On Mon, Aug 04, 2014 at 03:36:19PM +0800, chai wen wrote:
>
> 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.
I am not sure I fully understand the problem resolved.
>From the changelog I understood that two processes bouncing back and forth
could hog the cpu and could create a 'false negative' (a situation not
reported but should).
But looking at the patch below I was a little confused on the
__touch_watchdog addition. See below:
>
> Signed-off-by: chai wen <chaiw.f...@cn.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();
> + }
The above piece is what I am trying to understand. Are you saying that
when two different processes are hogging the cpu, undo the
soft_watchdog_warn and allow the second pid to be reported?
Just trying to understand the problem and see if this is the right
approach (because 3 or more processes could cause the same problem???).
Cheers,
Don
> 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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