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

[Konstantin Khlebnikov]

On 13/11/2019 10.35, Dave Chiluk wrote:
> 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?

I haven't thought about that yet.

Burst size should have reasonable default, like quota for 4 periods.
So, per-container configuration might be redundant.

> 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.

I've loaded it with fio, something like:
fio --name=test --ioengine=cpuio --time_based=1 --runtime=600 --cpuload=10 --cpuchunks=100000 --numjobs=10

Statistics in cpu.stat allows to see behaviour even without feedback from workload.

Precise testing isn't trivial though - per-cpu slices adds much noise.

> 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.

Makes sense.

> >          /* 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
> >   };