[PATCH] Hotplug CPUs: Take cpu Lock Around Migration

Grab cpu lock around sched_migrate_task() and sys_sched_setaffinity().
This is a noop without CONFIG_HOTPLUG_CPU.

The sched_migrate_task may have a performance penalty on NUMA if lots
of exec rebalancing is happening, however this only applies to
CONFIG_NUMA and CONFIG_HOTPLUG_CPU, which noone does at the moment
anyway.

Also, the scheduler in -mm solves the race another way, so this will
vanish then.

diff --git a/kernel/sched.c b/kernel/sched.c
index ad549c9bad36d0b069f7ff4648d4cb4acc8a17c7..3097153d43a03f6bdd8dec284604dc28fb1f66da 100644 (file)
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1012,6 +1012,7 @@ static void sched_migrate_task(task_t *p, int dest_cpu)
        unsigned long flags;
        cpumask_t old_mask, new_mask = cpumask_of_cpu(dest_cpu);
 
+       lock_cpu_hotplug();
        rq = task_rq_lock(p, &flags);
        old_mask = p->cpus_allowed;
        if (!cpu_isset(dest_cpu, old_mask) || !cpu_online(dest_cpu))
@@ -1035,6 +1036,7 @@ static void sched_migrate_task(task_t *p, int dest_cpu)
        }
 out:
        task_rq_unlock(rq, &flags);
+       unlock_cpu_hotplug();
 }
 
 /*
@@ -2309,11 +2311,13 @@ asmlinkage long sys_sched_setaffinity(pid_t pid, unsigned int len,
        if (copy_from_user(&new_mask, user_mask_ptr, sizeof(new_mask)))
                return -EFAULT;
 
+       lock_cpu_hotplug();
        read_lock(&tasklist_lock);
 
        p = find_process_by_pid(pid);
        if (!p) {
                read_unlock(&tasklist_lock);
+               unlock_cpu_hotplug();
                return -ESRCH;
        }
 
@@ -2334,6 +2338,7 @@ asmlinkage long sys_sched_setaffinity(pid_t pid, unsigned int len,
 
 out_unlock:
        put_task_struct(p);
+       unlock_cpu_hotplug();
        return retval;
 }