Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754333AbdDMQPr (ORCPT ); Thu, 13 Apr 2017 12:15:47 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:49846 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751605AbdDMQPm (ORCPT ); Thu, 13 Apr 2017 12:15:42 -0400 Date: Thu, 13 Apr 2017 09:15:34 -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, bobby.prani@gmail.com Subject: Re: [PATCH tip/core/rcu 01/40] rcu: Maintain special bits at bottom of ->dynticks counter Reply-To: paulmck@linux.vnet.ibm.com References: <20170412174003.GA23207@linux.vnet.ibm.com> <1492018825-25634-1-git-send-email-paulmck@linux.vnet.ibm.com> <20170413092044.ur7nzbzst3jdlepx@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170413092044.ur7nzbzst3jdlepx@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17041316-0040-0000-0000-00000314F03B X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006929; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000208; SDB=6.00847055; UDB=6.00417879; IPR=6.00625489; BA=6.00005288; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00015033; XFM=3.00000013; UTC=2017-04-13 16:15:38 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17041316-0041-0000-0000-00000708FC3F Message-Id: <20170413161534.GB3956@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-04-13_11:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1702020001 definitions=main-1704130137 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1939 Lines: 35 On Thu, Apr 13, 2017 at 11:20:44AM +0200, Peter Zijlstra wrote: > On Wed, Apr 12, 2017 at 10:39:46AM -0700, Paul E. McKenney wrote: > > Currently, IPIs are used to force other CPUs to invalidate their TLBs > > in response to a kernel virtual-memory mapping change. This works, but > > degrades both battery lifetime (for idle CPUs) and real-time response > > (for nohz_full CPUs), and in addition results in unnecessary IPIs due to > > the fact that CPUs executing in usermode are unaffected by stale kernel > > mappings. It would be better to cause a CPU executing in usermode to > > wait until it is entering kernel mode to do the flush, first to avoid > > interrupting usemode tasks and second to handle multiple flush requests > > with a single flush in the case of a long-running user task. > > > > This commit therefore reserves a bit at the bottom of the ->dynticks > > counter, which is checked upon exit from extended quiescent states. > > If it is set, it is cleared and then a new rcu_eqs_special_exit() macro is > > invoked, which, if not supplied, is an empty single-pass do-while loop. > > If this bottom bit is set on -entry- to an extended quiescent state, > > then a WARN_ON_ONCE() triggers. > > > > This bottom bit may be set using a new rcu_eqs_special_set() function, > > which returns true if the bit was set, or false if the CPU turned > > out to not be in an extended quiescent state. Please note that this > > function refuses to set the bit for a non-nohz_full CPU when that CPU > > is executing in usermode because usermode execution is tracked by RCU > > as a dyntick-idle extended quiescent state only for nohz_full CPUs. > > > > Reported-by: Andy Lutomirski > > Isn't that more a: Requested-by ? I am not too worried about the distinction. Request a feature, report the lack of a needed feature, or report a bug, but either way I had to write the code. ;-) Thanx, Paul