Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: WARNING in __mmdrop
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     syzbot <syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com>,
        aarcange@redhat.com, akpm@linux-foundation.org,
        christian@brauner.io, davem@davemloft.net, ebiederm@xmission.com,
        elena.reshetova@intel.com, guro@fb.com, hch@infradead.org,
        james.bottomley@hansenpartnership.com, jglisse@redhat.com,
        keescook@chromium.org, ldv@altlinux.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-parisc@vger.kernel.org,
        luto@amacapital.net, mhocko@suse.com, mingo@kernel.org,
        namit@vmware.com, peterz@infradead.org,
        syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk,
        wad@chromium.org
References: <20190725012149-mutt-send-email-mst@kernel.org>
 <55e8930c-2695-365f-a07b-3ad169654d28@redhat.com>
 <20190725042651-mutt-send-email-mst@kernel.org>
 <84bb2e31-0606-adff-cf2a-e1878225a847@redhat.com>
 <20190725092332-mutt-send-email-mst@kernel.org>
 <11802a8a-ce41-f427-63d5-b6a4cf96bb3f@redhat.com>
 <20190726074644-mutt-send-email-mst@kernel.org>
 <5cc94f15-b229-a290-55f3-8295266edb2b@redhat.com>
 <20190726082837-mutt-send-email-mst@kernel.org>
 <ada10dc9-6cab-e189-5289-6f9d3ff8fed2@redhat.com>
 <20190726094353-mutt-send-email-mst@kernel.org>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <63754251-a39a-1e0e-952d-658102682094@redhat.com>
Date:   Fri, 26 Jul 2019 22:00:20 +0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.8.0
MIME-Version: 1.0
In-Reply-To: <20190726094353-mutt-send-email-mst@kernel.org>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 2019/7/26 下午9:47, Michael S. Tsirkin wrote:
> On Fri, Jul 26, 2019 at 08:53:18PM +0800, Jason Wang wrote:
>> On 2019/7/26 下午8:38, Michael S. Tsirkin wrote:
>>> On Fri, Jul 26, 2019 at 08:00:58PM +0800, Jason Wang wrote:
>>>> On 2019/7/26 下午7:49, Michael S. Tsirkin wrote:
>>>>> On Thu, Jul 25, 2019 at 10:25:25PM +0800, Jason Wang wrote:
>>>>>> On 2019/7/25 下午9:26, Michael S. Tsirkin wrote:
>>>>>>>> Exactly, and that's the reason actually I use synchronize_rcu() there.
>>>>>>>>
>>>>>>>> So the concern is still the possible synchronize_expedited()?
>>>>>>> I think synchronize_srcu_expedited.
>>>>>>>
>>>>>>> synchronize_expedited sends lots of IPI and is bad for realtime VMs.
>>>>>>>
>>>>>>>> Can I do this
>>>>>>>> on through another series on top of the incoming V2?
>>>>>>>>
>>>>>>>> Thanks
>>>>>>>>
>>>>>>> The question is this: is this still a gain if we switch to the
>>>>>>> more expensive srcu? If yes then we can keep the feature on,
>>>>>> I think we only care about the cost on srcu_read_lock() which looks pretty
>>>>>> tiny form my point of view. Which is basically a READ_ONCE() + WRITE_ONCE().
>>>>>>
>>>>>> Of course I can benchmark to see the difference.
>>>>>>
>>>>>>
>>>>>>> if not we'll put it off until next release and think
>>>>>>> of better solutions. rcu->srcu is just a find and replace,
>>>>>>> don't see why we need to defer that. can be a separate patch
>>>>>>> for sure, but we need to know how well it works.
>>>>>> I think I get here, let me try to do that in V2 and let's see the numbers.
>>>>>>
>>>>>> Thanks
>>>> It looks to me for tree rcu, its srcu_read_lock() have a mb() which is too
>>>> expensive for us.
>>> I will try to ponder using vq lock in some way.
>>> Maybe with trylock somehow ...
>>
>> Ok, let me retry if necessary (but I do remember I end up with deadlocks
>> last try).
>>
>>
>>>
>>>> If we just worry about the IPI,
>>> With synchronize_rcu what I would worry about is that guest is stalled
>>
>> Can this synchronize_rcu() be triggered by guest? If yes, there are several
>> other MMU notifiers that can block. Is vhost something special here?
> Sorry, let me explain: guests (and tasks in general)
> can trigger activity that will
> make synchronize_rcu take a long time.


Yes, I get this.


>   Thus blocking
> an mmu notifier until synchronize_rcu finishes
> is a bad idea.


The question is, MMU notifier are allowed to be blocked on 
invalidate_range_start() which could be much slower than 
synchronize_rcu() to finish.

Looking at amdgpu_mn_invalidate_range_start_gfx() which calls 
amdgpu_mn_invalidate_node() which did:

                 r = reservation_object_wait_timeout_rcu(bo->tbo.resv,
                         true, false, MAX_SCHEDULE_TIMEOUT);

...


>>> because system is busy because of other guests.
>>> With expedited it's the IPIs...
>>>
>> The current synchronize_rcu()  can force a expedited grace period:
>>
>> void synchronize_rcu(void)
>> {
>>          ...
>>          if (rcu_blocking_is_gp())
>> return;
>>          if (rcu_gp_is_expedited())
>> synchronize_rcu_expedited();
>> else
>> wait_rcu_gp(call_rcu);
>> }
>> EXPORT_SYMBOL_GPL(synchronize_rcu);
>
> An admin can force rcu to finish faster, trading
> interrupts for responsiveness.


Yes, so when set, all each synchronize_rcu() will go for 
synchronize_rcu_expedited().


>
>>>> can we do something like in
>>>> vhost_invalidate_vq_start()?
>>>>
>>>>           if (map) {
>>>>                   /* In order to avoid possible IPIs with
>>>>                    * synchronize_rcu_expedited() we use call_rcu() +
>>>>                    * completion.
>>>> */
>>>> init_completion(&c.completion);
>>>>                   call_rcu(&c.rcu_head, vhost_finish_vq_invalidation);
>>>> wait_for_completion(&c.completion);
>>>>                   vhost_set_map_dirty(vq, map, index);
>>>> vhost_map_unprefetch(map);
>>>>           }
>>>>
>>>> ?
>>> Why would that be faster than synchronize_rcu?
>>
>> No faster but no IPI.
>>
> Sorry I still don't see the point.
> synchronize_rcu doesn't normally do an IPI either.
>

Not the case of when rcu_expedited is set. This can just 100% make sure 
there's no IPI.


>>>
>>>>> There's one other thing that bothers me, and that is that
>>>>> for large rings which are not physically contiguous
>>>>> we don't implement the optimization.
>>>>>
>>>>> For sure, that can wait, but I think eventually we should
>>>>> vmap large rings.
>>>> Yes, worth to try. But using direct map has its own advantage: it can use
>>>> hugepage that vmap can't
>>>>
>>>> Thanks
>>> Sure, so we can do that for small rings.
>>
>> Yes, that's possible but should be done on top.
>>
>> Thanks
> Absolutely. Need to fix up the bugs first.
>

Yes.

Thanks