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Mon, 21 Oct 2024 19:21:01 +0000 Received: from CO6PR10MB5409.namprd10.prod.outlook.com ([fe80::25a9:32c2:a7b0:de9e]) by CO6PR10MB5409.namprd10.prod.outlook.com ([fe80::25a9:32c2:a7b0:de9e%4]) with mapi id 15.20.8069.024; Mon, 21 Oct 2024 19:21:01 +0000 References: <20241011081847.r2x73XIr@linutronix.de> <20241011144341.mQKXkGkm@linutronix.de> <20241015112224.KdvzKo80@linutronix.de> <2a2a3ae6-ed0b-4afe-b48a-489cf19667a3@paulmck-laptop> <20241017070710.U9bTJFMS@linutronix.de> <0313c8c5-a6a0-4d09-9f85-ac5afa379041@paulmck-laptop> <20241021112755.m7dWNbc0@linutronix.de> User-agent: mu4e 1.4.10; emacs 27.2 From: Ankur Arora To: paulmck@kernel.org Cc: Sebastian Andrzej Siewior , Peter Zijlstra , Ankur Arora , linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@kernel.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, frederic@kernel.org, efault@gmx.de Subject: Re: [PATCH 2/7] rcu: limit PREEMPT_RCU configurations In-reply-to: Date: Mon, 21 Oct 2024 12:20:58 -0700 Message-ID: <87ldyhi7ol.fsf@oracle.com> Content-Type: text/plain X-ClientProxiedBy: MW4PR03CA0292.namprd03.prod.outlook.com (2603:10b6:303:b5::27) To CO6PR10MB5409.namprd10.prod.outlook.com (2603:10b6:5:357::14) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CO6PR10MB5409:EE_|DM4PR10MB6839:EE_ X-MS-Office365-Filtering-Correlation-Id: 7ed4805f-1bbc-4b26-81a5-08dcf2057fb7 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|366016|7416014; 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McKenney writes: > On Mon, Oct 21, 2024 at 01:27:55PM +0200, Sebastian Andrzej Siewior wrote: >> On 2024-10-18 10:38:15 [-0700], Paul E. McKenney wrote: >> > > > > I don't think this is always the case because the "preemptible" users >> > > > > would also get this and this is an unexpected change for them. >> > > > >> > > > Is this series now removing PREEMPT_NONE and PREEMPT_VOLUNTARY? >> > > no, not yet. It is only adding PREEMPT_LAZY as new model, next to >> > > PREEMPT_NONE and PREEMPT_VOLUNTARY. But is is likely to be on schedule. >> > > >> > > > As conceived last time around, the change would affect only kernels >> > > > built with one of the other of those two Kconfig options, which will >> > > > not be users expecting preemption. >> > > >> > > If you continue to use PREEMPT_NONE/ PREEMPT_VOLUNTARY nothing changes >> > > right now. >> > >> > Good, thank you! >> > >> > Presumably PREEMPT_NONE=y && PREEMPT_LAZY=y enables lazy preemption, >> > but retains non-preemptible RCU. >> >> PREEMPT_NONE=y && PREEMPT_LAZY=y is exclusive. Either NONE or LAZY. > > Ah, I was thinking in terms of the previous lazy-preemption patch series, > and just now got around to looking at the new one. Apologies for my > confusion! Minor point, but you might be thinking of PREEMPT_AUTO=y && PREEMPT_LAZY=y. >> > > > If PREEMPT_NONE and PREEMPT_VOLUNTARY are still around, it would be >> > > > far better to make PREEMPT_RCU depend on neither of those being set. >> > > > That would leave the RCU Kconfig settings fully automatic, and this >> > > > automation is not to be abandoned lightly. >> > > >> > > Yes, that was my intention - only to make is selectable with >> > > LAZY-preemption enabled but without dynamic. >> > > So you are not complete against it. >> > >> > Help me out here. In what situation is it beneficial to make PREEMPT_RCU >> > visible, given that PREEMPT_NONE, PREEMPT_VOLUNTARY, PREEMPT, and >> > PREEMPT_FULL already take care of this automatically? >> >> We have now NONE, VOLUNTARY, PREEMPT, PREEMPT_RT (as in choose one). >> >> This series changes it to NONE, VOLUNTARY, PREEMPT, LAZY, LAZIEST. >> Ignore LAZIEST. PREEMPT_RT is a on/ off bool. > > In terms of preemptibility, isn't the order from least to most preemptible > NONE, VOLUNTARY, LAZIEST, LAZY, PREEMPT, and PREEMPT_RT? After all, > PREEMPT will preempt more aggressively than will LAZY which in turn > preempts more aggressively than LAZIEST. > > And I finally do see the later patch in Peter's series that removes > PREEMPT_RT from the choice. Plus I need to look more at LAZIEST in > order to work out whether Peter's suckage is our robustness. ;-) > >> Based on my understanding so far, you have nothing to worry about. >> >> With NONE + VOLUNTARY removed in favor of LAZY or without the removal >> (yet) you ask yourself what happens to those using NONE, go to LAZY and >> want to stay with !PREEMPT_RCU, right? >> With LAZY and !PREEMPT_DYNAMIC there is still PREEMPT_RCU as of now. >> And you say people using !PREEMPT_DYNAMIC + LAZY are the old NONE/ >> VOLUNTARY users and want !PREEMPT_RCU. > > Yes. > >> This could be true but it could also attract people from PREEMPT which >> expect additional performance gain due to LAZY and the same "preemption" >> level. Additionally if PREEMPT gets removed because LAZY turns out to be >> superior then PREEMPT_DYNAMIC makes probably no sense since there is >> nothing to switch from/ to. > > We definitely have users that would migrate from NONE to LAZY. Shouldn't Indeed. This was the original intent behind Thomas's proposal of preempt lazy. > their needs outweigh the possible future users that might (or might not) > find that (1) LAZY might be preferable to PREEMPT for some users and > (2) That those users would prefer that RCU be preemptible? Users who care about low latency already have perfectly good options: PREEMPT, PREEMPT_DYNAMIC and now PREEMPT_RT. I don't see the point of elevating low latency needs in all preemption models -- even those which are meant to be througput oriented. >> Therefore I would suggest to make PREEMPT_RCU >> - selectable for LAZY && !PREEMPT_DYNAMIC, default yes >> - selected for LAZY && PREEMPT_DYNAMIC >> - the current unchanged state for NONE, VOLUNTARY, PREEMPT (with >> !PREEMPT_DYNAMIC) >> >> Does this make sense to you? > > Not really. At the very least, default no. > > Unless LAZIEST makes the most sense for us (which will take time to > figure out), in which case make PREMPT_RCU: I don't think laziest was ever meant to be a serious option. Peter mentioned that he'll be dropping it: https://lore.kernel.org/lkml/20241008144049.GF14587@noisy.programming.kicks-ass.net/ Ankur > - hard-defined =n for LAZIEST. > - selectable for LAZY && !PREEMPT_DYNAMIC, default yes > - selected for LAZY && PREEMPT_DYNAMIC > - the current unchanged state for NONE, VOLUNTARY, PREEMPT (with > !PREEMPT_DYNAMIC) > > Or am I still missing some aspect of this series? > > Thanx, Paul -- ankur