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Subject[081/123] sched: Fix sched_exec() balancing
From: Peter Zijlstra <a.p.zijlstra@chello.nl>

commit 3802290628348674985d14914f9bfee7b9084548 upstream

sched: Fix sched_exec() balancing

Since we access ->cpus_allowed without holding rq->lock we need
a retry loop to validate the result, this comes for near free
when we merge sched_migrate_task() into sched_exec() since that
already does the needed check.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <20091216170517.884743662@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
kernel/sched.c | 45 +++++++++++++++++++++++----------------------
1 file changed, 23 insertions(+), 22 deletions(-)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2322,7 +2322,7 @@ void task_oncpu_function_call(struct tas
*
* - fork, @p is stable because it isn't on the tasklist yet
*
- * - exec, @p is unstable XXX
+ * - exec, @p is unstable, retry loop
*
* - wake-up, we serialize ->cpus_allowed against TASK_WAKING so
* we should be good.
@@ -3133,21 +3133,36 @@ static void double_rq_unlock(struct rq *
}

/*
- * If dest_cpu is allowed for this process, migrate the task to it.
- * This is accomplished by forcing the cpu_allowed mask to only
- * allow dest_cpu, which will force the cpu onto dest_cpu. Then
- * the cpu_allowed mask is restored.
+ * sched_exec - execve() is a valuable balancing opportunity, because at
+ * this point the task has the smallest effective memory and cache footprint.
*/
-static void sched_migrate_task(struct task_struct *p, int dest_cpu)
+void sched_exec(void)
{
+ struct task_struct *p = current;
struct migration_req req;
+ int dest_cpu, this_cpu;
unsigned long flags;
struct rq *rq;

+again:
+ this_cpu = get_cpu();
+ dest_cpu = select_task_rq(p, SD_BALANCE_EXEC, 0);
+ if (dest_cpu == this_cpu) {
+ put_cpu();
+ return;
+ }
+
rq = task_rq_lock(p, &flags);
+ put_cpu();
+
+ /*
+ * select_task_rq() can race against ->cpus_allowed
+ */
if (!cpumask_test_cpu(dest_cpu, &p->cpus_allowed)
- || unlikely(!cpu_active(dest_cpu)))
- goto out;
+ || unlikely(!cpu_active(dest_cpu))) {
+ task_rq_unlock(rq, &flags);
+ goto again;
+ }

/* force the process onto the specified CPU */
if (migrate_task(p, dest_cpu, &req)) {
@@ -3162,24 +3177,10 @@ static void sched_migrate_task(struct ta

return;
}
-out:
task_rq_unlock(rq, &flags);
}

/*
- * sched_exec - execve() is a valuable balancing opportunity, because at
- * this point the task has the smallest effective memory and cache footprint.
- */
-void sched_exec(void)
-{
- int new_cpu, this_cpu = get_cpu();
- new_cpu = select_task_rq(current, SD_BALANCE_EXEC, 0);
- put_cpu();
- if (new_cpu != this_cpu)
- sched_migrate_task(current, new_cpu);
-}
-
-/*
* pull_task - move a task from a remote runqueue to the local runqueue.
* Both runqueues must be locked.
*/



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