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Sat, 19 Oct 2024 01:07:58 +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; Sat, 19 Oct 2024 01:07:58 +0000 References: <20241011081847.r2x73XIr@linutronix.de> <20241011144341.mQKXkGkm@linutronix.de> <20241015112224.KdvzKo80@linutronix.de> <87ed4hrp45.fsf@oracle.com> <20241017080439.9od9eoBO@linutronix.de> <871q0emji1.fsf@oracle.com> <0c04a986-91d0-4f6d-bd4e-ca00d1cd710a@paulmck-laptop> <87zfn1jk43.fsf@oracle.com> User-agent: mu4e 1.4.10; emacs 27.2 From: Ankur Arora To: paulmck@kernel.org Cc: Ankur Arora , Sebastian Andrzej Siewior , Peter Zijlstra , 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: Fri, 18 Oct 2024 18:07:52 -0700 Message-ID: <87iktokihj.fsf@oracle.com> Content-Type: text/plain X-ClientProxiedBy: LO4P302CA0018.GBRP302.PROD.OUTLOOK.COM (2603:10a6:600:2c1::14) 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_|CY8PR10MB6634:EE_ X-MS-Office365-Filtering-Correlation-Id: b0e1563c-f32c-47e1-df23-08dcefda7860 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|1800799024|7416014|366016; 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McKenney writes: > On Fri, Oct 18, 2024 at 12:18:04PM -0700, Ankur Arora wrote: >> >> Paul E. McKenney writes: >> >> > On Thu, Oct 17, 2024 at 03:50:46PM -0700, Ankur Arora wrote: >> >> Sebastian Andrzej Siewior writes: >> >> > On 2024-10-15 15:13:46 [-0700], Ankur Arora wrote: >> >> >> Sebastian Andrzej Siewior writes: >> >> >> >> [ ... ] >> >> Sure. But, that's just begging the question. >> >> >> >> We want _NONE and _VOLUNTARY to go away because keeping cond_resched() >> >> around incurs a cost. >> > >> > When you say "go away", do you mean for your use cases? Or globally? >> >> When I say "want _ to go away" I mean: cond_resched() is deleterious >> to performance since you are forced to have code which can do the >> rescheduling check synchronously -- when this could easily be done >> asynchronously (as the non voluntary models do). >> >> And this either means poor performance (ex. in the page zeroing code >> where it would be more optimal to work on continguous ranges) or >> gyrations like the ones that xen_pv_evtchn_do_upcall() and the >> Xen hypervisor have to go through. >> >> And, as we've discussed before, the cond_resched() interface, while it >> works, is not ideal. > > I would expect that many instances of cond_resched() could go away given > lazy preemption, but I would not be surprised if there were some that > needed to stay around. > > Your thought being that if *all* instance of cond_resched() go away, > then PREEMPT_NONE also goes away? If *all* instances of cond_resched() go away, is there anything left of PREEMPT_NONE? > Given how long PREEMPT_NONE has been > around, this would need to be done (and communicated) quite carefully. I don't think there's any question about that. >> Also, a man can dream! > > Fair enough, just be very careful to distinguish dreams from reality. ;-) I've generally not found that to be a problem, but thanks for the warning. -- ankur