[PATCH] softlockup: Make detector be aware of task switch of processes hogging cpu

From: Don Zickus
Date: Thu Aug 28 2014 - 00:52:56 EST

From: chai wen <chaiw.fnst@xxxxxxxxxxxxxx>

For now, soft lockup detector warns once for each case of process softlockup.
But the thread 'watchdog/n' may not always get the cpu at the time slot between
the task switch of two processes hogging that cpu to reset soft_watchdog_warn.

An example would be two processes hogging the cpu. Process A causes the
softlockup warning and is killed manually by a user. Process B immediately
becomes the new process hogging the cpu preventing the softlockup code from
resetting the soft_watchdog_warn variable.

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. Resolve this by
saving/checking the task pointer of the hogging process and use that to reset
soft_watchdog_warn too.

Signed-off-by: chai wen <chaiw.fnst@xxxxxxxxxxxxxx>
Signed-off-by: Don Zickus <dzickus@xxxxxxxxxx>
kernel/watchdog.c | 16 +++++++++++++++-
1 files changed, 15 insertions(+), 1 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index c3319bd..499f65f 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -47,6 +47,7 @@ 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(struct task_struct *, softlockup_task_ptr_saved);
static DEFINE_PER_CPU(bool, hard_watchdog_warn);
static DEFINE_PER_CPU(bool, watchdog_nmi_touch);
@@ -331,8 +332,20 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)

/* only warn once */
- if (__this_cpu_read(soft_watchdog_warn) == true)
+ if (__this_cpu_read(soft_watchdog_warn) == true) {
+ /*
+ * Handle the case where multiple processes are
+ * causing softlockups but the duration is small
+ * enough, the softlockup detector can not reset
+ * itself in time. Use task pointers to detect this.
+ */
+ if (__this_cpu_read(softlockup_task_ptr_saved) !=
+ current) {
+ __this_cpu_write(soft_watchdog_warn, false);
+ __touch_watchdog();
+ }
+ }

if (softlockup_all_cpu_backtrace) {
/* Prevent multiple soft-lockup reports if one cpu is already
@@ -348,6 +361,7 @@ 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));
+ __this_cpu_write(softlockup_task_ptr_saved, current);
if (regs)

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