Re: [PATCH 5.10 1/3] torture: Fix hang during kthread shutdown phase

[Joel Fernandes <joel@joelfernandes.org>]

On Sun, Aug 13, 2023 at 06:34:27PM +0200, Greg KH wrote:
> On Sun, Aug 13, 2023 at 03:15:34AM +0000, Joel Fernandes (Google) wrote:
> > From: Joel Fernandes <joel@joelfernandes.org>
> > 
> > During shutdown of rcutorture, the shutdown thread in
> > rcu_torture_cleanup() calls torture_cleanup_begin() which sets fullstop
> > to FULLSTOP_RMMOD. This is enough to cause the rcutorture threads for
> > readers and fakewriters to breakout of their main while loop and start
> > shutting down.
> > 
> > Once out of their main loop, they then call torture_kthread_stopping()
> > which in turn waits for kthread_stop() to be called, however
> > rcu_torture_cleanup() has not even called kthread_stop() on those
> > threads yet, it does that a bit later.  However, before it gets a chance
> > to do so, torture_kthread_stopping() calls
> > schedule_timeout_interruptible(1) in a tight loop. Tracing confirmed
> > this makes the timer softirq constantly execute timer callbacks, while
> > never returning back to the softirq exit path and is essentially "locked
> > up" because of that. If the softirq preempts the shutdown thread,
> > kthread_stop() may never be called.
> > 
> > This commit improves the situation dramatically, by increasing timeout
> > passed to schedule_timeout_interruptible() 1/20th of a second. This
> > causes the timer softirq to not lock up a CPU and everything works fine.
> > Testing has shown 100 runs of TREE07 passing reliably, which was not the
> > case before because of RCU stalls.
> > 
> > Cc: Paul McKenney <paulmck@kernel.org>
> > Cc: Frederic Weisbecker <fweisbec@gmail.com>
> > Cc: Zhouyi Zhou <zhouzhouyi@gmail.com>
> > Cc: <stable@vger.kernel.org> # 6.0.x
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
> > Tested-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
> > ---
> >  kernel/torture.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Any hint as to what the git commit id in Linus's tree for this, and the
> other patches you just sent, are?  I kind of need that to keep track of
> things...

Apologies, I added the SHA to the 5.15 ones but not 5.10. Here they are for 5.10:

1/3
d52d3a2bf408ff86f3a79560b5cce80efb340239
("torture: Fix hang during kthread shutdown phase")

2/3
a1ff03cd6fb9c501fff63a4a2bface9adcfa81cd
("tick: Detect and fix jiffies update stall")

3/3
62c1256d544747b38e77ca9b5bfe3a26f9592576
("timers/nohz: Switch to ONESHOT_STOPPED in the low-res handler when the tick is stopped")

In case you wish to pull them in via git, I have uploaded them to:
Git: https://github.com/joelagnel/linux-kernel.git
Branch: rcu/linux-5.10.y.aug13.greg


thanks,

 - Joel