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SubjectRe: [PATCH?] Livelock in pick_next_task_fair() / idle_balance()
Hi Peter,

What about this patch?

Adding:
Tested-by: Stefan Ekenberg <stefan.ekenberg@axis.com>

Thanks,
Yuyang

On Mon, Jul 06, 2015 at 06:11:51AM +0800, Yuyang Du wrote:
> On Fri, Jul 03, 2015 at 06:39:28PM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 02, 2015 at 07:25:11AM +0800, Yuyang Du wrote:
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 40a7fcb..f7cc1ef 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -5898,6 +5898,10 @@ static int detach_tasks(struct lb_env *env)
> > > return 0;
> > >
> > > while (!list_empty(tasks)) {
> > > +
> > > + if (env->idle == CPU_NEWLY_IDLE && env->src_rq->nr_running <= 1)
> >
> > Should we make that ->idle != CPU_NOT_IDLE ?
>
> I think including CPU_IDLE is good.
>
> --
> Subject: [PATCH] sched: Avoid pulling all tasks in idle balancing
>
> In idle balancing where a CPU going idle pulls tasks from another CPU,
> a livelock may happen if the CPU pulls all tasks from another, makes
> it idle, and this iterates. So just avoid this.
>
> Reported-by: Rabin Vincent <rabin.vincent@axis.com>
> Signed-off-by: Yuyang Du <yuyang.du@intel.com>
> ---
> kernel/sched/fair.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 40a7fcb..769d591 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5898,6 +5898,13 @@ static int detach_tasks(struct lb_env *env)
> return 0;
>
> while (!list_empty(tasks)) {
> + /*
> + * We don't want to steal all, otherwise we may be treated likewise,
> + * which could at worst lead to a livelock crash.
> + */
> + if (env->idle != CPU_NOT_IDLE && env->src_rq->nr_running <= 1)
> + break;
> +
> p = list_first_entry(tasks, struct task_struct, se.group_node);
>
> env->loop++;
> --
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