have the same ability to run processes,
> lowering the overall throughput.
> 
> Signed-off-by: Glauber Costa <glommer@...hat.com>
> CC: Rik van Riel <riel@...hat.com>
> CC: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
> CC: Peter Zijlstra <peterz@...radead.org>
> CC: Avi Kivity <avi@...hat.com>
> ---
>  include/linux/sched.h |    1 +
>  kernel/sched.c        |    4 ++++
>  kernel/sched_fair.c   |   10 ++++++++++
>  3 files changed, 15 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index d747f94..beab72d 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -302,6 +302,7 @@ long io_schedule_timeout(long timeout);
>  extern void cpu_init (void);
>  extern void trap_init(void);
>  extern void update_process_times(int user);
> +extern cputime_t (*hypervisor_steal_time)(void);
>  extern void scheduler_tick(void);
>  
>  extern void sched_show_task(struct task_struct *p);
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 3b3e88d..c460f0d 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -501,6 +501,8 @@ struct rq {
>  	struct sched_domain *sd;
>  
>  	unsigned long cpu_power;
> +	unsigned long real_ticks;
> +	unsigned long total_ticks;
>  
>  	unsigned char idle_at_tick;
>  	/* For active balancing */
> @@ -3533,10 +3535,12 @@ static int touch_steal_time(int is_idle)
>  	if (is_idle)
>  		return 0;
>  
> +	rq->total_ticks++;
>  	if (steal) {
>  		account_steal_time(steal);
>  		return 1;
>  	}
> +	rq->real_ticks++;
>  	return 0;
>  }

yuck!! is ticks really the best you can do?

I thought kvm had a ns resolution steal-time clock?

> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index c62ebae..1364c28 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -2509,6 +2509,16 @@ static void update_cpu_power(struct sched_domain *sd, int cpu)
>  		power >>= SCHED_LOAD_SHIFT;
>  	}
>  
> +	WARN_ON(cpu_rq(cpu)->real_ticks > cpu_rq(cpu)->total_ticks);
> +
> +	if (cpu_rq(cpu)->total_ticks) {
> +		power *= cpu_rq(cpu)->real_ticks;
> +		power /= cpu_rq(cpu)->total_ticks;
> +	}
> +
> +	cpu_rq(cpu)->total_ticks = 0;
> +	cpu_rq(cpu)->real_ticks = 0;
> +
>  	sdg->cpu_power_orig = power;
>  
>  	if (sched_feat(ARCH_POWER))

I would really much rather see you change update_rq_clock_task() and
subtract your ns resolution steal time from our wall-time,
update_rq_clock_task() already updates the cpu_power relative to the
remaining time available.
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