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    SubjectRe: [PATCH 2/5] softlockup: make detector be aware of task switch of processes hogging cpu

    * Don Zickus <dzickus@redhat.com> wrote:

    > On Mon, Aug 18, 2014 at 11:03:19AM +0200, Ingo Molnar wrote:
    > > * Don Zickus <dzickus@redhat.com> wrote:
    > >
    > > > From: chai wen <chaiw.fnst@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.fnst@cn.fujitsu.com>
    > > > [modified the comment and changelog to be more specific]
    > > > Signed-off-by: Don Zickus <dzickus@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?
    >
    > What namespace are you worried about colliding with? I thought
    > softlockup_ would provide the safety?? Maybe I am missing something
    > obvious. :-(

    I meant PID namespaces - a PID in itself isn't guaranteed to be
    unique across the system.

    Thanks,

    Ingo


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