Re: [PATCH RFC] sched/fair: add burst to cgroup cpu bandwidth controller

[Dave Chiluk]

I was going to write this exact patch, but you beat me to it.  I like
that it's sequestered into a separate setting.  This would be very
useful for interactive applications that have cfs bandwidth control
enabled *(cough* kubernetes).  Are you planning on submitting the
plumbing for this feature back into docker/containerd/lxc/other
container orchestrators?

Also this is very similar in desired behavior to a patchset submitted
by Cong Wang https://lore.kernel.org/patchwork/patch/907450/
.Although, I like this implementation better due to it's simplicity.
I guess a lot of the simplification comes from not having to deal with
the expiration logic.

Additional comments inlined.  Documentation edits, and my preference
for a full burst bank on setting.

Otherwise looks good to me.
+ Reviewed-by: Dave Chiluk <chiluk+linux@xxxxxxxxxx>

I also tested this with my simple test application I use for
validating runtime accuracy https://github.com/indeedeng/fibtest.  And
everything looks reasonable here.  I'll try to do a bit more testing
but so far I'm happy.  Hopefully Ben will give a +1 to this as well.

Thanks!

On Thu, Nov 7, 2019 at 6:12 AM Konstantin Khlebnikov
<khlebnikov@xxxxxxxxxxxxxx> wrote:
>
> Currently CFS bandwidth controller assigns cpu.cfs_quota_us runtime into
> global pool every cpu.cfs_period_us. All unused runtime is expired.
>
> Since commit de53fd7aedb1 ("sched/fair: Fix low cpu usage with high
> throttling by removing expiration of cpu-local slices") slice assigned
> to cpu does not expire. This allows to serve tiny bursts (upto 1ms),
> but this runtime pool is cpu-bound and not transferred between cpus.
>
> Setup for interactive workload with irregular cpu consumption have to set
> quota according to relatively short spikes of cpu usage. This eliminates
> possibility of control for average cpu usage. Increasing period and quota
> proportionally for getting bigger runtime chunks is not an option because
> if even bigger spike deplete global pool then execution will stuck until
> end of period and next refill.
>
> This patch adds limited accumulation of unused runtime from past periods.
> Accumulated runtime does not expire. It stays in global pool and could
> be used by any cpu. Average cpu usage stays limited with quota / period,
> but spiky workload could use more cpu power for a short period of time.
>
> Size of pool for burst runtime is set in attribute cpu.cfs_burst_us.
> Default is 0, which reflects current behavior.
>
> Statistics for used bust runtime is shown in cpu.stat as "burst_time".
>
> Example setup:
> cpu.cfs_period_us = 100ms
> cpu.cfs_quota_us = 200ms
> cpu.cfs_burst_us = 300ms
>
> Average cpu usage stays limited with 2 cpus (quota / period), but cgroup
> could accumulate runtime (burst) and for 100ms could utilize up to 5 cpus
> (quota / period + burst / 100ms), or 3 cpus for 300ms, an so on.
>
> Implementation is simple. All logic is in __refill_cfs_bandwidth_runtime().
> The rest changes are interface for cgroup and cgroup2.
>
> For cgroup2 burst is set as third number in attribute cpu.max:
> cpu.max = $QUOTA $PERIOD $BURST
>
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> ---
>  Documentation/admin-guide/cgroup-v2.rst |   15 +++--
>  Documentation/scheduler/sched-bwc.rst   |    8 ++-
>  kernel/sched/core.c                     |   88 +++++++++++++++++++++++++------
>  kernel/sched/fair.c                     |   34 ++++++++++--
>  kernel/sched/sched.h                    |    4 +
>  5 files changed, 119 insertions(+), 30 deletions(-)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 5361ebec3361..8c3cc3d882ba 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -981,11 +981,12 @@ All time durations are in microseconds.
>         - user_usec
>         - system_usec
>
> -       and the following three when the controller is enabled:
> +       and the following four when the controller is enabled:
>
>         - nr_periods
>         - nr_throttled
>         - throttled_usec
> +       - burst_usec
>
>    cpu.weight
>         A read-write single value file which exists on non-root
> @@ -1006,16 +1007,18 @@ All time durations are in microseconds.
>         the closest approximation of the current weight.
>
>    cpu.max
> -       A read-write two value file which exists on non-root cgroups.
> -       The default is "max 100000".
> +       A read-write 1..3 values file which exists on non-root cgroups.
> +       The default is "max 100000 0".
>
>         The maximum bandwidth limit.  It's in the following format::
>
> -         $MAX $PERIOD
> +         $MAX $PERIOD $BURST
>
>         which indicates that the group may consume upto $MAX in each
> -       $PERIOD duration.  "max" for $MAX indicates no limit.  If only
> -       one number is written, $MAX is updated.
> +       $PERIOD duration and accumulates upto $BURST time for bursts.
> +
> +       "max" for $MAX indicates no limit.
> +       If only one number is written, $MAX is updated.
>
>    cpu.pressure
>         A read-only nested-key file which exists on non-root cgroups.
> diff --git a/Documentation/scheduler/sched-bwc.rst b/Documentation/scheduler/sched-bwc.rst
> index 9801d6b284b1..5f1cac7b2d44 100644
> --- a/Documentation/scheduler/sched-bwc.rst
> +++ b/Documentation/scheduler/sched-bwc.rst
> @@ -27,12 +27,14 @@ Quota and period are managed within the cpu subsystem via cgroupfs.
>
>  cpu.cfs_quota_us: the total available run-time within a period (in microseconds)
>  cpu.cfs_period_us: the length of a period (in microseconds)
> +cpu.cfs_burst_us: the maxumum size of burst run-time pool (in microseconds)

