linux-kernel - Re: [PATCH 1/2] oom: do not live lock on frozen tasks

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Date:	Mon, 26 Sep 2011 13:05:59 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	David Rientjes <rientjes@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 1/2] oom: do not live lock on frozen tasks

On Mon 26-09-11 19:58:50, Rusty Russell wrote:
> On Mon, 26 Sep 2011 10:28:37 +0200, Michal Hocko <mhocko@...e.cz> wrote:
> > On Fri 26-08-11 11:13:40, David Rientjes wrote:
> > > I'd love to be able to do a thaw on a PF_FROZEN task in the oom killer 
> > > followed by a SIGKILL if that task is selected for oom kill without an 
> > > heuristic change.  Not sure if that's possible, so we'll wait for Rafael 
> > > to chime in.
> > 
> > We have discussed that with Rafael and it should be safe to do that. See
> > the patch bellow.
> > The only place I am not entirely sure about is run_guest
> > (drivers/lguest/core.c). It seems that the code is able to cope with
> > signals but it also calls lguest_arch_run_guest after try_to_freeze.
> 
> Yes; if you want to kill things in the refrigerator(), then will a
> 
> 		if (cpu->lg->dead || task_is_dead(current))
>                         break;
> 
> Work?  

The task is not dead yet. We should rather check for pending signals.
Can we just move try_to_freeze up before the pending signals check?

diff --git a/drivers/lguest/core.c b/drivers/lguest/core.c
index 2535933..a513509 100644
--- a/drivers/lguest/core.c
+++ b/drivers/lguest/core.c
@@ -232,6 +232,12 @@ int run_guest(struct lg_cpu *cpu, unsigned long __user *user)
 			}
 		}
 
+		/*
+		 * All long-lived kernel loops need to check with this horrible
+		 * thing called the freezer.  If the Host is trying to suspend,
+		 * it stops us.
+		 */
+		try_to_freeze();
 		/* Check for signals */
 		if (signal_pending(current))
 			return -ERESTARTSYS;
@@ -246,13 +252,6 @@ int run_guest(struct lg_cpu *cpu, unsigned long __user *user)
 			try_deliver_interrupt(cpu, irq, more);
 
 		/*
-		 * All long-lived kernel loops need to check with this horrible
-		 * thing called the freezer.  If the Host is trying to suspend,
-		 * it stops us.
-		 */
-		try_to_freeze();
-
-		/*
 		 * Just make absolutely sure the Guest is still alive.  One of
 		 * those hypercalls could have been fatal, for example.
 		 */

> That break means we return to the read() syscall pretty much
> immediately.
> 
> Thanks for the CC,
> Rusty.

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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