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

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Message-ID: <9d81ac79-1cc4-fba0-5aad-7acd8578d957@redhat.com>
Date:   Mon, 5 Aug 2019 16:22:15 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier
 with worker


On 2019/8/5 下午2:30, Michael S. Tsirkin wrote:
> On Mon, Aug 05, 2019 at 12:36:40PM +0800, 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?
> Depending on CONFIG_BROKEN? I'm not sure it's a good idea.


Ok.


>>> 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
> We want THP and NUMA to work. Both are important for performance.
>

Yes.

Thanks