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

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Date:   Mon, 5 Aug 2019 12:39:34 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>
Cc:     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 2019/8/4 下午4:07, Michael S. Tsirkin wrote:
> On Sat, Aug 03, 2019 at 09:14:00PM -0300, Jason Gunthorpe wrote:
>> On Sat, Aug 03, 2019 at 05:36:13PM -0400, Michael S. Tsirkin wrote:
>>> On Fri, Aug 02, 2019 at 02:24:18PM -0300, Jason Gunthorpe wrote:
>>>> On Fri, Aug 02, 2019 at 10:27:21AM -0400, 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.
>>>> Yikes, I'm not sure you can ever win against copy_from_user using
>>>> mmu_notifiers?
>>> Ever since copy_from_user started playing with flags (for SMAP) and
>>> added speculation barriers there's a chance we can win by accessing
>>> memory through the kernel address.
>> You think copy_to_user will be more expensive than the minimum two
>> atomics required to synchronize with another thread?
> I frankly don't know. With SMAP you flip flags twice, and with spectre
> you flush the pipeline. Is that cheaper or more expensive than an atomic
> operation? Testing is the only way to tell.


Let me test, I only did test on a non SMAP machine. Switching to 
spinlock kills all performance improvement.

Thanks


>
>>>> Also, why can't this just permanently GUP the pages? In fact, where
>>>> does it put_page them anyhow? Worrying that 7f466 adds a get_user page
>>>> but does not add a put_page??
>> You didn't answer this.. Why not just use GUP?
>>
>> Jason
> Sorry I misunderstood the question. Permanent GUP breaks lots of
> functionality we need such as THP and numa balancing.
>
> release_pages is used instead of put_page.
>
>
>
>