Re: [RFC PATCH v1 1/4] sched: introduce primitives to account for CFS bandwidth tracking

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

On Thu, Feb 25, 2010 at 02:30:44AM -0800, Paul Turner wrote:
> On Thu, Feb 25, 2010 at 12:14 AM, Bharata B Rao
> <bharata@linux.vnet.ibm.com> wrote:
> > On Fri, Feb 12, 2010 at 06:54:57PM -0800, Paul wrote:
> >> --- a/kernel/sched.c
> >> +++ b/kernel/sched.c
> >> @@ -190,10 +190,28 @@ static inline int rt_bandwidth_enabled(void)
> >>       return sysctl_sched_rt_runtime >= 0;
> >>  }
> >>
> >> -static void start_rt_bandwidth(struct rt_bandwidth *rt_b)
> >> +static void start_bandwidth_timer(struct hrtimer *period_timer, ktime_t period)
> >>  {
> >> -     ktime_t now;
> >> +     unsigned long delta;
> >> +     ktime_t soft, hard, now;
> >> +
> >> +     for (;;) {
> >> +             if (hrtimer_active(period_timer))
> >> +                     break;
> >> +
> >> +             now = hrtimer_cb_get_time(period_timer);
> >> +             hrtimer_forward(period_timer, now, period);
> >> +
> >> +             soft = hrtimer_get_softexpires(period_timer);
> >> +             hard = hrtimer_get_expires(period_timer);
> >> +             delta = ktime_to_ns(ktime_sub(hard, soft));
> >> +             __hrtimer_start_range_ns(period_timer, soft, delta,
> >> +                                      HRTIMER_MODE_ABS_PINNED, 0);
> >> +     }
> >> +}
> >>
> >> +static void start_rt_bandwidth(struct rt_bandwidth *rt_b)
> >> +{
> >>       if (!rt_bandwidth_enabled() || rt_b->rt_runtime == RUNTIME_INF)
> >>               return;
> >>
> >> @@ -201,22 +219,7 @@ static void start_rt_bandwidth(struct rt_bandwidth *rt_b)
> >>               return;
> >>
> >>       raw_spin_lock(&rt_b->rt_runtime_lock);
> >> -     for (;;) {
> >> -             unsigned long delta;
> >> -             ktime_t soft, hard;
> >> -
> >> -             if (hrtimer_active(&rt_b->rt_period_timer))
> >> -                     break;
> >> -
> >> -             now = hrtimer_cb_get_time(&rt_b->rt_period_timer);
> >> -             hrtimer_forward(&rt_b->rt_period_timer, now, rt_b->rt_period);
> >> -
> >> -             soft = hrtimer_get_softexpires(&rt_b->rt_period_timer);
> >> -             hard = hrtimer_get_expires(&rt_b->rt_period_timer);
> >> -             delta = ktime_to_ns(ktime_sub(hard, soft));
> >> -             __hrtimer_start_range_ns(&rt_b->rt_period_timer, soft, delta,
> >> -                             HRTIMER_MODE_ABS_PINNED, 0);
> >> -     }
> >> +     start_bandwidth_timer(&rt_b->rt_period_timer, rt_b->rt_period);
> >>       raw_spin_unlock(&rt_b->rt_runtime_lock);
> >>  }
> >>
> >> @@ -241,6 +244,15 @@ struct cfs_rq;
> >>
> >>  static LIST_HEAD(task_groups);
> >>
> >> +#ifdef CONFIG_CFS_BANDWIDTH
> >> +struct cfs_bandwidth {
> >> +     raw_spinlock_t          lock;
> >> +     ktime_t                 period;
> >> +     u64                     runtime, quota;
> >> +     struct hrtimer          period_timer;
> >> +};
> >> +#endif
> >> +
> >>  /* task group related information */
> >>  struct task_group {
> >>  #ifdef CONFIG_CGROUP_SCHED
