From: Christian Borntraeger <borntraeger@de.ibm.com>
To: paulmck@linux.vnet.ibm.com
Cc: David Woodhouse <dwmw2@infradead.org>,
peterz@infradead.org, mhillenb@amazon.de,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode
Date: Thu, 12 Jul 2018 08:21:06 +0200 [thread overview]
Message-ID: <2c0ad6f0-cf2d-acaa-1162-2a3d80952464@de.ibm.com> (raw)
In-Reply-To: <20180711233727.GA9888@linux.vnet.ibm.com>
On 07/12/2018 01:37 AM, Paul E. McKenney wrote:
> On Wed, Jul 11, 2018 at 02:32:59PM -0700, Paul E. McKenney wrote:
>> On Wed, Jul 11, 2018 at 11:11:19PM +0200, Christian Borntraeger wrote:
>>>
>>>
>>> On 07/11/2018 10:27 PM, Paul E. McKenney wrote:
>>>> On Wed, Jul 11, 2018 at 08:39:36PM +0200, Christian Borntraeger wrote:
>>>>>
>>>>>
>>>>> On 07/11/2018 08:36 PM, Paul E. McKenney wrote:
>>>>>> On Wed, Jul 11, 2018 at 11:20:53AM -0700, Paul E. McKenney wrote:
>>>>>>> On Wed, Jul 11, 2018 at 07:01:01PM +0100, David Woodhouse wrote:
>>>>>>>> From: David Woodhouse <dwmw@amazon.co.uk>
>>>>>>>>
>>>>>>>> RCU can spend long periods of time waiting for a CPU which is actually in
>>>>>>>> KVM guest mode, entirely pointlessly. Treat it like the idle and userspace
>>>>>>>> modes, and don't wait for it.
>>>>>>>>
>>>>>>>> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
>>>>>>>
>>>>>>> And idiot here forgot about some of the debugging code in RCU's dyntick-idle
>>>>>>> code. I will reply with a fixed patch.
>>>>>>>
>>>>>>> The code below works just fine as long as you don't enable CONFIG_RCU_EQS_DEBUG,
>>>>>>> so should be OK for testing, just not for mainline.
>>>>>>
>>>>>> And here is the updated code that allegedly avoids splatting when run with
>>>>>> CONFIG_RCU_EQS_DEBUG.
>>>>>>
>>>>>> Thoughts?
>>>>>>
>>>>>> Thanx, Paul
>>>>>>
>>>>>> ------------------------------------------------------------------------
>>>>>>
>>>>>> commit 12cd59e49cf734f907f44b696e2c6e4b46a291c3
>>>>>> Author: David Woodhouse <dwmw@amazon.co.uk>
>>>>>> Date: Wed Jul 11 19:01:01 2018 +0100
>>>>>>
>>>>>> kvm/x86: Inform RCU of quiescent state when entering guest mode
>>>>>>
>>>>>> RCU can spend long periods of time waiting for a CPU which is actually in
>>>>>> KVM guest mode, entirely pointlessly. Treat it like the idle and userspace
>>>>>> modes, and don't wait for it.
>>>>>>
>>>>>> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
>>>>>> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>>>>>> [ paulmck: Adjust to avoid bad advice I gave to dwmw, avoid WARN_ON()s. ]
>>>>>>
>>>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>>>>> index 0046aa70205a..b0c82f70afa7 100644
>>>>>> --- a/arch/x86/kvm/x86.c
>>>>>> +++ b/arch/x86/kvm/x86.c
>>>>>> @@ -7458,7 +7458,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>>>>> vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
>>>>>> }
>>>>>>
>>>>>> + rcu_kvm_enter();
>>>>>> kvm_x86_ops->run(vcpu);
>>>>>> + rcu_kvm_exit();
>>>>>
>>>>> As indicated in my other mail. This is supposed to be handled in the guest_enter|exit_ calls around
>>>>> the run function. This would also handle other architectures. So if the guest_enter_irqoff code is
>>>>> not good enough, we should rather fix that instead of adding another rcu hint.
>>>>
>>>> Something like this, on top of the earlier patch? I am not at all
>>>> confident of this patch because there might be other entry/exit
>>>> paths I am missing. Plus there might be RCU uses on the arch-specific
>>>> patch to and from the guest OS.
>>>>
>>>> Thoughts?
>>>>
>>>
>>> If you instrment guest_enter/exit, you should cover all cases and all architectures as far
>>> as I can tell. FWIW, we did this rcu_note thing back then actually handling this particular
>>> case of long running guests blocking rcu for many seconds. And I am pretty sure that
>>> this did help back then.
>>
>> And my second patch on the email you replied to replaced the only call
>> to rcu_virt_note_context_switch(). So maybe it covers what it needs to,
>> but yes, there might well be things I missed. Let's see what David
>> comes up with.
>>
>> What changed was RCU's reactions to longish grace periods. It used to
>> be very aggressive about forcing the scheduler to do otherwise-unneeded
>> context switches, which became a problem somewhere between v4.9 and v4.15.
>> I therefore reduced the number of such context switches, which in turn
>> caused KVM to tell RCU about quiescent states way too infrequently.
>>
>> The advantage of the rcu_kvm_enter()/rcu_kvm_exit() approach is that
>> it tells RCU of an extended duration in the guest, which means that
>> RCU can ignore the corresponding CPU, which in turn allows the guest
>> to proceed without any RCU-induced interruptions.
>>
>> Does that make sense, or am I missing something? I freely admit to
>> much ignorance of both kvm and s390! ;-)
>
> But I am getting some rcutorture near misses on the commit that
> introduces rcu_kvm_enter() and rcu_kvm_exit() to the x86 arch-specific
> vcpu_enter_guest() function. These near misses occur when running
> rcutorture scenarios TREE01 and TREE03, and in my -rcu tree rather
> than the v4.15 version of this patch.
>
> Given that I am making pervasive changes to the way that RCU works,
> it might well be that this commit is an innocent bystander. I will
> run tests overnight and let you know what comes up.
Is there a single patch that that I can test or do I have to combine all the pieces
that are sprinkled in this mail thread?
next prev parent reply other threads:[~2018-07-12 6:21 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 [this message]
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
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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