linux-kernel - [PATCH 2/2] softlockup: make detector be aware of task switch of processes hogging cpu

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Date:	Mon, 4 Aug 2014 15:36:19 +0800
From:	chai wen <chaiw.fnst@...fujitsu.com>
To:	<linux-kernel@...r.kernel.org>
CC:	<dzickus@...hat.com>, <mingo@...hat.com>,
	chai wen <chaiw.fnst@...fujitsu.com>
Subject: [PATCH 2/2] softlockup: make detector be aware of task switch of processes hogging cpu


For now, soft lockup detector warns once for each case of process softlockup.
But the thread 'watchdog/n' may can not always get cpu at the time slot between
the task switch of two processes hogging that cpu.
This case is a false negative of "warn only once for a process", as there may be
a different process that is going to hog the cpu. Is is better for detector to
be aware of it. 

Signed-off-by: chai wen <chaiw.fnst@...fujitsu.com>
---
 kernel/watchdog.c |   18 ++++++++++++++++--
 1 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 4c2e11c..908050c 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -42,6 +42,7 @@ static DEFINE_PER_CPU(bool, softlockup_touch_sync);
 static DEFINE_PER_CPU(bool, soft_watchdog_warn);
 static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts);
 static DEFINE_PER_CPU(unsigned long, soft_lockup_hrtimer_cnt);
+static DEFINE_PER_CPU(pid_t, softlockup_warn_pid_saved);
 #ifdef CONFIG_HARDLOCKUP_DETECTOR
 static DEFINE_PER_CPU(bool, hard_watchdog_warn);
 static DEFINE_PER_CPU(bool, watchdog_nmi_touch);
@@ -317,6 +318,8 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 	 */
 	duration = is_softlockup(touch_ts);
 	if (unlikely(duration)) {
+		pid_t pid = task_pid_nr(current);
+
 		/*
 		 * If a virtual machine is stopped by the host it can look to
 		 * the watchdog like a soft lockup, check to see if the host
@@ -326,8 +329,18 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 			return HRTIMER_RESTART;
 
 		/* only warn once */
-		if (__this_cpu_read(soft_watchdog_warn) == true)
+		if (__this_cpu_read(soft_watchdog_warn) == true) {
+			/*
+			 * soft lockup detector should be aware of that there
+			 * may be a task-swicth of two different processes
+			 * hogging the cpu continously
+			 */
+			if (__this_cpu_read(softlockup_warn_pid_saved) != pid) {
+				__this_cpu_write(soft_watchdog_warn, false);
+				__touch_watchdog();
+			}
 			return HRTIMER_RESTART;
+		}
 
 		if (softlockup_all_cpu_backtrace) {
 			/* Prevent multiple soft-lockup reports if one cpu is already
@@ -342,7 +355,8 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 
 		printk(KERN_EMERG "BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n",
 			smp_processor_id(), duration,
-			current->comm, task_pid_nr(current));
+			current->comm, pid);
+		__this_cpu_write(softlockup_warn_pid_saved, pid);
 		print_modules();
 		print_irqtrace_events(current);
 		if (regs)
-- 
1.7.1

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