Re: [PATCH 06/11] sched/irq: add irq utilization tracking

[Wanpeng Li <kernellwp@gmail.com>]

Hi Vincent,
On Fri, 29 Jun 2018 at 03:07, Vincent Guittot
<vincent.guittot@linaro.org> wrote:
>
> interrupt and steal time are the only remaining activities tracked by
> rt_avg. Like for sched classes, we can use PELT to track their average
> utilization of the CPU. But unlike sched class, we don't track when
> entering/leaving interrupt; Instead, we take into account the time spent
> under interrupt context when we update rqs' clock (rq_clock_task).
> This also means that we have to decay the normal context time and account
> for interrupt time during the update.
>
> That's also important to note that because
>   rq_clock == rq_clock_task + interrupt time
> and rq_clock_task is used by a sched class to compute its utilization, the
> util_avg of a sched class only reflects the utilization of the time spent
> in normal context and not of the whole time of the CPU. The utilization of
> interrupt gives an more accurate level of utilization of CPU.
> The CPU utilization is :
>   avg_irq + (1 - avg_irq / max capacity) * /Sum avg_rq
>
> Most of the time, avg_irq is small and neglictible so the use of the
> approximation CPU utilization = /Sum avg_rq was enough
>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>  kernel/sched/core.c  |  4 +++-
>  kernel/sched/fair.c  | 13 ++++++++++---
>  kernel/sched/pelt.c  | 40 ++++++++++++++++++++++++++++++++++++++++
>  kernel/sched/pelt.h  | 16 ++++++++++++++++
>  kernel/sched/sched.h |  3 +++
>  5 files changed, 72 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 78d8fac..e5263a4 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -18,6 +18,8 @@
>  #include "../workqueue_internal.h"
>  #include "../smpboot.h"
>
> +#include "pelt.h"
> +
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/sched.h>
>
> @@ -186,7 +188,7 @@ static void update_rq_clock_task(struct rq *rq, s64 delta)
>
>  #if defined(CONFIG_IRQ_TIME_ACCOUNTING) || defined(CONFIG_PARAVIRT_TIME_ACCOUNTING)
>         if ((irq_delta + steal) && sched_feat(NONTASK_CAPACITY))
> -               sched_rt_avg_update(rq, irq_delta + steal);
> +               update_irq_load_avg(rq, irq_delta + steal);

I think we should not add steal time into irq load tracking, steal
time is always 0 on native kernel which doesn't matter, what will
happen when guest disables IRQ_TIME_ACCOUNTING and enables
PARAVIRT_TIME_ACCOUNTING? Steal time is not the real irq util_avg. In
addition, we haven't exposed power management for performance which
means that e.g. schedutil governor can not cooperate with passive mode
intel_pstate driver to tune the OPP. To decay the old steal time avg
and add the new one just wastes cpu cycles.

