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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Thu, 12 Jul 2018 08:51:31 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w6CCpVDs9437568 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Thu, 12 Jul 2018 12:51:31 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 593E7B2064; Thu, 12 Jul 2018 08:51:29 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 37A12B205F; Thu, 12 Jul 2018 08:51:29 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.161.121]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Thu, 12 Jul 2018 08:51:29 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 7250716C41DE; Thu, 12 Jul 2018 05:53:51 -0700 (PDT) Date: Thu, 12 Jul 2018 05:53:51 -0700 From: "Paul E. McKenney" To: David Woodhouse Cc: Christian Borntraeger , Peter Zijlstra , mhillenb@amazon.de, linux-kernel , kvm Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested Reply-To: paulmck@linux.vnet.ibm.com References: <20180709220823.GA18045@linux.vnet.ibm.com> <1531319025.8759.57.camel@infradead.org> <20180711144303.GQ3593@linux.vnet.ibm.com> <20180711164952.GA29994@linux.vnet.ibm.com> <9ee4d6fd-02e2-2c73-36a7-36ef4f6413b0@de.ibm.com> <20180711201759.GB3593@linux.vnet.ibm.com> <1531340384.8759.86.camel@infradead.org> <20180711210828.GD3593@linux.vnet.ibm.com> <1531396842.8759.125.camel@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1531396842.8759.125.camel@infradead.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18071212-0040-0000-0000-0000044DC5D4 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009356; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01060263; UDB=6.00544225; IPR=6.00838181; MB=3.00022115; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-12 12:51:33 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18071212-0041-0000-0000-00000853EC06 Message-Id: <20180712125351.GP3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-12_05:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=817 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1807120135 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 12, 2018 at 01:00:42PM +0100, David Woodhouse wrote: > > > On Wed, 2018-07-11 at 14:08 -0700, Paul E. McKenney wrote: > > > > > Also... why in $DEITY's name was the existing > > > rcu_virt_note_context_switch() not actually sufficient? If we had that > > > there, why did we need an additional explicit calls to rcu_all_qs() in > > > the KVM loop, or the more complex fixes to need_resched() which > > > ultimately had the same effect, to avoid ten-second latencies? > > > > My guess is that this was because control passed through the > > rcu_virt_note_context_switch() only once, and then subsequent > > scheduling-clock interrupts bypassed this code. Gah! My guess was instead that the code did a rcu_kvm_enter() going in, but somehow managed to miss the rcu_kvm_exit() going out. But that makes absolutely no sense -- had that happened, rcutorture would likely have screamed bloody murder, loudly and often. No mere near misses! And besides, thus far, -ENOREPRODUCE. :-/ Which indicates that I have an opportunity to improve rcutorture and that this patch was with high probability an innocent bystander. > >   But that is just a guess. > > I need to defer to someone who understands the KVM code better than I do. > > I think it's more likely that we just never happened at all. It's > conditional. From the latest patch iteration (see it being removed): > > @@ -118,12 +118,12 @@ static inline void guest_enter_irqoff(void) >          * one time slice). Lets treat guest mode as quiescent state, just like >          * we do with user-mode execution. >          */ > -       if (!context_tracking_cpu_is_enabled()) > -               rcu_virt_note_context_switch(smp_processor_id()); > +       rcu_kvm_enter(); >  } > > > Given the vmexit overhead, I don't think we can do the currently- > proposed rcu_kvm_enter() thing except for CONFIG_NO_HZ_FULL where it's > really necessary. I'll make that conditional, but probably on the RCU > side. > > Without CONFIG_NO_HZ_FULL, rcu_kvm_exit() can do nothing, and > rcu_kvm_enter() can do rcu_virt_note_context_switch(). > > OK? Makes sense to me! And a big "thank you!" to Christian for testing and analyzing this in a timely fashion!!! Thanx, Paul