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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Wed, 11 Jul 2018 17:06:10 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23034.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w6BL69qQ3670506 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Wed, 11 Jul 2018 21:06:09 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D3F58B2065; Wed, 11 Jul 2018 17:06:08 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id B617FB2064; Wed, 11 Jul 2018 17:06:08 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.159]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Wed, 11 Jul 2018 17:06:08 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id DE35F16C3B7E; Wed, 11 Jul 2018 14:08:28 -0700 (PDT) Date: Wed, 11 Jul 2018 14:08:28 -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: <1531169145.26547.8.camel@infradead.org> <20180709210532.GH3593@linux.vnet.ibm.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1531340384.8759.86.camel@infradead.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18071121-0060-0000-0000-0000028AC1BA X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009352; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01059947; UDB=6.00544036; IPR=6.00837868; MB=3.00022107; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-11 21:06:12 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18071121-0061-0000-0000-000045C381CF Message-Id: <20180711210828.GD3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-11_06:,, 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=800 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1807110222 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 11, 2018 at 09:19:44PM +0100, David Woodhouse wrote: > On Wed, 2018-07-11 at 13:17 -0700, Paul E. McKenney wrote: > > As I understand it, they would like to have their guest run uninterrupted > > for extended times.  Because rcu_virt_note_context_switch() is a > > point-in-time quiescent state, it cannot tell RCU about the extended > > quiescent state. > > > > Should we replace the current calls to rcu_virt_note_context_switch() > > with rcu_kvm_enter() and rcu_kvm_exit()?  Would that be better > > than the below architecture-by-architecture approach? > > Yes it would. I was already starting to mutter about needing the same > for ARM and POWER. I'll do a v3 (incorporating your fixes too) in the > morning. > > Thanks. > > 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. But that is just a guess. I need to defer to someone who understands the KVM code better than I do. Thanx, Paul