Regards,
Wanpeng Li

>  #endif
>  }
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ffce4b2..d2758e3 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7289,7 +7289,7 @@ static inline bool cfs_rq_has_blocked(struct cfs_rq *cfs_rq)
>         return false;
>  }
>
> -static inline bool others_rqs_have_blocked(struct rq *rq)
> +static inline bool others_have_blocked(struct rq *rq)
>  {
>         if (READ_ONCE(rq->avg_rt.util_avg))
>                 return true;
> @@ -7297,6 +7297,11 @@ static inline bool others_rqs_have_blocked(struct rq *rq)
>         if (READ_ONCE(rq->avg_dl.util_avg))
>                 return true;
>
> +#if defined(CONFIG_IRQ_TIME_ACCOUNTING) || defined(CONFIG_PARAVIRT_TIME_ACCOUNTING)
> +       if (READ_ONCE(rq->avg_irq.util_avg))
> +               return true;
> +#endif
> +
>         return false;
>  }
>
> @@ -7361,8 +7366,9 @@ static void update_blocked_averages(int cpu)
>         }
>         update_rt_rq_load_avg(rq_clock_task(rq), rq, 0);
>         update_dl_rq_load_avg(rq_clock_task(rq), rq, 0);
> +       update_irq_load_avg(rq, 0);
>         /* Don't need periodic decay once load/util_avg are null */
> -       if (others_rqs_have_blocked(rq))
> +       if (others_have_blocked(rq))
>                 done = false;
>
>  #ifdef CONFIG_NO_HZ_COMMON
> @@ -7431,9 +7437,10 @@ static inline void update_blocked_averages(int cpu)
>         update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq);
>         update_rt_rq_load_avg(rq_clock_task(rq), rq, 0);
>         update_dl_rq_load_avg(rq_clock_task(rq), rq, 0);
> +       update_irq_load_avg(rq, 0);
>  #ifdef CONFIG_NO_HZ_COMMON
>         rq->last_blocked_load_update_tick = jiffies;
> -       if (!cfs_rq_has_blocked(cfs_rq) && !others_rqs_have_blocked(rq))
> +       if (!cfs_rq_has_blocked(cfs_rq) && !others_have_blocked(rq))
>                 rq->has_blocked_load = 0;
>  #endif
>         rq_unlock_irqrestore(rq, &rf);
> diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
> index 8b78b63..ead6d8b 100644
> --- a/kernel/sched/pelt.c
> +++ b/kernel/sched/pelt.c
> @@ -357,3 +357,43 @@ int update_dl_rq_load_avg(u64 now, struct rq *rq, int running)
>
>         return 0;
>  }
> +
> +#if defined(CONFIG_IRQ_TIME_ACCOUNTING) || defined(CONFIG_PARAVIRT_TIME_ACCOUNTING)
> +/*
> + * irq:
> + *
> + *   util_sum = \Sum se->avg.util_sum but se->avg.util_sum is not tracked
> + *   util_sum = cpu_scale * load_sum
> + *   runnable_load_sum = load_sum
> + *
> + */
> +
> +int update_irq_load_avg(struct rq *rq, u64 running)
> +{
> +       int ret = 0;
> +       /*
> +        * We know the time that has been used by interrupt since last update
> +        * but we don't when. Let be pessimistic and assume that interrupt has
> +        * happened just before the update. This is not so far from reality
> +        * because interrupt will most probably wake up task and trig an update
> +        * of rq clock during which the metric si updated.
> +        * We start to decay with normal context time and then we add the
> +        * interrupt context time.
> +        * We can safely remove running from rq->clock because
> +        * rq->clock += delta with delta >= running
> +        */
> +       ret = ___update_load_sum(rq->clock - running, rq->cpu, &rq->avg_irq,
> +                               0,
> +                               0,
> +                               0);
> +       ret += ___update_load_sum(rq->clock, rq->cpu, &rq->avg_irq,
> +                               1,
> +                               1,
> +                               1);
> +
> +       if (ret)
> +               ___update_load_avg(&rq->avg_irq, 1, 1);
> +
> +       return ret;
> +}
> +#endif
> diff --git a/kernel/sched/pelt.h b/kernel/sched/pelt.h
> index 0e4f912..d2894db 100644
> --- a/kernel/sched/pelt.h
> +++ b/kernel/sched/pelt.h
> @@ -6,6 +6,16 @@ int __update_load_avg_cfs_rq(u64 now, int cpu, struct cfs_rq *cfs_rq);
>  int update_rt_rq_load_avg(u64 now, struct rq *rq, int running);
>  int update_dl_rq_load_avg(u64 now, struct rq *rq, int running);
>
> +#if defined(CONFIG_IRQ_TIME_ACCOUNTING) || defined(CONFIG_PARAVIRT_TIME_ACCOUNTING)
> +int update_irq_load_avg(struct rq *rq, u64 running);
> +#else
> +static inline int
> +update_irq_load_avg(struct rq *rq, u64 running)
> +{
> +       return 0;
> +}
> +#endif
> +
>  /*
>   * When a task is dequeued, its estimated utilization should not be update if
>   * its util_avg has not been updated at least once.
> @@ -51,6 +61,12 @@ update_dl_rq_load_avg(u64 now, struct rq *rq, int running)
>  {
>         return 0;
>  }
> +
> +static inline int
> +update_irq_load_avg(struct rq *rq, u64 running)
> +{
> +       return 0;
> +}
>  #endif
>
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index ef5d6aa..377be2b 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -850,6 +850,9 @@ struct rq {
>         u64                     age_stamp;
>         struct sched_avg        avg_rt;
>         struct sched_avg        avg_dl;
> +#if defined(CONFIG_IRQ_TIME_ACCOUNTING) || defined(CONFIG_PARAVIRT_TIME_ACCOUNTING)
> +       struct sched_avg        avg_irq;
> +#endif
>         u64                     idle_stamp;
>         u64                     avg_idle;
>
> --
> 2.7.4
>