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 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.

[dzickus@redhat.com: update comment]
Signed-off-by: chai wen <chaiw.fnst@cn.fujitsu.com>
Signed-off-by: Don Zickus <dzickus@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 17 insertions, 1 deletions

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index a8d6914030fe..7b223b212683 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);
 #ifdef CONFIG_HARDLOCKUP_DETECTOR
 static DEFINE_PER_CPU(bool, hard_watchdog_warn);
 static DEFINE_PER_CPU(bool, watchdog_nmi_touch);
@@ -333,8 +334,22 @@ 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) {
+                        /*
+                         * When multiple processes are causing softlockups the
+                         * softlockup detector only warns on the first one
+                         * because the code relies on a full quiet cycle to
+                         * re-arm.  The second process prevents the quiet cycle
+                         * and never gets reported.  Use task pointers to detect
+                         * this.
+                         */
+                        if (__this_cpu_read(softlockup_task_ptr_saved) !=
+                            current) {
+                                __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
@@ -350,6 +365,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
                 pr_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);
                 print_modules();
                 print_irqtrace_events(current);
                 if (regs)