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?