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

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Message-ID: <410cf9c7-a993-4e8f-3279-caba802c6a5c@joelfernandes.org>
Date:   Thu, 27 Jul 2023 00:03:25 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     paulmck@...nel.org
Cc:     linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
        Josh Triplett <josh@...htriplett.org>, rcu@...r.kernel.org
Subject: Re: [PATCH 1/5] rcutorture: Fix stuttering races and other issues

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@...nel.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@...lfernandes.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.

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

thanks,

  - Joel