linux-kernel - Re: [PATCH?] Livelock in pick_next_task_fair() / idle

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Date:	Mon, 6 Jul 2015 06:11:51 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Rabin Vincent <rabin.vincent@...s.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>,
	Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH?] Livelock in pick_next_task_fair() / idle_balance()

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@...s.com>
Signed-off-by: Yuyang Du <yuyang.du@...el.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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