From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@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 08:38:13 -0400 [thread overview]
Message-ID: <20190726082837-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <5cc94f15-b229-a290-55f3-8295266edb2b@redhat.com>
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 ...
> If we just worry about the IPI,
With synchronize_rcu what I would worry about is that guest is stalled
because system is busy because of other guests.
With expedited it's the IPIs...
> 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?
>
> > 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.
--
MST
next prev parent reply other threads:[~2019-07-26 12:38 UTC|newest]
Thread overview: 88+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-19 3:35 WARNING in __mmdrop syzbot
2019-07-20 10:08 ` 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 [this message]
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
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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