Commit:     e1f47d891c0f00769d6d40ac5740f943e998d089
Parent:     a22d7fc187ed996b66d8439db27b2303f79a8e7b
Author:     Steven Rostedt <[EMAIL PROTECTED]>
AuthorDate: Fri Jan 25 21:08:12 2008 +0100
Committer:  Ingo Molnar <[EMAIL PROTECTED]>
CommitDate: Fri Jan 25 21:08:12 2008 +0100

    sched: RT-balance, avoid overloading
    This patch changes the searching for a run queue by a waking RT task
    to try to pick another runqueue if the currently running task
    is an RT task.
    The reason is that RT tasks behave different than normal
    tasks. Preempting a normal task to run a RT task to keep
    its cache hot is fine, because the preempted non-RT task
    may wait on that same runqueue to run again unless the
    migration thread comes along and pulls it off.
    RT tasks behave differently. If one is preempted, it makes
    an active effort to continue to run. So by having a high
    priority task preempt a lower priority RT task, that lower
    RT task will then quickly try to run on another runqueue.
    This will cause that lower RT task to replace its nice
    hot cache (and TLB) with a completely cold one. This is
    for the hope that the new high priority RT task will keep
     its cache hot.
    Remeber that this high priority RT task was just woken up.
    So it may likely have been sleeping for several milliseconds,
    and will end up with a cold cache anyway. RT tasks run till
    they voluntarily stop, or are preempted by a higher priority
    task. This means that it is unlikely that the woken RT task
    will have a hot cache to wake up to. So pushing off a lower
    RT task is just killing its cache for no good reason.
    Signed-off-by: Steven Rostedt <[EMAIL PROTECTED]>
    Signed-off-by: Ingo Molnar <[EMAIL PROTECTED]>
 kernel/sched_rt.c |   20 ++++++++++++++++----
 1 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 87d7b3f..9becc37 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -160,11 +160,23 @@ static int select_task_rq_rt(struct task_struct *p, int 
        struct rq *rq = task_rq(p);
-        * If the task will not preempt the RQ, try to find a better RQ
-        * before we even activate the task
+        * If the current task is an RT task, then
+        * try to see if we can wake this RT task up on another
+        * runqueue. Otherwise simply start this RT task
+        * on its current runqueue.
+        *
+        * We want to avoid overloading runqueues. Even if
+        * the RT task is of higher priority than the current RT task.
+        * RT tasks behave differently than other tasks. If
+        * one gets preempted, we try to push it off to another queue.
+        * So trying to keep a preempting RT task on the same
+        * cache hot CPU will force the running RT task to
+        * a cold CPU. So we waste all the cache for the lower
+        * RT task in hopes of saving some of a RT task
+        * that is just being woken and probably will have
+        * cold cache anyway.
-       if ((p->prio >= rq->rt.highest_prio)
-           && (p->nr_cpus_allowed > 1)) {
+       if (unlikely(rt_task(rq->curr))) {
                int cpu = find_lowest_rq(p);
                return (cpu == -1) ? task_cpu(p) : cpu;
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