Re: [PATCH 00/11] rcu: Make rcu_core() safe in PREEMPT_RT with NOCB + a few other fixes v2

[Frederic Weisbecker]

On Tue, Oct 12, 2021 at 08:28:32PM -0700, Paul E. McKenney wrote:
> On Tue, Oct 12, 2021 at 05:32:15PM -0700, Paul E. McKenney wrote:
> > On Mon, Oct 11, 2021 at 04:51:29PM +0200, Frederic Weisbecker wrote:
> > > Hi,
> > > 
> > > No code change in this v2, only changelogs:
> > > 
> > > * Add tags from Valentin and Sebastian
> > > 
> > > * Remove last reference to SEGCBLIST_SOFTIRQ_ONLY (thanks Valentin)
> > > 
> > > * Rewrite changelog for "rcu/nocb: Check a stable offloaded state to manipulate qlen_last_fqs_check"
> > >   after off-list debates with Paul.
> > > 
> > > * Remove the scenario with softirq interrupting rcuc on
> > >   "rcu/nocb: Limit number of softirq callbacks only on softirq" as it's
> > >   probably not possible (thanks Valentin).
> > > 
> > > * Remove the scenario with task spent scheduling out accounted on tlimit
> > >   as it's not possible (thanks Valentin)
> > >   (see "rcu: Apply callbacks processing time limit only on softirq")
> > > 
> > > * Fixed changelog of
> > >   "rcu/nocb: Don't invoke local rcu core on callback overload from nocb kthread"
> > >   (thanks Sebastian).
> > > 
> > > git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
> > > 	rcu/rt-v2
> > > 
> > > HEAD: 2c9349986d5f70a555195139665841cd98e9aba4
> > > 
> > > Thanks,
> > > 	Frederic
> > 
> > Nice!
> > 
> > I queued these for further review and testing.  I reworked the commit log
> > of 6/11 to give my idea of the reason, though I freely admit that this
> > reason is not as compelling as it no doubt seemed when I wrote that code.
> 
> But in initial tests TREE04.5, TREE04.6, and TREE04.9 all hit the
> WARN_ON(1) in rcu_torture_barrier(), which indicates rcu_barrier()
> breakage.  My best (but not so good) guess is a five-hour MTBF on a
> dual-socket system.
> 
> I started an automated "git bisect" with each step running 100 hours
> of TREE04, but I would be surprised if anything useful comes of it.
> Pleased, mind you, but surprised.

Ok I can reproduce.

I'm launching a bisect from my side as well.

Thanks.