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[209.132.183.28]) by mx.google.com with ESMTPS id u51si25149781qte.390.2019.07.21.10.53.33 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 21 Jul 2019 10:53:33 -0700 (PDT) Received-SPF: pass (google.com: domain of mst@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of mst@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=mst@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 6475685541; Sun, 21 Jul 2019 17:53:32 +0000 (UTC) Received: from redhat.com (ovpn-120-128.rdu2.redhat.com [10.10.120.128]) by smtp.corp.redhat.com (Postfix) with SMTP id DABC4600C0; Sun, 21 Jul 2019 17:53:24 +0000 (UTC) Date: Sun, 21 Jul 2019 13:53:23 -0400 From: "Michael S. Tsirkin" To: "Paul E. McKenney" Cc: 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) Message-ID: <20190721134614-mutt-send-email-mst@kernel.org> References: <0000000000008dd6bb058e006938@google.com> <000000000000964b0d058e1a0483@google.com> <20190721044615-mutt-send-email-mst@kernel.org> <20190721081933-mutt-send-email-mst@kernel.org> <20190721131725.GR14271@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190721131725.GR14271@linux.ibm.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.28]); Sun, 21 Jul 2019 17:53:32 +0000 (UTC) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sun, Jul 21, 2019 at 06:17:25AM -0700, Paul E. McKenney wrote: > On Sun, Jul 21, 2019 at 08:28:05AM -0400, Michael S. Tsirkin wrote: > > Hi Paul, others, > > > > So it seems that vhost needs to call kfree_rcu from an ioctl. My worry > > is what happens if userspace starts cycling through lots of these > > ioctls. Given we actually use rcu as an optimization, we could just > > disable the optimization temporarily - but the question would be how to > > detect an excessive rate without working too hard :) . > > > > I guess we could define as excessive any rate where callback is > > outstanding at the time when new structure is allocated. I have very > > little understanding of rcu internals - so I wanted to check that the > > following more or less implements this heuristic before I spend time > > actually testing it. > > > > Could others pls take a look and let me know? > > These look good as a way of seeing if there are any outstanding callbacks, > but in the case of Tree RCU, call_rcu_outstanding() would almost never > return false on a busy system. Hmm, ok. Maybe I could rename this to e.g. call_rcu_busy and change the tree one to do rcu_segcblist_n_lazy_cbs > 1000? > > Here are some alternatives: > > o RCU uses some pieces of Rao Shoaib kfree_rcu() patches. > The idea is to make kfree_rcu() locally buffer requests into > batches of (say) 1,000, but processing smaller batches when RCU > is idle, or when some smallish amout of time has passed with > no more kfree_rcu() request from that CPU. RCU than takes in > the batch using not call_rcu(), but rather queue_rcu_work(). > The resulting batch of kfree() calls would therefore execute in > workqueue context rather than in softirq context, which should > be much easier on the system. > > In theory, this would allow people to use kfree_rcu() without > worrying quite so much about overload. It would also not be > that hard to implement. > > o Subsystems vulnerable to user-induced kfree_rcu() flooding use > call_rcu() instead of kfree_rcu(). Keep a count of the number > of things waiting for a grace period, and when this gets too > large, disable the optimization. It will then drain down, at > which point the optimization can be re-enabled. > > But please note that callbacks are -not- guaranteed to run on > the CPU that queued them. So yes, you would need a per-CPU > counter, but you would need to periodically sum it up to check > against the global state. Or keep track of the CPU that > did the call_rcu() so that you can atomically decrement in > the callback the same counter that was atomically incremented > just before the call_rcu(). Or any number of other approaches. I'm really looking for something we can do this merge window and without adding too much code, and kfree_rcu is intended to fix a bug. Adding call_rcu and careful accounting is something that I'm not happy adding with merge window already open. > > Also, the overhead is important. For example, as far as I know, > current RCU gracefully handles close(open(...)) in a tight userspace > loop. But there might be trouble due to tight userspace loops around > lighter-weight operations. > > So an important question is "Just how fast is your ioctl?" If it takes > (say) 100 microseconds to execute, there should be absolutely no problem. > On the other hand, if it can execute in 50 nanoseconds, this very likely > does need serious attention. > > Other thoughts? > > Thanx, Paul Hmm the answer to this would be I'm not sure. It's setup time stuff we never tested it. > > Thanks! > > > > Signed-off-by: Michael S. Tsirkin > > > > > > diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c > > index 477b4eb44af5..067909521d72 100644 > > --- a/kernel/rcu/tiny.c > > +++ b/kernel/rcu/tiny.c > > @@ -125,6 +125,25 @@ void synchronize_rcu(void) > > } > > EXPORT_SYMBOL_GPL(synchronize_rcu); > > > > +/* > > + * Helpful for rate-limiting kfree_rcu/call_rcu callbacks. > > + */ > > +bool call_rcu_outstanding(void) > > +{ > > + unsigned long flags; > > + struct rcu_data *rdp; > > + bool outstanding; > > + > > + local_irq_save(flags); > > + rdp = this_cpu_ptr(&rcu_data); > > + outstanding = rcu_segcblist_empty(&rdp->cblist); > > + outstanding = rcu_ctrlblk.donetail != rcu_ctrlblk.curtail; > > + local_irq_restore(flags); > > + > > + return outstanding; > > +} > > +EXPORT_SYMBOL_GPL(call_rcu_outstanding); > > + > > /* > > * Post an RCU callback to be invoked after the end of an RCU grace > > * period. But since we have but one CPU, that would be after any > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > index a14e5fbbea46..d4b9d61e637d 100644 > > --- a/kernel/rcu/tree.c > > +++ b/kernel/rcu/tree.c > > @@ -2482,6 +2482,24 @@ static void rcu_leak_callback(struct rcu_head *rhp) > > { > > } > > > > +/* > > + * Helpful for rate-limiting kfree_rcu/call_rcu callbacks. > > + */ > > +bool call_rcu_outstanding(void) > > +{ > > + unsigned long flags; > > + struct rcu_data *rdp; > > + bool outstanding; > > + > > + local_irq_save(flags); > > + rdp = this_cpu_ptr(&rcu_data); > > + outstanding = rcu_segcblist_empty(&rdp->cblist); > > + local_irq_restore(flags); > > + > > + return outstanding; > > +} > > +EXPORT_SYMBOL_GPL(call_rcu_outstanding); > > + > > /* > > * Helper function for call_rcu() and friends. The cpu argument will > > * normally be -1, indicating "currently running CPU". It may specify