linux-kernel - Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker

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Message-ID: <494ac30d-b750-52c8-b927-16cd4b9414c4@redhat.com>
Date:   Mon, 5 Aug 2019 12:41:45 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>
Cc:     linux-mm@...ck.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier
 with worker

On 2019/8/5 下午12:36, Jason Wang wrote:
>
> On 2019/8/2 下午10:27, Michael S. Tsirkin wrote:
>> On Fri, Aug 02, 2019 at 09:46:13AM -0300, Jason Gunthorpe wrote:
>>> On Fri, Aug 02, 2019 at 05:40:07PM +0800, Jason Wang wrote:
>>>>> This must be a proper barrier, like a spinlock, mutex, or
>>>>> synchronize_rcu.
>>>>
>>>> I start with synchronize_rcu() but both you and Michael raise some
>>>> concern.
>>> I've also idly wondered if calling synchronize_rcu() under the various
>>> mm locks is a deadlock situation.
>>>
>>>> Then I try spinlock and mutex:
>>>>
>>>> 1) spinlock: add lots of overhead on datapath, this leads 0 
>>>> performance
>>>> improvement.
>>> I think the topic here is correctness not performance improvement
>> The topic is whether we should revert
>> commit 7f466032dc9 ("vhost: access vq metadata through kernel virtual 
>> address")
>>
>> or keep it in. The only reason to keep it is performance.
>
>
> Maybe it's time to introduce the config option?

Or does it make sense if I post a V3 with:

- introduce config option and disable the optimization by default

- switch from synchronize_rcu() to vhost_flush_work(), but the rest are 
the same

This can give us some breath to decide which way should go for next release?

Thanks

>
>
>>
>> Now as long as all this code is disabled anyway, we can experiment a
>> bit.
>>
>> I personally feel we would be best served by having two code paths:
>>
>> - Access to VM memory directly mapped into kernel
>> - Access to userspace
>>
>>
>> Having it all cleanly split will allow a bunch of optimizations, for
>> example for years now we planned to be able to process an incoming short
>> packet directly on softirq path, or an outgoing on directly within
>> eventfd.
>
>
> It's not hard consider we've already had our own accssors. But the 
> question is (as asked in another thread), do you want permanent GUP or 
> still use MMU notifiers.
>
> Thanks
>
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