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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 12 Nov 2018 02:24:55 -0000 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 wAC2OsUU46465264 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Mon, 12 Nov 2018 02:24:54 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 796D1B2064; Mon, 12 Nov 2018 02:24:54 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 418ADB205F; Mon, 12 Nov 2018 02:24:54 +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 02:24:54 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id B316D16C5D4F; Sun, 11 Nov 2018 18:24:55 -0800 (PST) Date: Sun, 11 Nov 2018 18:24:55 -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> <20181112014736.GB4170@linux.ibm.com> <20181112020710.GJ3056@worktop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181112020710.GJ3056@worktop> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18111202-0064-0000-0000-000003726D9E X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010030; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000270; SDB=6.01116145; UDB=6.00577118; IPR=6.00896253; MB=3.00024116; MTD=3.00000008; XFM=3.00000015; UTC=2018-11-12 02:24:59 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18111202-0065-0000-0000-00003B4E0BED Message-Id: <20181112022455.GD4170@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-11-12_01:,, 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=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1811120020 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 03:07:10AM +0100, Peter Zijlstra wrote: > On Sun, Nov 11, 2018 at 05:47:36PM -0800, Paul E. McKenney wrote: > > 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? > > The quoted bit only states that synchronize_rcu() is sufficient; it does > not say it is equivalent and the patch is a nop. It also doesn't say > that the purpose is to get rid of the synchronize_sched() function. > > > 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") > > The latter; it does not mention that this will possible make > synchronize_sched() quite a bit more expensive on PREEMPT=y builds :/ In theory, sure. In practice, people have switched any number of things from RCU-sched to RCU and back without problems. > > 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()? > > No, I still feel expedited should not exist at all ;-) I figured as much. ;-) > But for PREEMPT=y synchronize_sched() can be quite a bit shorter than > synchronize_rcu(), since we don't have to wait for preempted read side > stuff. Again, there are quite a few places that have managed that transition without issue. Why do you expect this change to have problems that have not been seen elsewhere? > > > > > 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(); > > } > > Again, the patch didn't say that. > > If the Changelog would've read something like: > > "Since synchronize_sched() is now equivalent to synchronize_rcu(), > replace the synchronize_sched() usage such that we can eventually remove > the interface." > > It would've been clear that the patch is a nop and what the purpose > was. I can easily make that change. Thanx, Paul