[PATCH] watchdog/mm: Allow dumping memory info in pretimeout

* [PATCH] watchdog/mm: Allow dumping memory info in pretimeout
@ 2023-06-09  6:45 Vincent Whitchurch
  2023-06-09  9:01 ` kernel test robot
                   ` (2 more replies)
  0 siblings, 3 replies; 4+ messages in thread
From: Vincent Whitchurch @ 2023-06-09  6:45 UTC (permalink / raw)
  To: Wim Van Sebroeck, Guenter Roeck, Andrew Morton
  Cc: linux-watchdog, linux-kernel, linux-mm, kernel, Vincent Whitchurch

On my (embedded) systems, the most common cause of hitting the watchdog
(pre)timeout is due to thrashing.  Diagnosing these problems is hard
without knowing the memory state at the point of the watchdog hit.  In
order to make this information available, add a module parameter to the
watchdog pretimeout panic governor to ask it to dump memory info and the
OOM task list (using a new helper in the OOM code) before triggering the
panic.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
---
 drivers/watchdog/pretimeout_panic.c | 15 +++++++++++
 include/linux/oom.h                 |  5 ++++
 include/linux/sched/task.h          |  5 ++++
 mm/oom_kill.c                       | 54 ++++++++++++++++++++++++++++++++++++-
 4 files changed, 78 insertions(+), 1 deletion(-)

diff --git a/drivers/watchdog/pretimeout_panic.c b/drivers/watchdog/pretimeout_panic.c
index 2cc3c41d2be5b..52d686fa541c7 100644
--- a/drivers/watchdog/pretimeout_panic.c
+++ b/drivers/watchdog/pretimeout_panic.c
@@ -5,10 +5,15 @@
 
 #include <linux/kernel.h>
 #include <linux/module.h>
+#include <linux/mm.h>
+#include <linux/oom.h>
 #include <linux/watchdog.h>
 
 #include "watchdog_pretimeout.h"
 
+static unsigned long dump_min_rss_bytes;
+module_param(dump_min_rss_bytes, ulong, 0644);
+
 /**
  * pretimeout_panic - Panic on watchdog pretimeout event
  * @wdd - watchdog_device
@@ -17,6 +22,16 @@
  */
 static void pretimeout_panic(struct watchdog_device *wdd)
 {
+	/*
+	 * Since the root cause is not certain to be low memory, only print
+	 * tasks with RSS above a configurable limit, to avoid losing
+	 * potentially more important messages from the log.
+	 */
+	if (dump_min_rss_bytes) {
+		show_mem(SHOW_MEM_FILTER_NODES, NULL);
+		oom_dump_tasks(DIV_ROUND_UP(dump_min_rss_bytes, PAGE_SIZE));
+	}
+
 	panic("watchdog pretimeout event\n");
 }
 
diff --git a/include/linux/oom.h b/include/linux/oom.h
index 7d0c9c48a0c54..1451fe2c38d78 100644
--- a/include/linux/oom.h
+++ b/include/linux/oom.h
@@ -52,6 +52,9 @@ struct oom_control {
 
 	/* Used to print the constraint info. */
 	enum oom_constraint constraint;
+
+	bool dump_trylock;
+	unsigned long dump_min_rss_pages;
 };
 
 extern struct mutex oom_lock;
@@ -102,6 +105,8 @@ long oom_badness(struct task_struct *p,
 
 extern bool out_of_memory(struct oom_control *oc);
 
+extern void oom_dump_tasks(unsigned long min_rss_pages);
+
 extern void exit_oom_victim(void);
 
 extern int register_oom_notifier(struct notifier_block *nb);
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index e0f5ac90a228b..e8a68b2a3e829 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -183,6 +183,11 @@ static inline void task_lock(struct task_struct *p)
 	spin_lock(&p->alloc_lock);
 }
 
+static inline int task_trylock(struct task_struct *p)
+{
+	return spin_trylock(&p->alloc_lock);
+}
+
 static inline void task_unlock(struct task_struct *p)
 {
 	spin_unlock(&p->alloc_lock);
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 044e1eed720ee..87e554a1c152f 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -149,6 +149,30 @@ struct task_struct *find_lock_task_mm(struct task_struct *p)
 	return t;
 }
 
+/*
+ * Identical to the above, except that we avoid tasks which we can't lock, to
+ * avoid deadlocks when called from an interrupt handler.
+ */
+struct task_struct *find_trylock_task_mm(struct task_struct *p)
+{
+	struct task_struct *t;
+
+	rcu_read_lock();
+
+	for_each_thread(p, t) {
+		if (!task_trylock(t))
+			continue;
+		if (likely(t->mm))
+			goto found;
+		task_unlock(t);
+	}
+	t = NULL;
+found:
+	rcu_read_unlock();
+
+	return t;
+}
+
 /*
  * order == -1 means the oom kill is required by sysrq, otherwise only
  * for display purposes.
@@ -390,15 +414,26 @@ static int dump_task(struct task_struct *p, void *arg)
 	if (!is_memcg_oom(oc) && !oom_cpuset_eligible(p, oc))
 		return 0;
 
-	task = find_lock_task_mm(p);
+	task = oc->dump_trylock ? find_trylock_task_mm(p) :
+				  find_lock_task_mm(p);
 	if (!task) {
 		/*
 		 * All of p's threads have already detached their mm's. There's
 		 * no need to report them; they can't be oom killed anyway.
+		 *
+		 * Or we got here from an interrupt and the task lock is
+		 * locked, in which case we're forced to ignore this task to
+		 * avoid deadlocks.
 		 */
 		return 0;
 	}
 
+	if (oc->dump_min_rss_pages &&
+	    get_mm_rss(task->mm) < oc->dump_min_rss_pages) {
+		task_unlock(task);
+		return 0;
+	}
+
 	pr_info("[%7d] %5d %5d %8lu %8lu %8ld %8lu         %5hd %s\n",
 		task->pid, from_kuid(&init_user_ns, task_uid(task)),
 		task->tgid, task->mm->total_vm, get_mm_rss(task->mm),
@@ -437,6 +472,23 @@ static void dump_tasks(struct oom_control *oc)
 	}
 }
 
+void oom_dump_tasks(unsigned long min_rss_pages)
+{
+	const gfp_t gfp_mask = GFP_KERNEL;
+	struct oom_control oc = {
+		.zonelist = node_zonelist(first_memory_node, gfp_mask),
+		.nodemask = NULL,
+		.memcg = NULL,
+		.gfp_mask = gfp_mask,
+		.order = -1,
+		.dump_min_rss_pages = min_rss_pages,
+		.dump_trylock = in_interrupt(),
+	};
+
+	dump_tasks(&oc);
+}
+EXPORT_SYMBOL_GPL(oom_dump_tasks);
+
 static void dump_oom_summary(struct oom_control *oc, struct task_struct *victim)
 {
 	/* one line summary of the oom killer context. */

---
base-commit: 9561de3a55bed6bdd44a12820ba81ec416e705a7
change-id: 20230608-pretimeout-oom-99148438a1df

Best regards,
-- 
Vincent Whitchurch <vincent.whitchurch@axis.com>


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