maxumum -> maximum

>  cpu.stat: exports throttling statistics [explained further below]
>
>  The default values are::
>
>         cpu.cfs_period_us=100ms
> -       cpu.cfs_quota=-1
> +       cpu.cfs_quota_us=-1
> +       cpu.cfs_burst_us=0
>
>  A value of -1 for cpu.cfs_quota_us indicates that the group does not have any
>  bandwidth restriction in place, such a group is described as an unconstrained
> @@ -51,6 +53,9 @@ and return the group to an unconstrained state once more.
>  Any updates to a group's bandwidth specification will result in it becoming
>  unthrottled if it is in a constrained state.
>
> +Writing positive value into cpu.cfs_burst_us allows to allcumulate unused
> +run-time (upto this value) and use it later in addition to assigned quota.
Writing positive value into cpu.cfs_burst_us allows unused quota to
accumulate up to this value and be used later in addition to assigned
quota.

> +
>  System wide settings
>  --------------------
>  For efficiency run-time is transferred between the global pool and CPU local
> @@ -75,6 +80,7 @@ cpu.stat:
>  - nr_throttled: Number of times the group has been throttled/limited.
>  - throttled_time: The total time duration (in nanoseconds) for which entities
>    of the group have been throttled.
> +- burst_time: The total running time consumed from burst pool.
>
>  This interface is read-only.
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index dd05a378631a..010b79c45200 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7354,7 +7354,8 @@ static const u64 min_cfs_quota_period = 1 * NSEC_PER_MSEC; /* 1ms */
>
>  static int __cfs_schedulable(struct task_group *tg, u64 period, u64 runtime);
>
> -static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
> +static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota,
> +                               u64 burst)
>  {
>         int i, ret = 0, runtime_enabled, runtime_was_enabled;
>         struct cfs_bandwidth *cfs_b = &tg->cfs_bandwidth;
> @@ -7399,12 +7400,13 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
>         raw_spin_lock_irq(&cfs_b->lock);
>         cfs_b->period = ns_to_ktime(period);
>         cfs_b->quota = quota;
> -
> -       __refill_cfs_bandwidth_runtime(cfs_b);
> +       cfs_b->burst = burst;

+ cfs_b->burst_runtime = burst;
I think we should default to a full burst bank.  After all, during
startup applications very likely to need additional CPU.  This is a
regular pain point for our applications..  By providing burst bank
early we help to mitigate slow start-up times for cpu limited
applications.

