From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Joel Fernandes <joel@joelfernandes.org>,
Matthew Wilcox <willy@infradead.org>,
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, jasowang@redhat.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: RFC: call_rcu_outstanding (was Re: WARNING in __mmdrop)
Date: Mon, 22 Jul 2019 11:58:38 -0700 [thread overview]
Message-ID: <20190722185838.GN14271@linux.ibm.com> (raw)
In-Reply-To: <20190722123016-mutt-send-email-mst@kernel.org>
On Mon, Jul 22, 2019 at 12:32:17PM -0400, Michael S. Tsirkin wrote:
> On Mon, Jul 22, 2019 at 09:25:51AM -0700, Paul E. McKenney wrote:
> > On Mon, Jul 22, 2019 at 12:13:40PM -0400, Michael S. Tsirkin wrote:
> > > On Mon, Jul 22, 2019 at 08:55:34AM -0700, Paul E. McKenney wrote:
> > > > On Mon, Jul 22, 2019 at 11:47:24AM -0400, Michael S. Tsirkin wrote:
> > > > > On Mon, Jul 22, 2019 at 11:14:39AM -0400, Joel Fernandes wrote:
> > > > > > [snip]
> > > > > > > > Would it make sense to have call_rcu() check to see if there are many
> > > > > > > > outstanding requests on this CPU and if so process them before returning?
> > > > > > > > That would ensure that frequent callers usually ended up doing their
> > > > > > > > own processing.
> > > > > >
> > > > > > Other than what Paul already mentioned about deadlocks, I am not sure if this
> > > > > > would even work for all cases since call_rcu() has to wait for a grace
> > > > > > period.
> > > > > >
> > > > > > So, if the number of outstanding requests are higher than a certain amount,
> > > > > > then you *still* have to wait for some RCU configurations for the grace
> > > > > > period duration and cannot just execute the callback in-line. Did I miss
> > > > > > something?
> > > > > >
> > > > > > Can waiting in-line for a grace period duration be tolerated in the vhost case?
> > > > > >
> > > > > > thanks,
> > > > > >
> > > > > > - Joel
> > > > >
> > > > > No, but it has many other ways to recover (try again later, drop a
> > > > > packet, use a slower copy to/from user).
> > > >
> > > > True enough! And your idea of taking recovery action based on the number
> > > > of callbacks seems like a good one while we are getting RCU's callback
> > > > scheduling improved.
> > > >
> > > > By the way, was this a real problem that you could make happen on real
> > > > hardware?
> > >
> > > > If not, I would suggest just letting RCU get improved over
> > > > the next couple of releases.
> > >
> > > So basically use kfree_rcu but add a comment saying e.g. "WARNING:
> > > in the future callers of kfree_rcu might need to check that
> > > not too many callbacks get queued. In that case, we can
> > > disable the optimization, or recover in some other way.
> > > Watch this space."
> >
> > That sounds fair.
> >
> > > > If it is something that you actually made happen, please let me know
> > > > what (if anything) you need from me for your callback-counting EBUSY
> > > > scheme.
> > >
> > > If you mean kfree_rcu causing OOM then no, it's all theoretical.
> > > If you mean synchronize_rcu stalling to the point where guest will OOPs,
> > > then yes, that's not too hard to trigger.
> >
> > Is synchronize_rcu() being stalled by the userspace loop that is invoking
> > your ioctl that does kfree_rcu()? Or instead by the resulting callback
> > invocation?
>
> Sorry, let me clarify. We currently have synchronize_rcu in a userspace
> loop. I have a patch replacing that with kfree_rcu. This isn't the
> first time synchronize_rcu is stalling a VM for a long while so I didn't
> investigate further.
Ah, so a bunch of synchronize_rcu() calls within a single system call
inside the host is stalling the guest, correct?
If so, one straightforward approach is to do an rcu_barrier() every
(say) 1000 kfree_rcu() calls within that loop in the system call.
This will decrease the overhead by almost a factor of 1000 compared to
a synchronize_rcu() on each trip through that loop, and will prevent
callback overload.
Or if the situation is different (for example, the guest does a long
sequence of system calls, each of which does a single kfree_rcu() or
some such), please let me know what the situation is.
Thanx, Paul
next prev parent reply other threads:[~2019-07-22 18:58 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
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 [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190722185838.GN14271@linux.ibm.com \
--to=paulmck@linux.ibm.com \
--cc=aarcange@redhat.com \
--cc=akpm@linux-foundation.org \
--cc=christian@brauner.io \
--cc=davem@davemloft.net \
--cc=ebiederm@xmission.com \
--cc=elena.reshetova@intel.com \
--cc=guro@fb.com \
--cc=hch@infradead.org \
--cc=james.bottomley@hansenpartnership.com \
--cc=jasowang@redhat.com \
--cc=jglisse@redhat.com \
--cc=joel@joelfernandes.org \
--cc=keescook@chromium.org \
--cc=ldv@altlinux.org \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=linux-parisc@vger.kernel.org \
--cc=luto@amacapital.net \
--cc=mhocko@suse.com \
--cc=mingo@kernel.org \
--cc=mst@redhat.com \
--cc=namit@vmware.com \
--cc=peterz@infradead.org \
--cc=syzkaller-bugs@googlegroups.com \
--cc=viro@zeniv.linux.org.uk \
--cc=wad@chromium.org \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).