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Date:   Wed, 26 Jul 2023 21:01:33 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     linux-kernel@vger.kernel.org, Davidlohr Bueso <dave@stgolabs.net>,
        Josh Triplett <josh@joshtriplett.org>, rcu@vger.kernel.org
Subject: Re: [PATCH 1/5] rcutorture: Fix stuttering races and other issues
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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@kernel.org> 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@joelfernandes.org>
> > > ---
> > >  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.

							Thanx, Paul