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[209.132.183.28]) by mx.google.com with ESMTPS id y7si24804534qvp.116.2019.07.22.00.52.16 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 22 Jul 2019 00:52:16 -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-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 1176F86668; Mon, 22 Jul 2019 07:52:15 +0000 (UTC) Received: from redhat.com (ovpn-120-233.rdu2.redhat.com [10.10.120.233]) by smtp.corp.redhat.com (Postfix) with SMTP id 5B9085C221; Mon, 22 Jul 2019 07:52:06 +0000 (UTC) Date: Mon, 22 Jul 2019 03:52:05 -0400 From: "Michael S. Tsirkin" To: "Paul E. McKenney" Cc: Matthew Wilcox , 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: <20190722035042-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> <20190721210837.GC363@bombadil.infradead.org> <20190721233113.GV14271@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190721233113.GV14271@linux.ibm.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.26]); Mon, 22 Jul 2019 07:52:15 +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 04:31:13PM -0700, Paul E. McKenney wrote: > On Sun, Jul 21, 2019 at 02:08:37PM -0700, Matthew Wilcox wrote: > > On Sun, Jul 21, 2019 at 06:17:25AM -0700, Paul E. McKenney wrote: > > > 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. > > > > I thought you believed that RCU was antifragile, in that it would scale > > better as it was used more heavily? > > You are referring to this? https://paulmck.livejournal.com/47933.html > > If so, the last few paragraphs might be worth re-reading. ;-) > > And in this case, the heuristics RCU uses to decide when to schedule > invocation of the callbacks needs some help. One component of that help > is a time-based limit to the number of consecutive callback invocations > (see my crude prototype and Eric Dumazet's more polished patch). Another > component is an overload warning. > > Why would an overload warning be needed if RCU's callback-invocation > scheduling heurisitics were upgraded? Because someone could boot a > 100-CPU system with the rcu_nocbs=0-99, bind all of the resulting > rcuo kthreads to (say) CPU 0, and then run a callback-heavy workload > on all of the CPUs. Given the constraints, CPU 0 cannot keep up. > > So warnings are required as well. > > > 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. > > Unfortunately, no. Here is a code fragment illustrating why: > > void my_cb(struct rcu_head *rhp) > { > unsigned long flags; > > spin_lock_irqsave(&my_lock, flags); > handle_cb(rhp); > spin_unlock_irqrestore(&my_lock, flags); > } > > . . . > > spin_lock_irqsave(&my_lock, flags); > p = look_something_up(); > remove_that_something(p); > call_rcu(p, my_cb); > spin_unlock_irqrestore(&my_lock, flags); > > Invoking the extra callbacks directly from call_rcu() would thus result > in self-deadlock. Documentation/RCU/UP.txt contains a few more examples > along these lines. We could add an option that simply fails if overloaded, right? Have caller recover... -- MST