Re: [PATCH] sched: properly account IRQ and RT load in SCHED_OTHER load balancing

[Gregory Haskins <ghaskins@novell.com>]

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Peter Zijlstra wrote:
> Subject: sched: properly account IRQ and RT load in SCHED_OTHER load balancing
> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Thu Aug 14 09:31:20 CEST 2008
>
> We used to account for RT tasks in SCHED_OTHER load-balancing by giving
> them some phantom weight.
>
> This is incorrect because there is no saying how much time a RT task will
> actually consume. Also, it doesn't take IRQ time into account.
>
> This patch tries to solve this issue by accounting the time spend on both
> Real-Time tasks and IRQ handling, and using that to proportionally inflate
> the SCHED_OTHER load.
>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
>   

I haven't had a chance to review the code thoroughly yet, but I had been 
working on a similar fix and know that this is sorely needed.  So...

Acked-by: Gregory Haskins <ghaskins@novell.com>

> ---
>  include/linux/hardirq.h |   10 +++
>  include/linux/sched.h   |    1 
>  kernel/sched.c          |  126 +++++++++++++++++++++++++++++++++++++++++++-----
>  kernel/sched_debug.c    |    2 
>  kernel/sched_rt.c       |    8 +++
>  kernel/softirq.c        |    1 
>  kernel/sysctl.c         |    8 +++
>  7 files changed, 145 insertions(+), 11 deletions(-)
>
> Index: linux-2.6/include/linux/hardirq.h
> ===================================================================
> --- linux-2.6.orig/include/linux/hardirq.h
> +++ linux-2.6/include/linux/hardirq.h
> @@ -127,6 +127,14 @@ static inline void account_system_vtime(
>  }
>  #endif
>  
> +#ifdef CONFIG_SMP
> +extern void sched_irq_enter(void);
> +extern void sched_irq_exit(void);
> +#else
> +# define sched_irq_enter() do { } while (0)
> +# define sched_irq_exit() do { } while (0)
> +#endif
> +
>  #if defined(CONFIG_PREEMPT_RCU) && defined(CONFIG_NO_HZ)
>  extern void rcu_irq_enter(void);
>  extern void rcu_irq_exit(void);
> @@ -143,6 +151,7 @@ extern void rcu_irq_exit(void);
>   */
>  #define __irq_enter()					\
>  	do {						\
> +		sched_irq_enter();			\
>  		rcu_irq_enter();			\
>  		account_system_vtime(current);		\
>  		add_preempt_count(HARDIRQ_OFFSET);	\
> @@ -163,6 +172,7 @@ extern void irq_enter(void);
>  		account_system_vtime(current);		\
>  		sub_preempt_count(HARDIRQ_OFFSET);	\
>  		rcu_irq_exit();				\
> +		sched_irq_exit();			\
>  	} while (0)
>  
>  /*
> Index: linux-2.6/include/linux/sched.h
> ===================================================================
> --- linux-2.6.orig/include/linux/sched.h
> +++ linux-2.6/include/linux/sched.h
> @@ -1614,6 +1614,7 @@ extern unsigned int sysctl_sched_feature
>  extern unsigned int sysctl_sched_migration_cost;
>  extern unsigned int sysctl_sched_nr_migrate;
>  extern unsigned int sysctl_sched_shares_ratelimit;
> +extern unsigned int sysctl_sched_time_avg;
>  
>  int sched_nr_latency_handler(struct ctl_table *table, int write,
>  		struct file *file, void __user *buffer, size_t *length,
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -571,6 +571,12 @@ struct rq {
>  
>  	struct task_struct *migration_thread;
>  	struct list_head migration_queue;
> +
> +	u64 irq_stamp;
> +	unsigned long irq_time;
> +	unsigned long rt_time;
> +	u64 age_stamp;
> +
>  #endif
>  
>  #ifdef CONFIG_SCHED_HRTICK
> @@ -816,14 +822,21 @@ const_debug unsigned int sysctl_sched_nr
>  unsigned int sysctl_sched_shares_ratelimit = 250000;
