From: Jason Wang <jasowang@redhat.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: mst@redhat.com, kvm@vger.kernel.org,
virtualization@lists.linux-foundation.org,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-mm@kvack.org
Subject: Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker
Date: Wed, 7 Aug 2019 14:49:57 +0800 [thread overview]
Message-ID: <4b448aa5-2c92-a6ca-67d6-d30fad67254c@redhat.com> (raw)
In-Reply-To: <20190806120416.GB11627@ziepe.ca>
On 2019/8/6 下午8:04, Jason Gunthorpe wrote:
> On Mon, Aug 05, 2019 at 12:20:45PM +0800, Jason Wang wrote:
>> On 2019/8/2 下午8:46, 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.
>>
>> Maybe, that's why I suggest to use vhost_work_flush() which is much
>> lightweight can can achieve the same function. It can guarantee all previous
>> work has been processed after vhost_work_flush() return.
> If things are already running in a work, then yes, you can piggyback
> on the existing spinlocks inside the workqueue and be Ok
>
> However, if that work is doing any copy_from_user, then the flush
> becomes dependent on swap and it won't work again...
Yes it do copy_from_user(), so we can't do this.
>
>>>> 1) spinlock: add lots of overhead on datapath, this leads 0 performance
>>>> improvement.
>>> I think the topic here is correctness not performance improvement>
>
>> But the whole series is to speed up vhost.
> So? Starting with a whole bunch of crazy, possibly broken, locking and
> claiming a performance win is not reasonable.
Yes, I admit this patch is tricky, I'm not going to push this. Will post
a V3.
>
>> Spinlock is correct but make the whole series meaningless consider it won't
>> bring any performance improvement.
> You can't invent a faster spinlock by opencoding some wild
> scheme. There is nothing special about the usage here, it needs a
> blocking lock, plain and simple.
>
> Jason
Will post V3. Let's see if you are happy with that version.
Thanks
next prev parent reply other threads:[~2019-08-07 6:50 UTC|newest]
Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-31 8:46 [PATCH V2 0/9] Fixes for metadata accelreation Jason Wang
2019-07-31 8:46 ` [PATCH V2 1/9] vhost: don't set uaddr for invalid address Jason Wang
2019-07-31 8:46 ` [PATCH V2 2/9] vhost: validate MMU notifier registration Jason Wang
2019-07-31 8:46 ` [PATCH V2 3/9] vhost: fix vhost map leak Jason Wang
2019-07-31 8:46 ` [PATCH V2 4/9] vhost: reset invalidate_count in vhost_set_vring_num_addr() Jason Wang
2019-07-31 12:41 ` Jason Gunthorpe
2019-07-31 13:29 ` Jason Wang
2019-07-31 19:32 ` Jason Gunthorpe
2019-07-31 19:37 ` Michael S. Tsirkin
2019-08-01 5:03 ` Jason Wang
2019-07-31 8:46 ` [PATCH V2 5/9] vhost: mark dirty pages during map uninit Jason Wang
2019-07-31 8:46 ` [PATCH V2 6/9] vhost: don't do synchronize_rcu() in vhost_uninit_vq_maps() Jason Wang
2019-07-31 8:46 ` [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker Jason Wang
2019-07-31 8:50 ` Jason Wang
2019-07-31 12:39 ` Jason Gunthorpe
2019-07-31 13:28 ` Jason Wang
2019-07-31 19:30 ` Jason Gunthorpe
2019-08-01 5:02 ` Jason Wang
2019-08-01 14:15 ` Jason Gunthorpe
2019-08-02 9:40 ` Jason Wang
2019-08-02 12:46 ` Jason Gunthorpe
2019-08-02 14:27 ` Michael S. Tsirkin
2019-08-02 17:24 ` Jason Gunthorpe
2019-08-03 21:36 ` Michael S. Tsirkin
2019-08-04 0:14 ` Jason Gunthorpe
2019-08-04 8:07 ` Michael S. Tsirkin
2019-08-05 4:39 ` Jason Wang
2019-08-06 11:53 ` Jason Gunthorpe
2019-08-06 13:36 ` Michael S. Tsirkin
2019-08-06 13:40 ` Jason Gunthorpe
2019-08-05 4:36 ` Jason Wang
2019-08-05 4:41 ` Jason Wang
2019-08-05 6:40 ` Michael S. Tsirkin
2019-08-05 8:24 ` Jason Wang
2019-08-05 6:30 ` Michael S. Tsirkin
2019-08-05 8:22 ` Jason Wang
2019-08-05 4:20 ` Jason Wang
2019-08-06 12:04 ` Jason Gunthorpe
2019-08-07 6:49 ` Jason Wang [this message]
2019-08-02 14:03 ` Michael S. Tsirkin
2019-08-05 4:33 ` Jason Wang
2019-08-05 6:28 ` Michael S. Tsirkin
2019-08-05 8:21 ` Jason Wang
2019-07-31 18:29 ` Michael S. Tsirkin
2019-08-01 8:06 ` Jason Wang
2019-08-03 21:54 ` Michael S. Tsirkin
2019-08-05 8:18 ` Jason Wang
2019-07-31 8:46 ` [PATCH V2 8/9] vhost: correctly set dirty pages in MMU notifiers callback Jason Wang
2019-07-31 8:46 ` [PATCH V2 9/9] vhost: do not return -EAGIAN for non blocking invalidation too early Jason Wang
2019-07-31 9:59 ` Stefano Garzarella
2019-07-31 10:05 ` Jason Wang
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