chen Shang wrote:
Given the frequency of schedule() calling, there is a margin to
improve it. In step of recalculate task priority, it first dequeues
from one priority queue, calls recalc_task_prio(), then enqueue the
task into another priority queue. However, statistics shows only
around 0.5% of recalculation changed task priority (see below). While
rest of 99.5% of recalculation do not change priority, it is
reasonably to use requeue_task() to avoid overhead of dequeue and
enqueue on the same priority queue.
The patch is implemented with above idea. Note, a new help function,
change_queue_task(), to combine dequeue() and enqueue() to reduce one
function call overhead. Two statistics fields, sched_prio_changed and
sched_prio_unchanged, are added to provide statistic data on priority
recalculation.
Thanks,
chen
[email protected]
Hi Chen,
With the added branch and the extra icache footprint, it isn't clear
that this would be a win.
Also, you didn't say where your statistics came from (what workload).
So you really need to start by demonstrating some increase on some workload.
Also, minor comments on the patch: please work against mm kernels,
please follow
kernel coding style, and don't change schedstat output format in the
same patch
(makes it easier for those with schedstat parsing tools).
+static void change_queue_task(struct task_struct *p, prio_array_t *array,
+ int old_prio)
+{
+ list_del(&p->run_list);
+ if (list_empty(array->queue + old_prio))
+ __clear_bit(old_prio, array->bitmap);
+
+ sched_info_queued(p);
+ list_add_tail(&p->run_list, array->queue + p->prio);
+ __set_bit(p->prio, array->bitmap);
+ p->array = array;
+}
/*
* Put task to the end of the run list without the overhead of dequeue
* followed by enqueue.
@@ -2668,7 +2690,7 @@
struct list_head *queue;
unsigned long long now;
unsigned long run_time;
- int cpu, idx;
+ int cpu, idx, prio;
/*
* Test if we are atomic. Since do_exit() needs to call into
@@ -2787,9 +2809,19 @@
delta = delta * (ON_RUNQUEUE_WEIGHT * 128 / 100) / 128;
array = next->array;
- dequeue_task(next, array);
+ prio = next->prio;
recalc_task_prio(next, next->timestamp + delta);
- enqueue_task(next, array);
+
+ if (unlikely(prio != next->prio))
+ {
+ change_queue_task(next, array, prio);
+ schedstat_inc(rq, sched_prio_changed);
+ }
+ else
+ {
+ requeue_task(next, array);
+ schedstat_inc(rq, sched_prio_unchanged);
+ }
}
next->activated = 0;
switch_tasks:
-
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