linux-kernel - Re: [PATCH] mm,vmscan: Use accurate values for zone

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Message-Id: <201511031132.GBB09374.JQFOVSFLOtHFMO@I-love.SAKURA.ne.jp>
Date:	Tue, 3 Nov 2015 11:32:06 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	mhocko@...nel.org
Cc:	htejun@...il.com, cl@...ux.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	rientjes@...gle.com, oleg@...hat.com, kwalker@...hat.com,
	akpm@...ux-foundation.org, hannes@...xchg.org,
	vdavydov@...allels.com, skozina@...hat.com, mgorman@...e.de,
	riel@...hat.com
Subject: Re: [PATCH] mm,vmscan: Use accurate values for zone_reclaimable() checks

Tejun Heo wrote:
>                                                                  If
> the possibility of sysrq getting stuck behind concurrency management
> is an issue, queueing them on an unbound or highpri workqueue should
> be good enough.

Regarding SysRq-f, we could do like below. Though I think that converting
the OOM killer into a dedicated kernel thread would allow more things to do
(e.g. Oleg's memory zapping code, my timeout based next victim selection).

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 5381a72..46b951aa 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -47,6 +47,7 @@
 #include <linux/syscalls.h>
 #include <linux/of.h>
 #include <linux/rcupdate.h>
+#include <linux/kthread.h>
 
 #include <asm/ptrace.h>
 #include <asm/irq_regs.h>
@@ -351,27 +352,35 @@ static struct sysrq_key_op sysrq_term_op = {
 	.enable_mask	= SYSRQ_ENABLE_SIGNAL,
 };
 
-static void moom_callback(struct work_struct *ignored)
+static DECLARE_WAIT_QUEUE_HEAD(moom_wait);
+
+static int moom_callback(void *unused)
 {
 	const gfp_t gfp_mask = GFP_KERNEL;
-	struct oom_control oc = {
-		.zonelist = node_zonelist(first_memory_node, gfp_mask),
-		.nodemask = NULL,
-		.gfp_mask = gfp_mask,
-		.order = -1,
-	};
-
-	mutex_lock(&oom_lock);
-	if (!out_of_memory(&oc))
-		pr_info("OOM request ignored because killer is disabled\n");
-	mutex_unlock(&oom_lock);
+	DEFINE_WAIT(wait);
+
+	while (1) {
+		struct oom_control oc = {
+			.zonelist = node_zonelist(first_memory_node, gfp_mask),
+			.nodemask = NULL,
+			.gfp_mask = gfp_mask,
+			.order = -1,
+		};
+
+		prepare_to_wait(&moom_wait, &wait, TASK_INTERRUPTIBLE);
+		schedule();
+		finish_wait(&moom_wait, &wait);
+		mutex_lock(&oom_lock);
+		if (!out_of_memory(&oc))
+			pr_info("OOM request ignored because killer is disabled\n");
+		mutex_unlock(&oom_lock);
+	}
+	return 0;
 }
 
-static DECLARE_WORK(moom_work, moom_callback);
-
 static void sysrq_handle_moom(int key)
 {
-	schedule_work(&moom_work);
+	wake_up(&moom_wait);
 }
 static struct sysrq_key_op sysrq_moom_op = {
 	.handler	= sysrq_handle_moom,
@@ -1116,6 +1125,9 @@ static inline void sysrq_init_procfs(void)
 
 static int __init sysrq_init(void)
 {
+	struct task_struct *task = kthread_run(moom_callback, NULL,
+					       "manual_oom");
+	BUG_ON(IS_ERR(task));
 	sysrq_init_procfs();
 
 	if (sysrq_on())
--
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