>
>         /* Restart the period timer (if active) to handle new period expiry: */
> -       if (runtime_enabled)
> +       if (runtime_enabled) {
> +               __refill_cfs_bandwidth_runtime(cfs_b);
>                 start_cfs_bandwidth(cfs_b);
> +       }
>
>         raw_spin_unlock_irq(&cfs_b->lock);
>
> @@ -7432,9 +7434,10 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
>
>  static int tg_set_cfs_quota(struct task_group *tg, long cfs_quota_us)
>  {
> -       u64 quota, period;
> +       u64 quota, period, burst;
>
>         period = ktime_to_ns(tg->cfs_bandwidth.period);
> +       burst = tg->cfs_bandwidth.burst;
>         if (cfs_quota_us < 0)
>                 quota = RUNTIME_INF;
>         else if ((u64)cfs_quota_us <= U64_MAX / NSEC_PER_USEC)
> @@ -7442,7 +7445,7 @@ static int tg_set_cfs_quota(struct task_group *tg, long cfs_quota_us)
>         else
>                 return -EINVAL;
>
> -       return tg_set_cfs_bandwidth(tg, period, quota);
> +       return tg_set_cfs_bandwidth(tg, period, quota, burst);
>  }
>
>  static long tg_get_cfs_quota(struct task_group *tg)
> @@ -7460,15 +7463,16 @@ static long tg_get_cfs_quota(struct task_group *tg)
>
>  static int tg_set_cfs_period(struct task_group *tg, long cfs_period_us)
>  {
> -       u64 quota, period;
> +       u64 quota, period, burst;
>
>         if ((u64)cfs_period_us > U64_MAX / NSEC_PER_USEC)
>                 return -EINVAL;
>
>         period = (u64)cfs_period_us * NSEC_PER_USEC;
>         quota = tg->cfs_bandwidth.quota;
> +       burst = tg->cfs_bandwidth.burst;
>
> -       return tg_set_cfs_bandwidth(tg, period, quota);
> +       return tg_set_cfs_bandwidth(tg, period, quota, burst);
>  }
>
>  static long tg_get_cfs_period(struct task_group *tg)
> @@ -7481,6 +7485,28 @@ static long tg_get_cfs_period(struct task_group *tg)
>         return cfs_period_us;
>  }
>
> +static long tg_get_cfs_burst(struct task_group *tg)
> +{
> +       u64 cfs_burst_us = tg->cfs_bandwidth.burst;
> +
> +       do_div(cfs_burst_us, NSEC_PER_USEC);
> +       return cfs_burst_us;
> +}
> +
> +static int tg_set_cfs_burst(struct task_group *tg, long cfs_burst_us)
> +{
> +       u64 quota, period, burst;
> +
> +       if ((u64)cfs_burst_us > U64_MAX / NSEC_PER_USEC)
> +               return -EINVAL;
> +
> +       period = ktime_to_ns(tg->cfs_bandwidth.period);
> +       quota = tg->cfs_bandwidth.quota;
> +       burst = (u64)cfs_burst_us * NSEC_PER_USEC;
> +
> +       return tg_set_cfs_bandwidth(tg, period, quota, burst);
> +}
> +
>  static s64 cpu_cfs_quota_read_s64(struct cgroup_subsys_state *css,
>                                   struct cftype *cft)
>  {
> @@ -7505,6 +7531,18 @@ static int cpu_cfs_period_write_u64(struct cgroup_subsys_state *css,
>         return tg_set_cfs_period(css_tg(css), cfs_period_us);
>  }
>
> +static u64 cpu_cfs_burst_read_u64(struct cgroup_subsys_state *css,
> +                                 struct cftype *cft)
> +{
> +       return tg_get_cfs_burst(css_tg(css));
> +}
> +
> +static int cpu_cfs_burst_write_u64(struct cgroup_subsys_state *css,
> +                                  struct cftype *cftype, u64 cfs_burst_us)
> +{
> +       return tg_set_cfs_burst(css_tg(css), cfs_burst_us);
> +}
> +
>  struct cfs_schedulable_data {
>         struct task_group *tg;
>         u64 period, quota;
> @@ -7596,6 +7634,7 @@ static int cpu_cfs_stat_show(struct seq_file *sf, void *v)
>         seq_printf(sf, "nr_periods %d\n", cfs_b->nr_periods);
>         seq_printf(sf, "nr_throttled %d\n", cfs_b->nr_throttled);
>         seq_printf(sf, "throttled_time %llu\n", cfs_b->throttled_time);
