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

[Joel Fernandes]

On 7/27/23 00:01, Paul E. McKenney wrote:
> On Wed, Jul 26, 2023 at 11:01:40PM -0400, Joel Fernandes wrote:
> > 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.
>
> I queued the patch on the -rcu tree's "dev" branch.  It turns out that
> torture_hrtimeout_ns() isn't called very many times, so adding the
> parameter was straightforward.  Plus the compiler might well optimize
> it away anyway.

Ok sounds good, I will make use of it in this patch and send it again after testing.

thanks,

 - Joel