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Date:   Mon, 24 Jul 2023 00:32:57 +0000
From:   Joel Fernandes <joel@joelfernandes.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        rcu@vger.kernel.org, Greg KH <gregkh@linuxfoundation.org>
Subject: Re: [BUG] Re: Linux 6.4.4
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On Sun, Jul 23, 2023 at 10:19:27AM -0700, Paul E. McKenney wrote:
> On Sun, Jul 23, 2023 at 10:50:26AM -0400, Joel Fernandes wrote:
> > 
> > 
> > On 7/22/23 13:27, Paul E. McKenney wrote:
> > [..]
> > > 
> > > OK, if this kernel is non-preemptible, you are not running TREE03,
> > > correct?
> > > 
> > >> Next plan of action is to get sched_waking stack traces since I have a
> > >> very reliable repro of this now.
> > > 
> > > Too much fun!  ;-)
> > 
> > For TREE07 issue, it is actually the schedule_timeout_interruptible(1)
> > in stutter_wait() that is beating up the CPU0 for 4 seconds.
> > 
> > This is very similar to the issue I fixed in New year in d52d3a2bf408
> > ("torture: Fix hang during kthread shutdown phase")
> 
> Agreed, if there are enough kthreads, and all the kthreads are on a
> single CPU, this could consume that CPU.
> 
> > Adding a cond_resched() there also did not help.
> > 
> > I think the issue is the stutter thread fails to move spt forward
> > because it does not get CPU time. But spt == 1 should be very brief
> > AFAIU. I was wondering if we could set that to RT.
> 
> Or just use a single hrtimer-based wait for each kthread?

[Joel]
Yes this might be better, but there's still the issue that spt may not be set
back to 0 in some future release where the thread gets starved.

> > But also maybe the following will cure it like it did for the shutdown
> > issue, giving the stutter thread just enough CPU time to move spt forward.
> > 
> > Now I am trying the following and will let it run while I go do other
> > family related things. ;)
> 
> Good point, if this avoids the problem, that gives a strong indication
> that your hypothesis on the root cause is correct.

[Joel]
And the TREE07 issue is gone with that change! So I think I'll roll into a
patch and send it to you. But I am also hoping that you are Ok with me
setting the stutter thread to RT in addition to the longer schedule_timeout.
That's just to make it more robust since I think it is crucial that it does
not stutter threads indefinitely due to the scheduler (for any unforeseen
reason in the future, such as scheduler issues). And maybe, as a part of
that I could also tackle that other TODO item about cleaning up
torture_create_kthead() as well to add support to it for setting things to
RT and use it for that.

Let me know what you think, thanks!

 - Joel