> >> @@ -272,6 +284,10 @@ struct task_group {
> >>       struct task_group *parent;
> >>       struct list_head siblings;
> >>       struct list_head children;
> >> +
> >> +#ifdef CONFIG_CFS_BANDWIDTH
> >> +     struct cfs_bandwidth cfs_bandwidth;
> >> +#endif
> >>  };
> >>
> >>  #ifdef CONFIG_USER_SCHED
> >> @@ -445,9 +461,76 @@ struct cfs_rq {
> >>        */
> >>       unsigned long rq_weight;
> >>  #endif
> >> +#ifdef CONFIG_CFS_BANDWIDTH
> >> +     u64 quota_assigned, quota_used;
> >> +#endif
> >>  #endif
> >>  };
> >>
> >> +#ifdef CONFIG_CFS_BANDWIDTH
> >> +static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun);
> >> +
> >> +static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
> >> +{
> >> +     struct cfs_bandwidth *cfs_b =
> >> +             container_of(timer, struct cfs_bandwidth, period_timer);
> >> +     ktime_t now;
> >> +     int overrun;
> >> +     int idle = 0;
> >> +
> >> +     for (;;) {
> >> +             now = hrtimer_cb_get_time(timer);
> >> +             overrun = hrtimer_forward(timer, now, cfs_b->period);
> >> +
> >> +             if (!overrun)
> >> +                     break;
> >> +
> >> +             idle = do_sched_cfs_period_timer(cfs_b, overrun);
> >> +     }
> >> +
> >> +     return idle ? HRTIMER_NORESTART : HRTIMER_RESTART;
> >> +}
> >> +
> >> +static
> >> +void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b, u64 quota, u64 period)
> >> +{
> >> +     raw_spin_lock_init(&cfs_b->lock);
> >> +     cfs_b->quota = cfs_b->runtime = quota;
> >> +     cfs_b->period = ns_to_ktime(period);
> >> +
> >> +     hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> >> +     cfs_b->period_timer.function = sched_cfs_period_timer;
> >> +}
> >> +
> >> +static
> >> +void init_cfs_rq_quota(struct cfs_rq *cfs_rq)
> >> +{
> >> +     cfs_rq->quota_used = 0;
> >> +     if (cfs_rq->tg->cfs_bandwidth.quota == RUNTIME_INF)
> >> +             cfs_rq->quota_assigned = RUNTIME_INF;
> >> +     else
> >> +             cfs_rq->quota_assigned = 0;
> >> +}
> >> +
> >> +static void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
> >> +{
> >> +     if (cfs_b->quota == RUNTIME_INF)
> >> +             return;
> >> +
> >> +     if (hrtimer_active(&cfs_b->period_timer))
> >> +             return;
> >> +
> >> +     raw_spin_lock(&cfs_b->lock);
> >> +     start_bandwidth_timer(&cfs_b->period_timer, cfs_b->period);
> >> +     raw_spin_unlock(&cfs_b->lock);
> >> +}
> >> +
> >> +static void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
> >> +{
> >> +     hrtimer_cancel(&cfs_b->period_timer);
> >> +}
> >> +#endif
> >
> > May be you could define some of this functions for !CONFIG_CFS_BANDWIDTH case
> > and avoid them calling under #ifdef ? I was given this comment during my
> > initial iterations.
> >
> 
> Was it for init or run-time functions?  We try to maintain the empty
> def style for most of our run-time functions (e.g. cfs_throttled); for
> init it seems more descriptive (and in keeping with the rest of sched
> init) to ifdef specific initialization.
> 
> Regardless,  I will definitely give this a pass-over to see what I can clean up.