> +       seq_printf(sf, "burst_time %llu\n", cfs_b->burst_time);
>
>         if (schedstat_enabled() && tg != &root_task_group) {
>                 u64 ws = 0;
> @@ -7657,6 +7696,11 @@ static struct cftype cpu_legacy_files[] = {
>                 .read_u64 = cpu_cfs_period_read_u64,
>                 .write_u64 = cpu_cfs_period_write_u64,
>         },
> +       {
> +               .name = "cfs_burst_us",
> +               .read_u64 = cpu_cfs_burst_read_u64,
> +               .write_u64 = cpu_cfs_burst_write_u64,
> +       },
>         {
>                 .name = "stat",
>                 .seq_show = cpu_cfs_stat_show,
> @@ -7699,15 +7743,20 @@ static int cpu_extra_stat_show(struct seq_file *sf,
>                 struct task_group *tg = css_tg(css);
>                 struct cfs_bandwidth *cfs_b = &tg->cfs_bandwidth;
>                 u64 throttled_usec;
> +               u64 burst_usec;
>
>                 throttled_usec = cfs_b->throttled_time;
>                 do_div(throttled_usec, NSEC_PER_USEC);
>
> +               burst_usec = cfs_b->burst_time;
> +               do_div(burst_usec, NSEC_PER_USEC);
> +
>                 seq_printf(sf, "nr_periods %d\n"
>                            "nr_throttled %d\n"
> -                          "throttled_usec %llu\n",
> +                          "throttled_usec %llu\n"
> +                          "burst_usec %llu\n",
>                            cfs_b->nr_periods, cfs_b->nr_throttled,
> -                          throttled_usec);
> +                          throttled_usec, burst_usec);
>         }
>  #endif
>         return 0;
> @@ -7777,26 +7826,29 @@ static int cpu_weight_nice_write_s64(struct cgroup_subsys_state *css,
>  #endif
>
>  static void __maybe_unused cpu_period_quota_print(struct seq_file *sf,
> -                                                 long period, long quota)
> +                                                 long period, long quota,
> +                                                 long burst)
>  {
>         if (quota < 0)
>                 seq_puts(sf, "max");
>         else
>                 seq_printf(sf, "%ld", quota);
>
> -       seq_printf(sf, " %ld\n", period);
> +       seq_printf(sf, " %ld %ld\n", period, burst);
>  }
>
>  /* caller should put the current value in *@periodp before calling */
>  static int __maybe_unused cpu_period_quota_parse(char *buf,
> -                                                u64 *periodp, u64 *quotap)
> +                                                u64 *periodp, u64 *quotap,
> +                                                s64 *burstp)
>  {
>         char tok[21];   /* U64_MAX */
>
> -       if (sscanf(buf, "%20s %llu", tok, periodp) < 1)
> +       if (sscanf(buf, "%20s %llu %llu", tok, periodp, burstp) < 1)
>                 return -EINVAL;
>
>         *periodp *= NSEC_PER_USEC;
> +       *burstp *= NSEC_PER_USEC;
>
>         if (sscanf(tok, "%llu", quotap))
>                 *quotap *= NSEC_PER_USEC;
> @@ -7813,7 +7865,8 @@ static int cpu_max_show(struct seq_file *sf, void *v)
>  {
>         struct task_group *tg = css_tg(seq_css(sf));
>
> -       cpu_period_quota_print(sf, tg_get_cfs_period(tg), tg_get_cfs_quota(tg));
> +       cpu_period_quota_print(sf, tg_get_cfs_period(tg), tg_get_cfs_quota(tg),
> +                              tg_get_cfs_burst(tg));
>         return 0;
>  }
>
> @@ -7822,12 +7875,13 @@ static ssize_t cpu_max_write(struct kernfs_open_file *of,
>  {
>         struct task_group *tg = css_tg(of_css(of));
>         u64 period = tg_get_cfs_period(tg);