>  
>  /*
> - * period over which we measure -rt task cpu usage in us.
> + * period over which we average the IRQ and RT cpu consumption, measured in
> + * jiffies.
>   * default: 1s
>   */
> -unsigned int sysctl_sched_rt_period = 1000000;
> +const_debug unsigned int sysctl_sched_time_avg = MSEC_PER_SEC;
>  
>  static __read_mostly int scheduler_running;
>  
>  /*
> + * period over which we measure -rt task cpu usage in us.
> + * default: 1s
> + */
> +unsigned int sysctl_sched_rt_period = 1000000;
> +
> +/*
>   * part of the period that we allow rt tasks to run in us.
>   * default: 9.5s
>   */
> @@ -1143,6 +1156,82 @@ static inline void init_hrtick(void)
>  }
>  #endif
>  
> +#ifdef CONFIG_SMP
> +/*
> + * Measure IRQ time, we start when we first enter IRQ state
> + * and stop when we last leave IRQ state (nested IRQs).
> + */
> +void sched_irq_enter(void)
> +{
> +	if (!in_irq()) {
> +		struct rq *rq = this_rq();
> +
> +		update_rq_clock(rq);
> +		rq->irq_stamp = rq->clock;
> +	}
> +}
> +
> +void sched_irq_exit(void)
> +{
> +	if (!in_irq()) {
> +		struct rq *rq = this_rq();
> +
> +		update_rq_clock(rq);
> +		rq->irq_time += rq->clock - rq->irq_stamp;
> +	}
> +}
> +
> +static inline u64 sched_avg_period(void)
> +{
> +	return (u64)sysctl_sched_time_avg * (NSEC_PER_MSEC / 2);
> +}
> +
> +/*
> + * Every period/2 we half the accumulated time. See lib/proportions.c
> + */
> +static void sched_age_time(struct rq *rq)
> +{
> +	if (rq->clock - rq->age_stamp >= sched_avg_period()) {
> +		rq->irq_time /= 2;
> +		rq->rt_time /= 2;
> +		rq->age_stamp = rq->clock;
> +	}
> +}
> +
> +/*
> + * Scale the SCHED_OTHER load on this rq up to compensate for the pressure
> + * of IRQ and RT usage of this CPU.
> + *
> + * See lib/proportions.c
> + */
> +static unsigned long sched_scale_load(struct rq *rq, u64 load)
> +{
> +	u64 total = sched_avg_period() + (rq->clock - rq->age_stamp);
> +	u64 available = total - rq->irq_time - rq->rt_time;
> +
> +	/*
> +	 * Shift back to roughly us scale, so that the divisor fits in u32.
> +	 */
> +	total >>= 10;
> +	available >>= 10;
> +
> +	if (unlikely((s64)available <= 0))
> +		available = 1;
> +
> +	load *= total;
> +	load = div_u64(load, available);
> +
> +	/*
> +	 * Clip the maximal load value to something plenty high.
> +	 */
> +	return min_t(unsigned long, load, 1UL << 22);
> +}
> +#else
> +static inline void sched_age_time(struct rq *rq)
> +{
> +}
> +#endif
> +
>  /*
>   * resched_task - mark a task 'to be rescheduled now'.
>   *
> @@ -1635,8 +1724,12 @@ static void dec_nr_running(struct rq *rq
>  static void set_load_weight(struct task_struct *p)
>  {
>  	if (task_has_rt_policy(p)) {
> -		p->se.load.weight = prio_to_weight[0] * 2;
> -		p->se.load.inv_weight = prio_to_wmult[0] >> 1;
> +		/*
> +		 * Real-time tasks do not contribute to SCHED_OTHER load
> +		 * this is compensated by sched_scale_load() usage.
> +		 */
> +		p->se.load.weight = 0;
> +		p->se.load.inv_weight = 0;
>  		return;
>  	}
>  
> @@ -2028,10 +2121,10 @@ static unsigned long source_load(int cpu
>  	struct rq *rq = cpu_rq(cpu);
>  	unsigned long total = weighted_cpuload(cpu);
>  
> -	if (type == 0 || !sched_feat(LB_BIAS))
> -		return total;
> +	if (type && sched_feat(LB_BIAS))
> +		total = min(rq->cpu_load[type-1], total);
>  
> -	return min(rq->cpu_load[type-1], total);
> +	return sched_scale_load(rq, total);
>  }
>  
>  /*
> @@ -2043,10 +2136,10 @@ static unsigned long target_load(int cpu
>  	struct rq *rq = cpu_rq(cpu);
>  	unsigned long total = weighted_cpuload(cpu);
