linux-kernel - Re: [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor

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Message-ID: <51E6A66D.7090407@linux.vnet.ibm.com>
Date:	Wed, 17 Jul 2013 19:43:01 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	mingo@...hat.com, jeremy@...p.org, x86@...nel.org,
	konrad.wilk@...cle.com, hpa@...or.com, pbonzini@...hat.com,
	linux-doc@...r.kernel.org, habanero@...ux.vnet.ibm.com,
	xen-devel@...ts.xensource.com, peterz@...radead.org,
	mtosatti@...hat.com, stefano.stabellini@...citrix.com,
	andi@...stfloor.org, ouyang@...pitt.edu, agraf@...e.de,
	chegu_vinod@...com, torvalds@...ux-foundation.org,
	avi.kivity@...il.com, tglx@...utronix.de, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, riel@...hat.com, drjones@...hat.com,
	virtualization@...ts.linux-foundation.org,
	srivatsa.vaddagiri@...il.com
Subject: Re: [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support for
 linux guests running on KVM hypervisor

On 07/17/2013 06:55 PM, Gleb Natapov wrote:
> On Wed, Jul 17, 2013 at 06:25:05PM +0530, Raghavendra K T wrote:
>> On 07/17/2013 06:15 PM, Gleb Natapov wrote:
>>> On Wed, Jul 17, 2013 at 03:35:37PM +0530, Raghavendra K T wrote:
>>>>>> Instead of halt we started with a sleep hypercall in those
>>>>>>   versions. Changed to halt() once Avi suggested to reuse existing sleep.
>>>>>>
>>>>>> If we use older hypercall with few changes like below:
>>>>>>
>>>>>> kvm_pv_wait_for_kick_op(flags, vcpu, w->lock )
>>>>>> {
>>>>>>   // a0 reserved for flags
>>>>>> if (!w->lock)
>>>>>> return;
>>>>>> DEFINE_WAIT
>>>>>> ...
>>>>>> end_wait
>>>>>> }
>>>>>>
>>>>> How would this help if NMI takes lock in critical section. The thing
>>>>> that may happen is that lock_waiting->want may have NMI lock value, but
>>>>> lock_waiting->lock will point to non NMI lock. Setting of want and lock
>>>>> have to be atomic.
>>>>
>>>> True. so we are here
>>>>
>>>>          non NMI lock(a)
>>>>          w->lock = NULL;
>>>>          smp_wmb();
>>>>          w->want = want;
>>>>                                 NMI
>>>>                           <---------------------
>>>>                            NMI lock(b)
>>>>                            w->lock = NULL;
>>>>                            smp_wmb();
>>>>                            w->want = want;
>>>>                            smp_wmb();
>>>>                            w->lock = lock;
>>>>                           ---------------------->
>>>>          smp_wmb();
>>>>          w->lock = lock;
>>>>
>>>> so how about fixing like this?
>>>>
>>>> again:
>>>>          w->lock = NULL;
>>>>          smp_wmb();
>>>>          w->want = want;
>>>>          smp_wmb();
>>>>          w->lock = lock;
>>>>
>>>> if (!lock || w->want != want) goto again;
>>>>
>>> NMI can happen after the if() but before halt and the same situation
>>> we are trying to prevent with IRQs will occur.
>>
>> True, we can not fix that. I thought to fix the inconsistency of
>> lock,want pair.
>> But NMI could happen after the first OR condition also.
>> /me thinks again
>>
> lock_spinning() can check that it is called in nmi context and bail out.

Good point.
I think we can check for even irq context and bailout so that in irq 
context we continue spinning instead of slowpath. no ?

> How often this will happens anyway.
>

I know NMIs occur frequently with watchdogs. or used by sysrq-trigger
etc.. But I am not an expert how frequent it is otherwise. But even
then if they do not use spinlock, we have no problem as already pointed.

I can measure with debugfs counter how often it happens.

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