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

Gregory Haskins
2008-08-21T11:39:18Z

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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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