Re: [PATCH 0/3] rcu: synchronize_rcu[_expedited]() related fixes

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Marco Elver <elver@google.com>,
	Neeraj Upadhyay <quic_neeraju@quicinc.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Uladzislau Rezki <uladzislau.rezki@sony.com>,
	Joel Fernandes <joel@joelfernandes.org>
Subject: Re: [PATCH 0/3] rcu: synchronize_rcu[_expedited]() related fixes
Date: Mon, 14 Mar 2022 15:35:04 -0700	[thread overview]
Message-ID: <20220314223504.GA1150279@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <20220314202610.GW4285@paulmck-ThinkPad-P17-Gen-1>

On Mon, Mar 14, 2022 at 01:26:10PM -0700, Paul E. McKenney wrote:
> On Mon, Mar 14, 2022 at 02:37:35PM +0100, Frederic Weisbecker wrote:
> > 
> > A few fixes especially for expedited GP polling causing a stall on TREE07,
> > as reported by Paul.
> > 
> > We may still want to optimize start_poll_synchronize_rcu_expedited() on
> > UP-no-preempt but I think Paul may be implying this while doing other
> > fixes.
> > 
> > git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
> > 	rcu/dev
> > 
> > HEAD: 6e5fd7e614fd5c8f0fffeaa140b7ea697bfeb096
> > 
> > Thanks,
> > 	Frederic
> 
> I have pulled these in for review and testing, thank you!!!  I have
> started a ~90-minute test and will let you know how that goes.

And TREE05 doesn't like this much.  I get too-short grace periods.
TREE05 is unusual in being the one with the kernel build with
CONFIG_PREEMPT_NONE=y but also with CONFIG_PREEMPT_DYNAMIC=n.

Running tests of the commits individually.

							Thanx, Paul

> > ---
> > 
> > Frederic Weisbecker (2):
> >       rcu: Fix expedited GP polling against UP/no-preempt environment
> 
> I have some concerns with this one due to the fact that it acquires locks
> in cases where the old code would not.  (I did have problems with this
> in both recent SRCU changes and the normal-grace-period counterpart to
> this series.)
> 
> But let's see what rcutorture and kbuild test robot think about it.
> 
> >       rcu: Fix preemption mode check on synchronize_rcu[_expedited]()
> 
> This one looks good.
> 
> > Valentin Schneider (1):
> >       preempt/dynamic: Introduce preempt mode accessors
> 
> I am guessing that this one is a compact placeholder for my convenience
> (in which case thank you!).  I will be marking it "EXP" on my next rebase.
> 
> 							Thanx, Paul
> 
> >  include/linux/sched.h | 16 +++++++++++++++
> >  kernel/rcu/tree.c     |  2 +-
> >  kernel/rcu/tree_exp.h | 57 +++++++++++++++++++++++++++++++--------------------
> >  kernel/sched/core.c   | 11 ++++++++++
> >  4 files changed, 63 insertions(+), 23 deletions(-)