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Subject[PATCH] 2.6.1 Hyperthread smart "nice" 2
Date
Following on from the previous hyperthread smart nice patch;

>A while back we had an lkml thread about the problem of running low priority
>tasks on hyperthread enabled cpus in SMP mode. Brief summary: If you run a
>P4HT in uniprocessor mode and run a cpu intensive task at nice +20 (like
>setiathome), the most cpu it will get during periods of heavy usage is about
>8%. If you boot a P4HT in SMP mode and run a cpu intensive task at nice +20
>then if you run a task even at nice -20 concurrently, the nice +20 task will
>get 50% of the cpu time even though you have a very high priority task. So
>ironically booting in SMP mode makes your machine slower for running
>background tasks.

Criticism was laid at the previous patch for the way a more "nice" task might
never run on the sibling cpu if a high priority task was running. This patch
is a much better solution.

What this one does is the following; If there is a "nice" difference between
tasks running on logical cores of the same cpu, the more "nice" one will run
a proportion of time equal to the timeslice it would have been given relative
to the less "nice" task.
ie a nice 19 task running on one core and the nice 0 task running on the other
core will let the nice 0 task run continuously (102ms is normal timeslice)
and the nice 19 task will only run for the last 10ms of time the nice 0 task
is running. This makes for a much more balanced resource distribution, gives
significant preference to the higher priority task, but allows them to
benefit from running on both logical cores.

This seems to me a satisfactory solution to the hyperthread vs nice problem.
Once again this is too arch. specific a change to sched.c for mainline, but
as proof of concept I believe it works well for those who need something that
works that they can use now.

http://ck.kolivas.org/patches/2.6/2.6.1/experimental/

The stuff on my website is incremental with my other experiments, but the
attached patch applies cleanly to 2.6.1

Con
--- linux-2.6.1-base/kernel/sched.c 2004-01-09 22:57:04.000000000 +1100
+++ linux-2.6.1-htn2/kernel/sched.c 2004-02-02 20:01:17.042394133 +1100
@@ -208,6 +208,7 @@ struct runqueue {
atomic_t *node_nr_running;
int prev_node_load[MAX_NUMNODES];
#endif
+ unsigned long cpu;
task_t *migration_thread;
struct list_head migration_queue;

@@ -221,6 +222,10 @@ static DEFINE_PER_CPU(struct runqueue, r
#define task_rq(p) cpu_rq(task_cpu(p))
#define cpu_curr(cpu) (cpu_rq(cpu)->curr)

+#define ht_active (cpu_has_ht && smp_num_siblings > 1)
+#define ht_siblings(cpu1, cpu2) (ht_active && \
+ cpu_sibling_map[(cpu1)] == (cpu2))
+
/*
* Default context-switch locking:
*/
@@ -1380,6 +1385,10 @@ void scheduler_tick(int user_ticks, int
cpustat->iowait += sys_ticks;
else
cpustat->idle += sys_ticks;
+ if (rq->nr_running) {
+ resched_task(p);
+ goto out;
+ }
rebalance_tick(rq, 1);
return;
}
@@ -1536,6 +1545,20 @@ need_resched:
if (!rq->nr_running) {
next = rq->idle;
rq->expired_timestamp = 0;
+#ifdef CONFIG_SMP
+ if (ht_active) {
+ /*
+ * If a HT sibling task is sleeping due to
+ * priority reasons wake it up now
+ */
+ runqueue_t *htrq;
+ htrq = cpu_rq(cpu_sibling_map[(rq->cpu)]);
+
+ if (htrq->curr == htrq->idle &&
+ htrq->nr_running)
+ resched_task(htrq->idle);
+ }
+#endif
goto switch_tasks;
}
}
@@ -1555,6 +1578,42 @@ need_resched:
queue = array->queue + idx;
next = list_entry(queue->next, task_t, run_list);

+#ifdef CONFIG_SMP
+ if (ht_active) {
+ runqueue_t *htrq;
+ htrq = cpu_rq(cpu_sibling_map[(rq->cpu)]);
+ task_t *htcurr;
+ htcurr = htrq->curr;
+
+ /*
+ * If a user task with lower static priority than the
+ * running task on the hyperthread sibling is trying
+ * to schedule, delay it till there is equal timeslice
+ * left of the hyperthread task to prevent a lower priority
+ * task from using an unfair proportion of the physical
+ * cpu's resources.
+ */
+ if (next->mm && htcurr->mm && !rt_task(next) &&
+ ((next->static_prio >
+ htcurr->static_prio && htcurr->time_slice >
+ task_timeslice(next)) || rt_task(htcurr))) {
+ next = rq->idle;
+ goto switch_tasks;
+ }
+
+ /*
+ * Reschedule a lower priority task
+ * on the HT sibling, or wake it up if it has been
+ * put to sleep for priority reasons.
+ */
+ if ((htcurr != htrq->idle &&
+ htcurr->static_prio > next->static_prio) ||
+ (rt_task(next) && !rt_task(htcurr)) ||
+ (htcurr == htrq->idle && htrq->nr_running))
+ resched_task(htcurr);
+ }
+#endif
+
if (next->activated > 0) {
unsigned long long delta = now - next->timestamp;

@@ -2809,6 +2868,7 @@ void __init sched_init(void)
prio_array_t *array;

rq = cpu_rq(i);
+ rq->cpu = (unsigned long)(i);
rq->active = rq->arrays;
rq->expired = rq->arrays + 1;
spin_lock_init(&rq->lock);
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