* Don Zickus <dzic...@redhat.com> wrote:
> From: chai wen <chaiw.f...@cn.fujitsu.com>
>
> 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 pid of the hogging process and use that to reset
> soft_watchdog_warn too.
>
> Signed-off-by: chai wen <chaiw.f...@cn.fujitsu.com>
> [modified the comment and changelog to be more specific]
> Signed-off-by: Don Zickus <dzic...@redhat.com>
> ---
> kernel/watchdog.c | 20 ++++++++++++++++++--
> 1 files changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 4c2e11c..6d0a891 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 +329,20 @@ 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) {
> +
> + /*
> + * 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 pids to detect this.
> + */
> + if (__this_cpu_read(softlockup_warn_pid_saved) != pid) {
So I agree with the motivation of this improvement, but is this
implementation namespace-safe?
Thanks,
Ingo
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