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: Wed, 11 Jul 2018 23:11:19 +0200 [thread overview]
Message-ID: <2f7ba67e-0442-13cc-628c-2dca56520c21@de.ibm.com> (raw)
In-Reply-To: <20180711202752.GC3593@linux.vnet.ibm.com>
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.
next prev parent reply other threads:[~2018-07-11 21:11 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 [this message]
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
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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