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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Tue, 10 Jul 2018 12:24:20 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22035.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w6AGOJfK5833314 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Tue, 10 Jul 2018 16:24:19 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C00E1B206A; Tue, 10 Jul 2018 12:23:52 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id AB61EB2065; Tue, 10 Jul 2018 12:23:52 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.159]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Tue, 10 Jul 2018 12:23:52 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 0158A16C1AF9; Tue, 10 Jul 2018 09:26:37 -0700 (PDT) Date: Tue, 10 Jul 2018 09:26:37 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: David Woodhouse , 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: <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> <20180709204248.GF3593@linux.vnet.ibm.com> <20180710092426.GC2476@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20180710092426.GC2476@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18071016-0052-0000-0000-0000030BCA54 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009345; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01059373; UDB=6.00543691; IPR=6.00837294; MB=3.00022089; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-10 16:24:21 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18071016-0053-0000-0000-00005D501D9A Message-Id: <20180710162637.GW3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-10_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=752 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1807100174 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 10, 2018 at 11:24:26AM +0200, Peter Zijlstra wrote: > On Mon, Jul 09, 2018 at 01:42:48PM -0700, Paul E. McKenney wrote: > > 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. > > Correct. Just getting to __schedule() means we can schedule and thus is > a valid point for RCU to progress. Even if we then end up selecting the > very same task and not switching at all. Thank you for the confirmation! Thanx, Paul