Re: [PATCH 1/5] rcutorture: Fix stuttering races and other issues

[Joel Fernandes]

On Wed, Jul 26, 2023 at 4:59 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Tue, Jul 25, 2023 at 11:29:06PM +0000, Joel Fernandes (Google) wrote:
> > The stuttering code isn't functioning as expected. Ideally, it should
> > pause the torture threads for a designated period before resuming. Yet,
> > it fails to halt the test for the correct duration. Additionally, a race
> > condition exists, potentially causing the stuttering code to pause for
> > an extended period if the 'spt' variable is non-zero due to the stutter
> > orchestration thread's inadequate CPU time.
> >
> > Moreover, over-stuttering can hinder RCU's progress on TREE07 kernels.
> > This happens as the stuttering code may run within a softirq due to RCU
> > callbacks. Consequently, ksoftirqd keeps a CPU busy for several seconds,
> > thus obstructing RCU's progress. This situation triggers a warning
> > message in the logs:
> >
> > [ 2169.481783] rcu_torture_writer: rtort_pipe_count: 9
> >
> > This warning suggests that an RCU torture object, although invisible to
> > RCU readers, couldn't make it past the pipe array and be freed -- a
> > strong indication that there weren't enough grace periods during the
> > stutter interval.
> >
> > To address these issues, this patch sets the "stutter end" time to an
> > absolute point in the future set by the main stutter thread. This is
> > then used for waiting in stutter_wait(). While the stutter thread still
> > defines this absolute time, the waiters' waiting logic doesn't rely on
> > the stutter thread receiving sufficient CPU time to halt the stuttering
> > as the halting is now self-controlled.
> >
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > ---
> >  kernel/torture.c | 46 +++++++++++++---------------------------------
> >  1 file changed, 13 insertions(+), 33 deletions(-)
> >
> > diff --git a/kernel/torture.c b/kernel/torture.c
> > index 68dba4ecab5c..63f8f2a7d960 100644
> > --- a/kernel/torture.c
> > +++ b/kernel/torture.c
> > @@ -719,7 +719,7 @@ static void torture_shutdown_cleanup(void)
> >   * suddenly applied to or removed from the system.
> >   */
> >  static struct task_struct *stutter_task;
> > -static int stutter_pause_test;
> > +static ktime_t stutter_till_abs_time;
> >  static int stutter;
> >  static int stutter_gap;
> >
> > @@ -729,30 +729,17 @@ static int stutter_gap;
> >   */
> >  bool stutter_wait(const char *title)
> >  {
> > -     unsigned int i = 0;
> >       bool ret = false;
> > -     int spt;
> > +     ktime_t now_ns, till_ns;
> >
> >       cond_resched_tasks_rcu_qs();
> > -     spt = READ_ONCE(stutter_pause_test);
> > -     for (; spt; spt = READ_ONCE(stutter_pause_test)) {
> > -             if (!ret && !rt_task(current)) {
> > -                     sched_set_normal(current, MAX_NICE);
> > -                     ret = true;
> > -             }
> > -             if (spt == 1) {
> > -                     torture_hrtimeout_jiffies(1, NULL);
> > -             } else if (spt == 2) {
> > -                     while (READ_ONCE(stutter_pause_test)) {
> > -                             if (!(i++ & 0xffff))
> > -                                     torture_hrtimeout_us(10, 0, NULL);
> > -                             cond_resched();
> > -                     }
> > -             } else {
> > -                     torture_hrtimeout_jiffies(round_jiffies_relative(HZ), NULL);
> > -             }
> > -             torture_shutdown_absorb(title);
> > +     now_ns = ktime_get();
> > +     till_ns = READ_ONCE(stutter_till_abs_time);
> > +     if (till_ns && ktime_before(now_ns, till_ns)) {
> > +             torture_hrtimeout_ns(ktime_sub(till_ns, now_ns), 0, NULL);
>
> This ktime_sub() is roughly cancelled out by a ktime_add_safe() in
> __hrtimer_start_range_ns().

Yes, functionally it is the same but your suggestion is more robust I think.

> Perhaps torture_hrtimeout_ns() needs to
> take a mode argument as in the patch at the end of this email, allowing
> you to ditch that ktime_sub() in favor of HRTIMER_MODE_ABS.

Sure, or we can add a new API and keep the default as relative?

Or have 2 APIs:
torture_hrtimeout_relative_ns();

and:
torture_hrtimeout_absolute_ns();

That makes it more readable IMHO.

Also, do you want me to make both changes (API and usage) in the same
patch? Or were you planning to have a separate patch yourself in -dev
which I can use? Let me know either way, and then I'll refresh the
patch.

thanks,

 - Joel

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