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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 12 Nov 2018 01:47:36 -0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id wAC1lZPJ29229310 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Mon, 12 Nov 2018 01:47:35 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id F1BCBB2066; Mon, 12 Nov 2018 01:47:34 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id CE2A0B2064; Mon, 12 Nov 2018 01:47:34 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.207.24]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Mon, 12 Nov 2018 01:47:34 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 44A6916C5D33; Sun, 11 Nov 2018 17:47:36 -0800 (PST) Date: Sun, 11 Nov 2018 17:47:36 -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, joel@joelfernandes.org, Ingo Molnar Subject: Re: [PATCH tip/core/rcu 23/41] sched: Replace synchronize_sched() with synchronize_rcu() Reply-To: paulmck@linux.ibm.com References: <20181111194104.GA4787@linux.ibm.com> <20181111194410.6368-23-paulmck@linux.ibm.com> <20181112001233.GC3056@worktop> <20181112004528.GA4170@linux.ibm.com> <20181112005329.GG3056@worktop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181112005329.GG3056@worktop> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18111201-0052-0000-0000-0000035452F1 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010030; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000270; SDB=6.01116132; UDB=6.00578805; IPR=6.00896241; MB=3.00024116; MTD=3.00000008; XFM=3.00000015; UTC=2018-11-12 01:47:39 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18111201-0053-0000-0000-00005EBBCFB9 Message-Id: <20181112014736.GB4170@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-11-11_17:,, 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=891 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1811120014 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 12, 2018 at 01:53:29AM +0100, Peter Zijlstra wrote: > On Sun, Nov 11, 2018 at 04:45:28PM -0800, Paul E. McKenney wrote: > > On Mon, Nov 12, 2018 at 01:12:33AM +0100, Peter Zijlstra wrote: > > > On Sun, Nov 11, 2018 at 11:43:52AM -0800, Paul E. McKenney wrote: > > > > Now that synchronize_rcu() waits for preempt-disable regions of code > > > > as well as RCU read-side critical sections, synchronize_sched() can be > > > > replaced by synchronize_rcu(). This commit therefore makes this change. > > > > > > Yes, but it also waits for an actual RCU quiestent state, which makes > > > synchoinize_rcu() potentially much more expensive than an actual > > > synchronize_sched(). > > > > None of the readers have changed. > > > > For the updaters, if CONFIG_PREEMPT=n, synchronize_rcu() and > > synchronize_sched() always were one and the same. When CONFIG_PREEMPT=y, > > synchronize_rcu() and synchronize_sched() are now one and the same. > > The Changelog does not state this; and does the commit that makes that > happen state the regression potential? The Changelog says this: Now that synchronize_rcu() waits for preempt-disable regions of code as well as RCU read-side critical sections, synchronize_sched() can be replaced by synchronize_rcu(). This commit therefore makes this change. The "synchronize_rcu() waits for preempt-disable regions of code as well as RCU read-side critical sections" seems pretty unambiguous to me. Exactly what more are you wanting said there? There were quite a few commits involved in making this happen. Perhaps the most pertinent are these: 3e3100989869 ("rcu: Defer reporting RCU-preempt quiescent states when disabled") 45975c7d21a1 ("rcu: Define RCU-sched API in terms of RCU for Tree RCU PREEMPT builds") Normal grace periods are almost always quite long compared to typical read-side critical sections, preempt-disable regions of code, and so on. So in the common case this should be OK. Or are you instead worried about synchronize_sched_expedited()? > > > So why are we doing this? > > > > Given that synchronize_rcu() and synchronize_sched() are now always one > > and the same, this is a distinction without a difference. > > The Changelog did not state a reason for the patch. Therefore it is a > bad patch. ??? Here is the current definition of synchronize_sched() in mainline: static inline void synchronize_sched(void) { synchronize_rcu(); } Thanx, Paul