From: Jason Wang <jasowang@redhat.com>
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
Subject: Re: WARNING in __mmdrop
Date: Fri, 26 Jul 2019 22:00:20 +0800 [thread overview]
Message-ID: <63754251-a39a-1e0e-952d-658102682094@redhat.com> (raw)
In-Reply-To: <20190726094353-mutt-send-email-mst@kernel.org>
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
next prev parent reply other threads:[~2019-07-26 14:00 UTC|newest]
Thread overview: 87+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <0000000000008dd6bb058e006938@google.com>
2019-07-20 10:08 ` WARNING in __mmdrop syzbot
2019-07-21 10:02 ` Michael S. Tsirkin
2019-07-21 12:18 ` Michael S. Tsirkin
2019-07-22 5:24 ` Jason Wang
2019-07-22 8:08 ` Michael S. Tsirkin
2019-07-23 4:01 ` Jason Wang
2019-07-23 5:01 ` Michael S. Tsirkin
2019-07-23 5:47 ` Jason Wang
2019-07-23 7:23 ` Michael S. Tsirkin
2019-07-23 7:53 ` Jason Wang
2019-07-23 8:10 ` Michael S. Tsirkin
2019-07-23 8:49 ` Jason Wang
2019-07-23 9:26 ` Michael S. Tsirkin
2019-07-23 13:31 ` Jason Wang
2019-07-25 5:52 ` Michael S. Tsirkin
2019-07-25 7:43 ` Jason Wang
2019-07-25 8:28 ` Michael S. Tsirkin
2019-07-25 13:21 ` Jason Wang
2019-07-25 13:26 ` Michael S. Tsirkin
2019-07-25 14:25 ` Jason Wang
2019-07-26 11:49 ` Michael S. Tsirkin
2019-07-26 12:00 ` Jason Wang
2019-07-26 12:38 ` Michael S. Tsirkin
2019-07-26 12:53 ` Jason Wang
2019-07-26 13:36 ` Jason Wang
2019-07-26 13:49 ` Michael S. Tsirkin
2019-07-29 5:54 ` Jason Wang
2019-07-29 8:59 ` Michael S. Tsirkin
2019-07-29 14:24 ` Jason Wang
2019-07-29 14:44 ` Michael S. Tsirkin
2019-07-30 7:44 ` Jason Wang
2019-07-30 8:03 ` Jason Wang
2019-07-30 15:08 ` Michael S. Tsirkin
2019-07-31 8:49 ` Jason Wang
2019-07-31 23:00 ` Jason Gunthorpe
2019-07-26 13:47 ` Michael S. Tsirkin
2019-07-26 14:00 ` Jason Wang [this message]
2019-07-26 14:10 ` Michael S. Tsirkin
2019-07-26 15:03 ` Jason Gunthorpe
2019-07-29 5:56 ` Jason Wang
2019-07-21 12:28 ` RFC: call_rcu_outstanding (was Re: WARNING in __mmdrop) Michael S. Tsirkin
2019-07-21 13:17 ` Paul E. McKenney
2019-07-21 17:53 ` Michael S. Tsirkin
2019-07-21 19:28 ` Paul E. McKenney
2019-07-22 7:56 ` Michael S. Tsirkin
2019-07-22 11:57 ` Paul E. McKenney
2019-07-21 21:08 ` Matthew Wilcox
2019-07-21 23:31 ` Paul E. McKenney
2019-07-22 7:52 ` Michael S. Tsirkin
2019-07-22 11:51 ` Paul E. McKenney
2019-07-22 13:41 ` Jason Gunthorpe
2019-07-22 15:52 ` Paul E. McKenney
2019-07-22 16:04 ` Jason Gunthorpe
2019-07-22 16:15 ` Michael S. Tsirkin
2019-07-22 16:15 ` Paul E. McKenney
2019-07-22 15:14 ` Joel Fernandes
2019-07-22 15:47 ` Michael S. Tsirkin
2019-07-22 15:55 ` Paul E. McKenney
2019-07-22 16:13 ` Michael S. Tsirkin
2019-07-22 16:25 ` Paul E. McKenney
2019-07-22 16:32 ` Michael S. Tsirkin
2019-07-22 18:58 ` Paul E. McKenney
2019-07-22 5:21 ` WARNING in __mmdrop Jason Wang
2019-07-22 8:02 ` Michael S. Tsirkin
2019-07-23 3:55 ` Jason Wang
2019-07-23 5:02 ` Michael S. Tsirkin
2019-07-23 5:48 ` Jason Wang
2019-07-23 7:25 ` Michael S. Tsirkin
2019-07-23 7:55 ` Jason Wang
2019-07-23 7:56 ` Michael S. Tsirkin
2019-07-23 8:42 ` Jason Wang
2019-07-23 10:27 ` Michael S. Tsirkin
2019-07-23 13:34 ` Jason Wang
2019-07-23 15:02 ` Michael S. Tsirkin
2019-07-24 2:17 ` Jason Wang
2019-07-24 8:05 ` Michael S. Tsirkin
2019-07-24 10:08 ` Jason Wang
2019-07-24 18:25 ` Michael S. Tsirkin
2019-07-25 3:44 ` Jason Wang
2019-07-25 5:09 ` Michael S. Tsirkin
2019-07-24 16:53 ` Jason Gunthorpe
2019-07-24 18:25 ` Michael S. Tsirkin
2019-07-23 10:42 ` Michael S. Tsirkin
2019-07-23 13:37 ` Jason Wang
2019-07-22 14:11 ` Jason Gunthorpe
2019-07-25 6:02 ` Michael S. Tsirkin
2019-07-25 7:44 ` Jason Wang
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