From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751911AbdLBMW1 (ORCPT ); Sat, 2 Dec 2017 07:22:27 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:59556 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751687AbdLBMWZ (ORCPT ); Sat, 2 Dec 2017 07:22:25 -0500 Date: Sat, 2 Dec 2017 04:22:20 -0800 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 Subject: Re: [PATCH tip/core/rcu 10/10] rcu: Account for rcu_all_qs() in cond_resched() Reply-To: paulmck@linux.vnet.ibm.com References: <20171201192122.GA19301@linux.vnet.ibm.com> <1512156104-20104-10-git-send-email-paulmck@linux.vnet.ibm.com> <20171202085626.GS3326@worktop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171202085626.GS3326@worktop> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17120212-0036-0000-0000-0000029543A3 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00008141; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000243; SDB=6.00954324; UDB=6.00482244; IPR=6.00734381; BA=6.00005727; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00018305; XFM=3.00000015; UTC=2017-12-02 12:22:23 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17120212-0037-0000-0000-0000428D4935 Message-Id: <20171202122220.GM7829@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-12-02_05:,, 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 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1712020182 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Dec 02, 2017 at 09:56:26AM +0100, Peter Zijlstra wrote: > On Fri, Dec 01, 2017 at 11:21:44AM -0800, Paul E. McKenney wrote: > > If cond_resched() returns false, then it has already invoked > > rcu_all_qs(). This is also invoked (now redundantly) by > > rcu_note_voluntary_context_switch(). This commit therefore changes > > cond_resched_rcu_qs() to invoke rcu_note_voluntary_context_switch_lite() > > instead of rcu_note_voluntary_context_switch() to avoid the redundant > > invocation of rcu_all_qs(). > > > > Signed-off-by: Paul E. McKenney > > --- > > include/linux/rcupdate.h | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > index a6ddc42f87a5..7bd8b5a6db10 100644 > > --- a/include/linux/rcupdate.h > > +++ b/include/linux/rcupdate.h > > @@ -197,7 +197,7 @@ static inline void exit_tasks_rcu_finish(void) { } > > #define cond_resched_rcu_qs() \ > > do { \ > > if (!cond_resched()) \ > > - rcu_note_voluntary_context_switch(current); \ > > + rcu_note_voluntary_context_switch_lite(current); \ > > } while (0) > > > > Maybe I'm confused, but why are we keeping cond_resched_rcu_qs() around > at all? Because there are a few key places within RCU and rcutorture that need it. Without it, there are scenarios where the new cond_resched() never gets activated, and thus doesn't take effect. The key point is that with this series in place, it should not be necessary to use cond_resched_rcu_qs() outside of kernel/rcu and kernel/torture.c. Which is a valuable step forward, right? Thanx, Paul