Even for init functions.

> 
> >> +
> >>  /* Real-Time classes' related field in a runqueue: */
> >>  struct rt_rq {
> >>       struct rt_prio_array active;
> >> @@ -1834,6 +1917,14 @@ static inline void __set_task_cpu(struct task_struct *p, unsigned int cpu)
> >>  #endif
> >>  }
> >>
> >> +#ifdef CONFIG_CFS_BANDWIDTH
> >> +/*
> >> + * default period for cfs group bandwidth.
> >> + * default: 0.5s
> >> + */
> >> +static u64 sched_cfs_bandwidth_period = 500000000ULL;
> >> +#endif
> >> +
> >>  #include "sched_stats.h"
> >>  #include "sched_idletask.c"
> >>  #include "sched_fair.c"
> >> @@ -9422,6 +9513,9 @@ static void init_tg_cfs_entry(struct task_group *tg, struct cfs_rq *cfs_rq,
> >>       tg->cfs_rq[cpu] = cfs_rq;
> >>       init_cfs_rq(cfs_rq, rq);
> >>       cfs_rq->tg = tg;
> >> +#ifdef CONFIG_CFS_BANDWIDTH
> >> +     init_cfs_rq_quota(cfs_rq);
> >> +#endif
> >>       if (add)
> >>               list_add(&cfs_rq->leaf_cfs_rq_list, &rq->leaf_cfs_rq_list);
> >>
> >> @@ -9594,6 +9688,10 @@ void __init sched_init(void)
> >>                * We achieve this by letting init_task_group's tasks sit
> >>                * directly in rq->cfs (i.e init_task_group->se[] = NULL).
> >>                */
> >> +#ifdef CONFIG_CFS_BANDWIDTH
> >> +             init_cfs_bandwidth(&init_task_group.cfs_bandwidth,
> >> +                             RUNTIME_INF, sched_cfs_bandwidth_period);
> >> +#endif
> >>               init_tg_cfs_entry(&init_task_group, &rq->cfs, NULL, i, 1, NULL);
> >>  #elif defined CONFIG_USER_SCHED
> >>               root_task_group.shares = NICE_0_LOAD;
> >> @@ -9851,6 +9949,10 @@ static void free_fair_sched_group(struct task_group *tg)
> >>  {
> >>       int i;
> >>
> >> +#ifdef CONFIG_CFS_BANDWIDTH
> >> +     destroy_cfs_bandwidth(&tg->cfs_bandwidth);
> >> +#endif
> >> +
> >>       for_each_possible_cpu(i) {
> >>               if (tg->cfs_rq)
> >>                       kfree(tg->cfs_rq[i]);
> >> @@ -9878,7 +9980,10 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
> >>               goto err;
> >>
> >>       tg->shares = NICE_0_LOAD;
> >> -
> >> +#ifdef CONFIG_CFS_BANDWIDTH
> >> +     init_cfs_bandwidth(&tg->cfs_bandwidth, RUNTIME_INF,
> >> +                     sched_cfs_bandwidth_period);
> >> +#endif
> >>       for_each_possible_cpu(i) {
> >>               rq = cpu_rq(i);
> >>
> >> @@ -10333,7 +10438,7 @@ static int __rt_schedulable(struct task_group *tg, u64 period, u64 runtime)
> >>       return walk_tg_tree(tg_schedulable, tg_nop, &data);
> >>  }
> >>
> >> -static int tg_set_bandwidth(struct task_group *tg,
> >> +static int tg_set_rt_bandwidth(struct task_group *tg,
> >>               u64 rt_period, u64 rt_runtime)
> >>  {
> >>       int i, err = 0;
> >> @@ -10372,7 +10477,7 @@ int sched_group_set_rt_runtime(struct task_group *tg, long rt_runtime_us)
> >>       if (rt_runtime_us < 0)
> >>               rt_runtime = RUNTIME_INF;
> >>
> >> -     return tg_set_bandwidth(tg, rt_period, rt_runtime);
> >> +     return tg_set_rt_bandwidth(tg, rt_period, rt_runtime);
> >>  }
> >>
> >>  long sched_group_rt_runtime(struct task_group *tg)
> >> @@ -10397,7 +10502,7 @@ int sched_group_set_rt_period(struct task_group *tg, long rt_period_us)
> >>       if (rt_period == 0)
> >>               return -EINVAL;
> >>
> >> -     return tg_set_bandwidth(tg, rt_period, rt_runtime);
> >> +     return tg_set_rt_bandwidth(tg, rt_period, rt_runtime);
> >>  }
> >>
> >>  long sched_group_rt_period(struct task_group *tg)
> >> @@ -10604,6 +10709,120 @@ static u64 cpu_shares_read_u64(struct cgroup *cgrp, struct cftype *cft)
> >>
> >>       return (u64) tg->shares;
> >>  }
> >> +
> >> +#ifdef CONFIG_CFS_BANDWIDTH
> >> +static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
> >> +{
> >> +     int i;
> >> +     static DEFINE_MUTEX(mutex);
> >> +
> >> +     if (tg == &init_task_group)
> >> +             return -EINVAL;
> >> +
> >> +     if (!period)
> >> +             return -EINVAL;
> >> +
> >> +     mutex_lock(&mutex);
> >
> > What is this mutex for ? So you essentially serializing the bandwidth
> > setting of all groups ? While that iself isn't an issue, just wondering if
> > cfs_bandwidth.lock isn't suffient ?
> >
> 
> The idea isn't to synchronize quota updates for all groups, but to
> synchronize it within a single group.  Consider the case of 2 parallel
> writes, one setting infinite bandwidth the other setting finite.
> Depending on rq->lock contention it's possible for both values to
> propagate to some of the cpus.
> 
> You sit on the bandwidth lock because then there is inversion with
> update_curr, e.g.
> 
> cpu1 -> rq->lock held, update_curr -> request bandwidth -> acquire
> cfs_bandwidth.lock
> cpu2 -> tg_set_cfs_bandwidth, hold cfs_bandwidth -> try to acquire cpu1 rq->lock
> 
> This mutex could be per-cgroup but users shouldn't be updating fast
> enough to the point where they require it, it also reduces rq->lock
> slamming when users update several cgroups in parallel.

I get it. cfs_bandwidth.lock was suffienct for me since I have per cfs_rq
locks protecting the runtime related fields of cfs_rq. When I started,
I too had a simple scheme like yours where a per-rq lock protected the
runtime related fields of all cfs_rqs under it, but when I added runtime
rebalancing, I found the need for per cfs_rq locks and hence followed
rt code more closely. But I can see that since you don't do runtime rebalancing
you can keep the locking simple with just per rq lock protecting the
runtime related fields of all cfs_rqs under it.

Regards,
Bharata.