> +       s64 burst = tg_get_cfs_burst(tg);
>         u64 quota;
>         int ret;
>
> -       ret = cpu_period_quota_parse(buf, &period, &quota);
> +       ret = cpu_period_quota_parse(buf, &period, &quota, &burst);
>         if (!ret)
> -               ret = tg_set_cfs_bandwidth(tg, period, quota);
> +               ret = tg_set_cfs_bandwidth(tg, period, quota, burst);
>         return ret ?: nbytes;
>  }
>  #endif
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 682a754ea3e1..26b8dac67c79 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4353,16 +4353,26 @@ static inline u64 sched_cfs_bandwidth_slice(void)
>  }
>
>  /*
> - * Replenish runtime according to assigned quota. We use sched_clock_cpu
> - * directly instead of rq->clock to avoid adding additional synchronization
> - * around rq->lock.
> + * Replenish runtime according to assigned quota.
> + * Called only if quota != RUNTIME_INF.
>   *
>   * requires cfs_b->lock
>   */
>  void __refill_cfs_bandwidth_runtime(struct cfs_bandwidth *cfs_b)
>  {
> -       if (cfs_b->quota != RUNTIME_INF)
> -               cfs_b->runtime = cfs_b->quota;
> +       u64 runtime = cfs_b->runtime;
> +
> +       /*
> +        * Preserve past runtime up to burst size. If remaining runtime lower
> +        * than previous burst runtime then account delta as used burst time.
> +        */
> +       if (runtime > cfs_b->burst)
> +               runtime = cfs_b->burst;
> +       else if (runtime < cfs_b->burst_runtime)
> +               cfs_b->burst_time += cfs_b->burst_runtime - runtime;
> +
> +       cfs_b->burst_runtime = runtime;
> +       cfs_b->runtime = runtime + cfs_b->quota;
>  }
>
>  static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
> @@ -4968,6 +4978,9 @@ void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
>         cfs_b->runtime = 0;
>         cfs_b->quota = RUNTIME_INF;
>         cfs_b->period = ns_to_ktime(default_cfs_period());
> +       cfs_b->burst = 0;
> +       cfs_b->burst_runtime = 0;
> +       cfs_b->burst_time = 0;
>
>         INIT_LIST_HEAD(&cfs_b->throttled_cfs_rq);
>         hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED);
> @@ -4986,14 +4999,23 @@ static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq)
>
>  void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
>  {
> +       u64 overrun;
> +
>         lockdep_assert_held(&cfs_b->lock);
>
>         if (cfs_b->period_active)
>                 return;
>
>         cfs_b->period_active = 1;
> -       hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);
> +       overrun = hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);
>         hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED);
> +
> +       /*
> +        * Refill runtime for periods of inactivity and current.
> +        * __refill_cfs_bandwidth_runtime() will cut excess.
> +        */
> +       cfs_b->runtime += cfs_b->quota * overrun;
> +       __refill_cfs_bandwidth_runtime(cfs_b);
>  }
>
>  static void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 0db2c1b3361e..eaecd7298f80 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -344,10 +344,14 @@ struct cfs_bandwidth {
>         struct hrtimer          slack_timer;
>         struct list_head        throttled_cfs_rq;
>
> +       u64                     burst;
> +       u64                     burst_runtime;
> +
>         /* Statistics: */
>         int                     nr_periods;
>         int                     nr_throttled;
>         u64                     throttled_time;
> +       u64                     burst_time;
>  #endif
>  };
>
>