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

[Stefan Bader]

Chase Douglas wrote:
> 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
> 
If you could just resend the patch with a karmic header. Then I can assign it to
my todo queue in patchworks and will less likely forget it. Thanks

Stefan