>  
> -	if (type == 0 || !sched_feat(LB_BIAS))
> -		return total;
> +	if (type && sched_feat(LB_BIAS))
> +		total = max(rq->cpu_load[type-1], total);
>  
> -	return max(rq->cpu_load[type-1], total);
> +	return sched_scale_load(rq, total);
>  }
>  
>  /*
> @@ -2956,10 +3049,20 @@ balance_tasks(struct rq *this_rq, int th
>  	int loops = 0, pulled = 0, pinned = 0;
>  	struct task_struct *p;
>  	long rem_load_move = max_load_move;
> +	unsigned long busy_weight, this_weight, weight_scale;
>  
>  	if (max_load_move == 0)
>  		goto out;
>  
> +	/*
> +	 * Compute a weight scale to properly account for the varying
> +	 * load inflation between these CPUs.
> +	 */
> +	busy_weight = sched_scale_load(busiest, NICE_0_LOAD);
> +	this_weight = sched_scale_load(this_rq, NICE_0_LOAD);
> +
> +	weight_scale = div_u64((u64)this_weight * NICE_0_LOAD, busy_weight);
> +
>  	pinned = 1;
>  
>  	/*
> @@ -2978,7 +3081,7 @@ next:
>  
>  	pull_task(busiest, p, this_rq, this_cpu);
>  	pulled++;
> -	rem_load_move -= p->se.load.weight;
> +	rem_load_move -= (weight_scale * p->se.load.weight) >> NICE_0_SHIFT;
>  
>  	/*
>  	 * We only want to steal up to the prescribed amount of weighted load.
> @@ -4211,6 +4314,7 @@ void scheduler_tick(void)
>  	spin_lock(&rq->lock);
>  	update_rq_clock(rq);
>  	update_cpu_load(rq);
> +	sched_age_time(rq);
>  	curr->sched_class->task_tick(rq, curr, 0);
>  	spin_unlock(&rq->lock);
>  
> Index: linux-2.6/kernel/sched_rt.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched_rt.c
> +++ linux-2.6/kernel/sched_rt.c
> @@ -478,6 +478,14 @@ static void update_curr_rt(struct rq *rq
>  	if (unlikely((s64)delta_exec < 0))
>  		delta_exec = 0;
>  
> +#ifdef CONFIG_SMP
> +	/*
> +	 * Account the time spend running RT tasks on this rq. Used to inflate
> +	 * this rq's load values.
> +	 */
> +	rq->rt_time += delta_exec;
> +#endif
> +
>  	schedstat_set(curr->se.exec_max, max(curr->se.exec_max, delta_exec));
>  
>  	curr->se.sum_exec_runtime += delta_exec;
> Index: linux-2.6/kernel/softirq.c
> ===================================================================
> --- linux-2.6.orig/kernel/softirq.c
> +++ linux-2.6/kernel/softirq.c
> @@ -280,6 +280,7 @@ void irq_exit(void)
>  	account_system_vtime(current);
>  	trace_hardirq_exit();
>  	sub_preempt_count(IRQ_EXIT_OFFSET);
> +	sched_irq_exit();
>  	if (!in_interrupt() && local_softirq_pending())
>  		invoke_softirq();
>  
> Index: linux-2.6/kernel/sysctl.c
> ===================================================================
> --- linux-2.6.orig/kernel/sysctl.c
> +++ linux-2.6/kernel/sysctl.c
> @@ -309,6 +309,14 @@ static struct ctl_table kern_table[] = {
>  		.mode		= 0644,
>  		.proc_handler	= &proc_dointvec,
>  	},
> +	{
> +		.ctl_name	= CTL_UNNUMBERED,
> +		.procname	= "sched_time_avg_ms",
> +		.data		= &sysctl_sched_time_avg,
> +		.maxlen		= sizeof(unsigned int),
> +		.mode		= 0644,
> +		.proc_handler	= &proc_dointvec,
> +	},
>  #endif
>  	{
>  		.ctl_name	= CTL_UNNUMBERED,
> Index: linux-2.6/kernel/sched_debug.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched_debug.c
> +++ linux-2.6/kernel/sched_debug.c
> @@ -245,6 +245,8 @@ static void print_cpu(struct seq_file *m
>  	P(nr_running);
>  	SEQ_printf(m, "  .%-30s: %lu\n", "load",
>  		   rq->load.weight);
> +	SEQ_printf(m, "  .%-30s: %ld\n", "scaled_load",
> +			sched_scale_load(rq, rq->load.weight));
>  	P(nr_switches);
>  	P(nr_load_updates);
>  	P(nr_uninterruptible);
>
>
>   



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