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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 9 Jul 2018 10:27:15 -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 w69ERE3e47186108 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Mon, 9 Jul 2018 14:27:14 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 528A4B2070; Mon, 9 Jul 2018 10:26:49 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 3E3AFB206C; Mon, 9 Jul 2018 10:26:49 -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 10:26:49 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 57E4916C0F7D; Mon, 9 Jul 2018 07:29:32 -0700 (PDT) Date: Mon, 9 Jul 2018 07:29:32 -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: <20180706162905.GZ2476@hirez.programming.kicks-ass.net> <20180706171150.GI3593@linux.vnet.ibm.com> <20180709085351.GC2476@hirez.programming.kicks-ass.net> <1531127935.18697.57.camel@infradead.org> <20180709104429.GI2476@hirez.programming.kicks-ass.net> <1531133801.18697.73.camel@infradead.org> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180709130227.GQ2512@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18070914-0064-0000-0000-000003274FB3 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009338; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01058854; UDB=6.00543380; IPR=6.00836775; MB=3.00022073; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-09 14:27:17 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18070914-0065-0000-0000-000039E00C6B Message-Id: <20180709142932.GO3593@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=985 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1807090165 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 03:02:27PM +0200, Peter Zijlstra wrote: > On Mon, Jul 09, 2018 at 02:55:16PM +0200, Peter Zijlstra wrote: > > On Mon, Jul 09, 2018 at 05:34:57AM -0700, Paul E. McKenney wrote: > > > But KVM defeats this by checking need_resched() before invoking > > > cond_resched(). > > > > That's not wrong or even uncommon I think. > > In fact, I think we recently put that pattern in crypto code in order to > break up very long kernel_fpu sections. OK, so here are our options: 1. Add the RCU conditional to need_resched(), as David suggests. Peter has concerns about overhead. 2. Create a new need_resched_rcu_qs() that is to be used when deciding whether or not to do cond_resched(). This would exact the overhead only where it is needed, but is one more thing for people to get wrong. 3. Revert my changes to de-emphasize cond_resched_rcu_qs(), and go back to sprinkling cond_resched_rcu_qs() throughout the code. This also is one more thing for people to get wrong, and might well eventually convert all cond_resched() calls to cond_resched_rcu_qs(), which sure seems like a failure mode to me. 4. Others? > Note that you also 'broke' cond_resched_lock() as that no longer matches > cond_resched(). Given that cond_resched_lock() was there first, I believe that you can just say "broke" without the quote marks. :-/ Given that this code is releasing and acquiring a lock, I believe that the patch below should cure this, aside from also needing to check whether RCU needs a quiescent state. Any other similar gotchas out there? Thanx, Paul ------------------------------------------------------------------------ diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 537bced8f4bc..b559b556f464 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -5017,6 +5017,7 @@ int __cond_resched_lock(spinlock_t *lock) preempt_schedule_common(); else cpu_relax(); + rcu_all_qs(); ret = 1; spin_lock(lock); }