On Thu, Apr 25, 2013 at 07:23:19PM +0200, Vincent Guittot wrote:
> +static bool is_buddy_busy(int cpu)
> +{
> + struct rq *rq = cpu_rq(cpu);
> + u32 sum = rq->avg.runnable_avg_sum;
> + u32 period = rq->avg.runnable_avg_period;
> +
> + /*
> + * If a CPU accesses the runnable_avg_sum and runnable_avg_period
> + * fields of its buddy CPU while the latter updates it, it can get the
> + * new version of a field and the old version of the other one. This
> + * can generate erroneous decisions. We don't want to use a lock
> + * mechanism for ensuring the coherency because of the overhead in
> + * this critical path.
> + * The runnable_avg_period of a runqueue tends to the max value in
> + * less than 345ms after plugging a CPU, which implies that we could
> + * use the max value instead of reading runnable_avg_period after
> + * 345ms. During the starting phase, we must ensure a minimum of
> + * coherency between the fields. A simple rule is runnable_avg_sum <=
> + * runnable_avg_period.
> + */
> + sum = min(sum, period);
> +
> + /*
> + * A busy buddy is a CPU with a high load or a small load with a lot of
> + * running tasks.
> + */
> + return (sum > (period / (rq->nr_running + 2)));
> +}
I'm still not sold on the entire nr_running thing and the comment doesn't say
why you're doing it.
I'm fairly sure there's software out there that wakes a gazillion threads at a
time only for a gazillion-1 to go back to sleep immediately. Patterns like that
completely defeat your heuristic.
Does that matter... I don't know.
What happens if you keep this thing simple and only put a cap on utilization
(say 80%) and drop the entire nr_running magic? Have you seen it make an actual
difference or did it just seem like a good (TM) thing to do?
> +static bool is_light_task(struct task_struct *p)
> +{
> + /* A light task runs less than 20% in average */
> + return ((p->se.avg.runnable_avg_sum * 5) <
> + (p->se.avg.runnable_avg_period));
> +}
There superfluous () and ' ' in there. Also why 20%.. seemed like a good
number? Do we want a SCHED_DEBUG sysctl for that?
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