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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Tue, 26 Jun 2018 14:00:59 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23032.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w5QI0wQT19005926 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Tue, 26 Jun 2018 18:00:58 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 94273B206B; Tue, 26 Jun 2018 14:00:51 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 739B9B2064; Tue, 26 Jun 2018 14:00:51 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.159]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Tue, 26 Jun 2018 14:00:51 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 6615316C15BE; Tue, 26 Jun 2018 11:03:03 -0700 (PDT) Date: Tue, 26 Jun 2018 11:03:03 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org Subject: Re: [PATCH tip/core/rcu 06/27] rcu: Mark task as .need_qs less aggressively Reply-To: paulmck@linux.vnet.ibm.com References: <20180626003448.GA26209@linux.vnet.ibm.com> <20180626003513.27812-6-paulmck@linux.vnet.ibm.com> <20180626170812.GH2494@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180626170812.GH2494@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18062618-0060-0000-0000-00000282D2C9 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009259; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01052686; UDB=6.00539679; IPR=6.00830610; MB=3.00021865; MTD=3.00000008; XFM=3.00000015; UTC=2018-06-26 18:01:02 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18062618-0061-0000-0000-0000459572C0 Message-Id: <20180626180303.GD3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-06-26_09:,, 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=938 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1806260202 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 26, 2018 at 07:08:12PM +0200, Peter Zijlstra wrote: > On Mon, Jun 25, 2018 at 05:34:52PM -0700, Paul E. McKenney wrote: > > If any scheduling-clock interrupt interrupts an RCU-preempt read-side > > critical section, the interrupted task's ->rcu_read_unlock_special.b.need_qs > > field is set. This causes the outermost rcu_read_unlock() to incur the > > extra overhead of calling into rcu_read_unlock_special(). This commit > > reduces that overhead by setting ->rcu_read_unlock_special.b.need_qs only > > if the grace period has been in effect for more than one second. > > Even less agressive is never setting it at all. True, but if the CPU has been in an RCU read-side critical section for a full second (which is the case with high probability when .b.need_qs is set after this change), we might want to respond to the end of that critical section sooner rather than later. > Changelog fails to explain why not setting it every tick is correct, nor > why 1s is a 'safe' value to use. The RCU CPU stall warning cannot be set to less than 3s, so 1s is reasonable. It is a tradeoff -- setting it lower causes a greater fraction of RCU read-side critical sections to incur extra overhead at rcu_read_unlock() time, while setting it higher keeps a lazy approach to reporting the quiescent state to core RCU for longer critical sections. The upcoming RCU-bh/RCU-preempt/RCU-sched consolidation will raise contention and overhead, so this is one of several things done to lower overhead and contention to compensate for that. Thanx, Paul