From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: David Woodhouse <dwmw2@infradead.org>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>,
Peter Zijlstra <peterz@infradead.org>,
mhillenb@amazon.de, linux-kernel <linux-kernel@vger.kernel.org>,
kvm <kvm@vger.kernel.org>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested
Date: Wed, 18 Jul 2018 08:36:28 -0700 [thread overview]
Message-ID: <20180718153628.GA24359@linux.vnet.ibm.com> (raw)
In-Reply-To: <20180717125653.GH12945@linux.vnet.ibm.com>
On Tue, Jul 17, 2018 at 05:56:53AM -0700, Paul E. McKenney wrote:
> On Tue, Jul 17, 2018 at 10:19:08AM +0200, David Woodhouse wrote:
> > On Mon, 2018-07-16 at 08:40 -0700, Paul E. McKenney wrote:
> > > 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. ;-)
> >
> > One thing we can try, if my patch is implicated, is moving the calls to
> > rcu_kvm_en{ter,xit} closer to the actual VM entry. Let's try putting
> > them around the large asm block in arch/x86/kvm/vmx.c::vmx_vcpu_run()
> > for example. If that fixes it, then we know we've missed something else
> > interesting that's happening in the middle.
>
> I don't have enough data to say anything with too much certainty, but
> my patch has rcu_kvm_en{ter,xit}() quite a bit farther apart than yours
> does, and I am not seeing massive increases in error rate in my patch
> compared to yours. Which again might or might not mean anything.
>
> Plus I haven't proven that your patch isn't an innocent bystander yet.
> If it isn't just an innocent bystander, that will take most of this
> week do demonstrate given current failure rates.
And I finally did get some near misses from an earlier commit, so we
should consider your patch to be officially off the hook.
Thanx, Paul
> I am also working on improving rcutorture diagnostics which should help
> me work out how to change rcutorture so as to find this more quickly.
>
> > Testing on Skylake shows a guest CPUID goes from ~3000 cycles to ~3500
> > with this patch, so in the next iteration it definitely needs to be
> > ifdef CONFIG_NO_HZ_FULL anyway, because it's actually required there
> > (AFAICT) and it's too expensive otherwise as Christian pointed out.
>
> Makes sense!
>
> Thanx, Paul
next prev parent reply other threads:[~2018-07-18 15:34 UTC|newest]
Thread overview: 93+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-06 14:53 [RFC] Make need_resched() return true when rcu_urgent_qs requested David Woodhouse
2018-07-06 16:29 ` Peter Zijlstra
2018-07-06 17:11 ` Paul E. McKenney
2018-07-06 17:14 ` David Woodhouse
2018-07-06 21:12 ` Paul E. McKenney
2018-07-09 8:58 ` Peter Zijlstra
2018-07-09 8:53 ` Peter Zijlstra
2018-07-09 9:18 ` David Woodhouse
2018-07-09 10:44 ` Peter Zijlstra
2018-07-09 10:56 ` David Woodhouse
2018-07-09 11:06 ` Peter Zijlstra
2018-07-09 11:12 ` David Woodhouse
2018-07-09 11:31 ` Peter Zijlstra
2018-07-09 12:34 ` Paul E. McKenney
2018-07-09 12:47 ` David Woodhouse
2018-07-09 14:30 ` Paul E. McKenney
