linux-kernel - Re: [RFC] Make need_resched() return true when rcu_urgent

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Message-Id: <20180716154015.GA21419@linux.vnet.ibm.com>
Date:   Mon, 16 Jul 2018 08:40:15 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        Peter Zijlstra <peterz@...radead.org>, mhillenb@...zon.de,
        linux-kernel <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs
 requested

On Thu, Jul 12, 2018 at 09:17:04AM -0700, Paul E. McKenney wrote:
> On Thu, Jul 12, 2018 at 05:53:51AM -0700, Paul E. McKenney wrote:
> > On Thu, Jul 12, 2018 at 01:00:42PM +0100, David Woodhouse wrote:
> > > 
> > > 
> > > On Wed, 2018-07-11 at 14:08 -0700, Paul E. McKenney wrote:
> > > > 
> > > > > Also... why in $DEITY's name was the existing
> > > > > rcu_virt_note_context_switch() not actually sufficient? If we had that
> > > > > there, why did we need an additional explicit calls to rcu_all_qs() in
> > > > > the KVM loop, or the more complex fixes to need_resched() which
> > > > > ultimately had the same effect, to avoid ten-second latencies?
> > > > 
> > > > My guess is that this was because control passed through the
> > > > rcu_virt_note_context_switch() only once, and then subsequent
> > > > scheduling-clock interrupts bypassed this code.
> > 
> > Gah!  My guess was instead that the code did a rcu_kvm_enter() going in,
> > but somehow managed to miss the rcu_kvm_exit() going out.  But that makes
> > absolutely no sense -- had that happened, rcutorture would likely have
> > screamed bloody murder, loudly and often.  No mere near misses!
> > 
> > And besides, thus far, -ENOREPRODUCE.  :-/
> 
> OK, one close call in 63 hours of rcutorture, this one on scenario TREE03
> (yesterday hit TREE01 and TREE03).  Time for probabilitistic long-runtime
> bisection.  Plus thought about how to get more information out of the near
> misses.  Fun!  ;-)

Most of the weekend was devoted to testing today's upcoming pull request,
but I did get a bit more testing done on this.

I was able to make this happen more often by tweaking rcutorture a
bit, but I still do not yet have statistically significant results.
Nevertheless, I have thus far only seen failures with David's patch or
with both David's and my patch.  And I actually got a full-up rcutorture
failure (a too-short grace period) in addition to the aforementioned
close calls.

Over this coming week I expect to devote significant testing time to
the commit just prior to David's in my stack.  If I don't see failures
on that commit, we will need to spent some quality time with the KVM
folks on whether or not kvm_x86_ops->run() and friends have the option of
failing to return, but instead causing control to pop up somewhere else.
Or someone could tell me how I am being blind to some obvious bug in
the two commits that allow RCU to treat KVM guest-OS execution as an
extended quiescent state.  ;-)

							Thanx, Paul