linux-kernel - Re: [PATCH 3/4] OOM, PM: OOM killed task shouldn't escape PM suspend

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Message-ID: <3987583.vdsuvlAsHc@vostro.rjw.lan>
Date:	Wed, 22 Oct 2014 16:39:12 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Cong Wang <xiyou.wangcong@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	Tejun Heo <tj@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH 3/4] OOM, PM: OOM killed task shouldn't escape PM suspend

On Tuesday, October 21, 2014 04:29:39 PM Michal Hocko wrote:
> On Tue 21-10-14 16:41:07, Rafael J. Wysocki wrote:
> > On Tuesday, October 21, 2014 04:11:59 PM Michal Hocko wrote:
> [...]
> > > OK, incremental diff on top. I will post the complete patch if you are
> > > happier with this change
> > 
> > Yes, I am.
> ---
> From 9ab46fe539cded8e7b6425b2cd23ba9184002fde Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@...e.cz>
> Date: Mon, 20 Oct 2014 18:12:32 +0200
> Subject: [PATCH -v2] OOM, PM: OOM killed task shouldn't escape PM suspend
> 
> PM freezer relies on having all tasks frozen by the time devices are
> getting frozen so that no task will touch them while they are getting
> frozen. But OOM killer is allowed to kill an already frozen task in
> order to handle OOM situtation. In order to protect from late wake ups
> OOM killer is disabled after all tasks are frozen. This, however, still
> keeps a window open when a killed task didn't manage to die by the time
> freeze_processes finishes.
> 
> Reduce the race window by checking all tasks after OOM killer has been
> disabled. This is still not race free completely unfortunately because
> oom_killer_disable cannot stop an already ongoing OOM killer so a task
> might still wake up from the fridge and get killed without
> freeze_processes noticing. Full synchronization of OOM and freezer is,
> however, too heavy weight for this highly unlikely case.
> 
> Introduce and check oom_kills counter which gets incremented early when
> the allocator enters __alloc_pages_may_oom path and only check all the
> tasks if the counter changes during the freezing attempt. The counter
> is updated so early to reduce the race window since allocator checked
> oom_killer_disabled which is set by PM-freezing code. A false positive
> will push the PM-freezer into a slow path but that is not a big deal.
> 
> Changes since v1
> - push the re-check loop out of freeze_processes into
>   check_frozen_processes and invert the condition to make the code more
>   readable as per Rafael

I've applied that along with the rest of the series, but what about the
following cleanup patch on top of it?

Rafael


---
 kernel/power/process.c |   31 ++++++++++++++++---------------
 1 file changed, 16 insertions(+), 15 deletions(-)

Index: linux-pm/kernel/power/process.c
===================================================================
--- linux-pm.orig/kernel/power/process.c
+++ linux-pm/kernel/power/process.c
@@ -108,25 +108,27 @@ static int try_to_freeze_tasks(bool user
 	return todo ? -EBUSY : 0;
 }
 
+static bool __check_frozen_processes(void)
+{
+	struct task_struct *g, *p;
+
+	for_each_process_thread(g, p)
+		if (p != current && !freezer_should_skip(p) && !frozen(p))
+			return false;
+
+	return true;
+}
+
 /*
  * Returns true if all freezable tasks (except for current) are frozen already
  */
 static bool check_frozen_processes(void)
 {
-	struct task_struct *g, *p;
-	bool ret = true;
+	bool ret;
 
 	read_lock(&tasklist_lock);
-	for_each_process_thread(g, p) {
-		if (p != current && !freezer_should_skip(p) &&
-		    !frozen(p)) {
-			ret = false;
-			goto done;
-		}
-	}
-done:
+	ret = __check_frozen_processes();
 	read_unlock(&tasklist_lock);
-
 	return ret;
 }
 
@@ -167,15 +169,14 @@ int freeze_processes(void)
 		 * on the way out so we have to double check for race.
 		 */
 		if (oom_kills_count() != oom_kills_saved &&
-				!check_frozen_processes()) {
+		    !check_frozen_processes()) {
 			__usermodehelper_set_disable_depth(UMH_ENABLED);
 			printk("OOM in progress.");
 			error = -EBUSY;
-			goto done;
+		} else {
+			printk("done.");
 		}
-		printk("done.");
 	}
-done:
 	printk("\n");
 	BUG_ON(in_atomic());
 

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