2018-07-09 12:55 ` Peter Zijlstra
2018-07-09 12:57 ` David Woodhouse
2018-07-09 13:02 ` Peter Zijlstra
2018-07-09 14:29 ` Paul E. McKenney
2018-07-09 14:43 ` Peter Zijlstra
2018-07-09 14:54 ` Paul E. McKenney
2018-07-09 15:26 ` Peter Zijlstra
2018-07-09 16:34 ` Paul E. McKenney
2018-07-09 16:44 ` Paul E. McKenney
2018-07-09 18:50 ` David Woodhouse
2018-07-09 20:34 ` Paul E. McKenney
2018-07-09 20:35 ` David Woodhouse
2018-07-09 20:42 ` Paul E. McKenney
2018-07-09 20:45 ` David Woodhouse
2018-07-09 21:05 ` Paul E. McKenney
2018-07-09 22:08 ` Paul E. McKenney
2018-07-11 10:57 ` David Woodhouse
2018-07-11 12:51 ` Paul E. McKenney
2018-07-11 12:58 ` David Woodhouse
2018-07-11 14:25 ` Paul E. McKenney
2018-07-11 14:23 ` David Woodhouse
2018-07-11 14:43 ` Paul E. McKenney
2018-07-11 16:49 ` Paul E. McKenney
2018-07-11 17:03 ` David Woodhouse
2018-07-11 17:48 ` Paul E. McKenney
2018-07-11 18:01 ` [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode David Woodhouse
2018-07-11 18:20 ` Paul E. McKenney
2018-07-11 18:36 ` Paul E. McKenney
2018-07-11 18:39 ` Christian Borntraeger
2018-07-11 20:27 ` Paul E. McKenney
2018-07-11 20:54 ` David Woodhouse
2018-07-11 21:09 ` Paul E. McKenney
2018-07-11 21:11 ` Christian Borntraeger
2018-07-11 21:32 ` Paul E. McKenney
2018-07-11 21:39 ` Christian Borntraeger
2018-07-11 23:47 ` Paul E. McKenney
2018-07-12 8:31 ` David Woodhouse
2018-07-12 11:00 ` Christian Borntraeger
2018-07-12 11:10 ` David Woodhouse
2018-07-12 11:58 ` Christian Borntraeger
2018-07-12 12:04 ` Christian Borntraeger
2018-07-11 23:37 ` Paul E. McKenney
2018-07-12 2:15 ` Paul E. McKenney
2018-07-12 6:21 ` Christian Borntraeger
2018-07-12 9:52 ` David Woodhouse
2018-07-11 18:31 ` [RFC] Make need_resched() return true when rcu_urgent_qs requested Christian Borntraeger
2018-07-11 20:17 ` Paul E. McKenney
2018-07-11 20:19 ` David Woodhouse
2018-07-11 21:08 ` Paul E. McKenney
2018-07-12 12:00 ` David Woodhouse
2018-07-12 12:53 ` Paul E. McKenney
2018-07-12 16:17 ` Paul E. McKenney
2018-07-16 15:40 ` Paul E. McKenney
2018-07-17 8:19 ` David Woodhouse
2018-07-17 12:56 ` Paul E. McKenney
2018-07-18 15:36 ` Paul E. McKenney [this message]
2018-07-18 16:01 ` David Woodhouse
2018-07-18 16:37 ` Paul E. McKenney
2018-07-18 19:41 ` David Woodhouse
2018-07-18 20:17 ` Paul E. McKenney
2018-07-19 0:26 ` Frederic Weisbecker
2018-07-19 6:45 ` Christian Borntraeger
2018-07-19 7:20 ` David Woodhouse
2018-07-19 10:23 ` Christian Borntraeger
2018-07-19 12:55 ` Paul E. McKenney
2018-07-19 13:14 ` Frederic Weisbecker
2018-07-19 13:36 ` David Woodhouse
2018-07-19 17:09 ` Paul E. McKenney
2018-07-23 8:08 ` David Woodhouse
2018-07-23 12:22 ` Paul E. McKenney
2018-07-19 0:32 ` Frederic Weisbecker
2018-07-19 3:11 ` Paul E. McKenney
2018-07-19 6:16 ` David Woodhouse
2018-07-19 13:17 ` Frederic Weisbecker
2018-07-19 13:15 ` Frederic Weisbecker
2018-07-10 9:24 ` Peter Zijlstra
2018-07-10 16:26 ` Paul E. McKenney
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