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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 9 Jul 2018 16:40:30 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w69KeT8Y15335760 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Mon, 9 Jul 2018 20:40:30 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id BC15BB2064; Mon, 9 Jul 2018 16:40:04 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 9DDA7B206C; Mon, 9 Jul 2018 16:40:04 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.159]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Mon, 9 Jul 2018 16:40:04 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 72F8216C1972; Mon, 9 Jul 2018 13:42:48 -0700 (PDT) Date: Mon, 9 Jul 2018 13:42:48 -0700 From: "Paul E. McKenney" To: David Woodhouse Cc: Peter Zijlstra , mhillenb@amazon.de, linux-kernel Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested Reply-To: paulmck@linux.vnet.ibm.com References: <20180709110657.GL2476@hirez.programming.kicks-ass.net> <20180709123457.GM3593@linux.vnet.ibm.com> <20180709125516.GO2476@hirez.programming.kicks-ass.net> <20180709130227.GQ2512@hirez.programming.kicks-ass.net> <20180709142932.GO3593@linux.vnet.ibm.com> <20180709152632.GX2476@hirez.programming.kicks-ass.net> <20180709163432.GV3593@linux.vnet.ibm.com> <1531162254.26547.3.camel@infradead.org> <20180709203441.GE3593@linux.vnet.ibm.com> <1531168538.26547.5.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: <1531168538.26547.5.camel@infradead.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18070920-0064-0000-0000-00000327727D X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009340; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01058979; UDB=6.00543455; IPR=6.00836899; MB=3.00022076; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-09 20:40:32 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18070920-0065-0000-0000-000039E0E4D3 Message-Id: <20180709204248.GF3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-09_07:,, 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=724 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1807090234 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 09, 2018 at 09:35:38PM +0100, David Woodhouse wrote: > > > On Mon, 2018-07-09 at 13:34 -0700, Paul E. McKenney wrote: > > > > So here are the possible code paths when .rcu_urgent_qs is set to true: > > > > 1.      A context switch will record the quiescent state and clear > >         .rcu_urgent_qs.  (The failure to do the clearing in current -rcu > >         for PREEMPT builds is a performance bug that I need to fix.) > > What if there's nothing else runnable and there is no actual context > switch? The scheduler invokes rcu_note_context_switch() before looking to see if there really will or won't be a context switch. I am sure that Peter will correct me if I am confused on this point. ;-) Thanx, Paul > > 2.      A cond_resched() will cause rcu_all_qs() to be invoked, which > >         will record a quiescent state and clear .rcu_urgent_qs. > > > > 3.      With the patch below, a scheduling-clock interrupt of a > >         non-idle non-userspace task will force a reschedule, which > >         will result in #1 above happening.