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

[Paul E. McKenney]

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.

							Thanx, Paul

> > ---
> > 
> > Frederic Weisbecker (10):
> >       rcu/nocb: Make local rcu_nocb_lock_irqsave() safe against concurrent deoffloading
> >       rcu/nocb: Prepare state machine for a new step
> >       rcu/nocb: Invoke rcu_core() at the start of deoffloading
> >       rcu/nocb: Make rcu_core() callbacks acceleration (de-)offloading safe
> >       rcu/nocb: Check a stable offloaded state to manipulate qlen_last_fqs_check
> >       rcu/nocb: Use appropriate rcu_nocb_lock_irqsave()
> >       rcu/nocb: Limit number of softirq callbacks only on softirq
> >       rcu: Fix callbacks processing time limit retaining cond_resched()
> >       rcu: Apply callbacks processing time limit only on softirq
> >       rcu/nocb: Don't invoke local rcu core on callback overload from nocb kthread
> > 
> > Thomas Gleixner (1):
> >       rcu/nocb: Make rcu_core() callbacks acceleration preempt-safe
> > 
> > 
> >  include/linux/rcu_segcblist.h | 51 ++++++++++++++++++-------
> >  kernel/rcu/rcu_segcblist.c    | 10 ++---
> >  kernel/rcu/rcu_segcblist.h    | 12 +++---
> >  kernel/rcu/tree.c             | 87 +++++++++++++++++++++++++++++--------------
> >  kernel/rcu/tree.h             | 16 +++++---
> >  kernel/rcu/tree_nocb.h        | 29 ++++++++++++---
> >  6 files changed, 141 insertions(+), 64 deletions(-)