[PATCH v3?] sched: update load count only once per cpu in 10 tick update window

Chase Douglas chase.douglas at canonical.com
Fri Apr 9 17:32:46 UTC 2010


On Thu, Apr 8, 2010 at 12:02 PM, Chase Douglas
<chase.douglas at canonical.com> wrote:
> There's a period of 10 ticks where calc_load_tasks is updated by all the
> cpus for the load avg. Usually all the cpus do this during the first
> tick. If any cpus go idle, calc_load_tasks is decremented accordingly.
> However, if they wake up calc_load_tasks is not incremented. Thus, if
> cpus go idle during the 10 tick period, calc_load_tasks may be
> decremented to a non-representative value. This issue can lead to
> systems having a load avg of exactly 0, even though the real load avg
> could theoretically be up to NR_CPUS.
>
> This change defers calc_load_tasks accounting after each cpu updates the
> count until after the 10 tick update window.
>
> A few points:
>
> * A global atomic deferral counter, and not per-cpu vars, is needed
>  because a cpu may go NOHZ idle and not be able to update the global
>  calc_load_tasks variable for subsequent load calculations.
> * It is not enough to add calls to account for the load when a cpu is
>  awakened:
>  - Load avg calculation must be independent of cpu load.
>  - If a cpu is awakend by one tasks, but then has more scheduled before
>    the end of the update window, only the first task will be accounted.
>
> BugLink: http://bugs.launchpad.net/bugs/513848
>
> Signed-off-by: Chase Douglas <chase.douglas at canonical.com>
> ---
>  kernel/sched.c |   24 ++++++++++++++++++++++--
>  1 files changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 81ede13..c372249 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2967,6 +2967,7 @@ unsigned long nr_iowait(void)
>
>  /* Variables and functions for calc_load */
>  static atomic_long_t calc_load_tasks;
> +static atomic_long_t calc_load_tasks_deferred;
>  static unsigned long calc_load_update;
>  unsigned long avenrun[3];
>  EXPORT_SYMBOL(avenrun);
> @@ -3021,7 +3022,7 @@ void calc_global_load(void)
>  */
>  static void calc_load_account_active(struct rq *this_rq)
>  {
> -       long nr_active, delta;
> +       long nr_active, delta, deferred;
>
>        nr_active = this_rq->nr_running;
>        nr_active += (long) this_rq->nr_uninterruptible;
> @@ -3029,6 +3030,25 @@ static void calc_load_account_active(struct rq *this_rq)
>        if (nr_active != this_rq->calc_load_active) {
>                delta = nr_active - this_rq->calc_load_active;
>                this_rq->calc_load_active = nr_active;
> +
> +               /*
> +                * Update calc_load_tasks only once per cpu in 10 tick update
> +                * window.
> +                */
> +               if (unlikely(time_before(jiffies, this_rq->calc_load_update) &&
> +                            time_after_eq(jiffies, calc_load_update))) {
> +                       if (delta)
> +                               atomic_long_add(delta,
> +                                               &calc_load_tasks_deferred);
> +                       return;
> +               }
> +
> +               if (atomic_long_read(&calc_load_tasks_deferred)) {
> +                       deferred = atomic_long_xchg(&calc_load_tasks_deferred,
> +                                                   0);
> +                       delta += deferred;
> +               }
> +
>                atomic_long_add(delta, &calc_load_tasks);
>        }
>  }
> @@ -3072,8 +3092,8 @@ static void update_cpu_load(struct rq *this_rq)
>        }
>
>        if (time_after_eq(jiffies, this_rq->calc_load_update)) {
> -               this_rq->calc_load_update += LOAD_FREQ;
>                calc_load_account_active(this_rq);
> +               this_rq->calc_load_update += LOAD_FREQ;
>        }
>  }
>
> --
> 1.7.0

Now that this has been accepted for Lucid, please consider this for
Karmic as well. SRU justification can be found in bug 513848. Thanks!

-- Chase




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