On Wed, 10 Oct 2007, Paul Menage wrote:
> On 10/6/07, David Rientjes <[EMAIL PROTECTED]> wrote:
> >
> > It can race with sched_setaffinity(). It has to give up tasklist_lock as
> > well to call set_cpus_allowed() and can race
> >
> > cpus_allowed = cpuset_cpus_allowed(p);
> > cpus_and(new_mask, new_mask, cpus_allowed);
> > retval = set_cpus_allowed(p, new_mask);
> >
> > and allow a task to have a cpu outside of the cpuset's new cpus_allowed if
> > you've taken it away between cpuset_cpus_allowed() and set_cpus_allowed().
>
> cpuset_cpus_allowed() takes callback_mutex, which is held by
> update_cpumask() when it updates cs->cpus_allowed. So if we continue
> to hold callback_mutex across the task update loop this wouldn't be a
> race. Having said that, holding callback mutex for that long might not
> be a good idea.
>
Moving the actual use of set_cpus_allowed() from the cgroup code to
sched.c would probably be the best in terms of a clean interface. Then
you could protect the whole thing by sched_hotcpu_mutex, which is
expressly designed for migrations.
Something like this:
struct cgroup_iter it;
struct task_struct *p, **tasks;
int i = 0;
cgroup_iter_start(cs->css.cgroup, &it);
while ((p = cgroup_iter_next(cs->css.cgroup, &it))) {
get_task_struct(p);
tasks[i++] = p;
}
cgroup_iter_end(cs->css.cgroup, &it);
while (--i >= 0) {
sched_migrate_task(tasks[i], cs->cpus_allowed);
put_task_struct(tasks[i]);
}
kernel/sched.c:
void sched_migrate_task(struct task_struct *task,
cpumask_t cpus_allowed)
{
mutex_lock(&sched_hotcpu_mutex);
set_cpus_allowed(task, cpus_allowed);
mutex_unlock(&sched_hotcpu_mutex);
}
It'd be faster to just pass **tasks to a sched.c function with the number
of tasks to migrate to reduce the contention on sched_hotcpu_mutex, but
then the put_task_struct() is left dangling over in cgroup code.
sched_hotcpu_mutex should be rarely contended, anyway.
David
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