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Wed, 8 Jan 2025 18:18:19 +0000 Received: from CO6PR10MB5409.namprd10.prod.outlook.com ([fe80::25a9:32c2:a7b0:de9e]) by CO6PR10MB5409.namprd10.prod.outlook.com ([fe80::25a9:32c2:a7b0:de9e%3]) with mapi id 15.20.8335.011; Wed, 8 Jan 2025 18:18:19 +0000 References: <20241213040658.2282640-1-ankur.a.arora@oracle.com> User-agent: mu4e 1.4.10; emacs 27.2 From: Ankur Arora To: paulmck@kernel.org Cc: Ankur Arora , linux-kernel@vger.kernel.org, peterz@infradead.org, tglx@linutronix.de, mingo@kernel.org, bigeasy@linutronix.de, 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, sshegde@linux.ibm.com, boris.ostrovsky@oracle.com Subject: Re: [PATCH v3 0/7] RCU changes for PREEMPT_LAZY In-reply-to: Date: Wed, 08 Jan 2025 10:18:16 -0800 Message-ID: <87frltcgpj.fsf@oracle.com> Content-Type: text/plain X-ClientProxiedBy: YQBPR01CA0092.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:3::28) 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_|CH0PR10MB7533:EE_ X-MS-Office365-Filtering-Correlation-Id: 40e46161-3113-4c86-8911-08dd3010d426 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014|7416014|7053199007; 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McKenney writes: > On Thu, Dec 12, 2024 at 08:06:51PM -0800, Ankur Arora wrote: >> This series adds RCU bits for lazy preemption. >> >> The problem addressed is that pre-PREEMPT_LAZY, PREEMPTION=y implied >> PREEMPT_RCU=y. With PREEMPT_LAZY, that's no longer true. >> >> That's because PREEMPT_RCU makes some trade-offs to optimize for >> latency as opposed to throughput, and configurations with limited >> preemption might prefer the stronger forward-progress guarantees of >> PREEMPT_RCU=n. >> >> Accordingly, with standalone PREEMPT_LAZY (much like PREEMPT_NONE, >> PREEMPT_VOLUNTARY) we want to use PREEMPT_RCU=n. And, when used in >> conjunction with PREEMPT_DYNAMIC, we continue to use PREEMPT_RCU=y. >> >> Patches 1-3 are cleanup patches: >> "rcu: fix header guard for rcu_all_qs()" >> "rcu: rename PREEMPT_AUTO to PREEMPT_LAZY" >> "sched: update __cond_resched comment about RCU quiescent states" >> >> Patch 4, >> "rcu: handle unstable rdp in rcu_read_unlock_strict()" >> >> handles a latent RCU bug rcu_report_qs_rdp() could be called with >> an unstable rdp. >> >> Patches 5 and 6, >> "rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT_COUNT=y" >> "osnoise: provide quiescent states" >> >> handle quiescent states for the (PREEMPT_LAZY=y, PREEMPT_RCU=n) >> configuration. >> >> And, finally patch 7, "rcu: limit PREEMPT_RCU configurations", >> explicitly limits PREEMPT_RCU=y to the PREEMPT_DYNAMIC or the latency >> oriented models. > > Pulled into my -rcu tree for further review and testing, with initial > tests passing. Apologies for the delay! Great. Thanks Paul! Ankur >> Changelog: >> >> v3: >> - moved patch-3 to be the last one in the series (suggested by Sebastian) >> - added "rcu: handle unstable rdp in rcu_read_unlock_strict()" >> (suggested by Frederic Weisbecker). >> - switched to a more robust check in rcu_flavor_sched_clock_irq() >> (suggested by Frederic Weisbecker). >> - simplified check in osnoise (suggested by Frederic Weisbecker). >> - dropped an unrelated scheduler patch. >> >> v2: >> - fixup incorrect usage of tif_need_resched_lazy() (comment from >> from Sebastian Andrzej Siewior) >> - massaged the commit messages a bit >> - drops the powerpc support for PREEMPT_LAZY as that was orthogonal >> to this series (Shrikanth will send that out separately.) >> >> Please review. >> >> Ankur Arora (7): >> rcu: fix header guard for rcu_all_qs() >> rcu: rename PREEMPT_AUTO to PREEMPT_LAZY >> sched: update __cond_resched comment about RCU quiescent states >> rcu: handle unstable rdp in rcu_read_unlock_strict() >> rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT_COUNT=y >> osnoise: provide quiescent states >> rcu: limit PREEMPT_RCU configurations >> >> include/linux/rcupdate.h | 2 +- >> include/linux/rcutree.h | 2 +- >> include/linux/srcutiny.h | 2 +- >> kernel/rcu/Kconfig | 4 ++-- >> kernel/rcu/srcutiny.c | 14 +++++++------- >> kernel/rcu/tree_plugin.h | 22 +++++++++++++++++----- >> kernel/sched/core.c | 4 +++- >> kernel/trace/trace_osnoise.c | 32 +++++++++++++++----------------- >> 8 files changed, 47 insertions(+), 35 deletions(-) >> >> -- >> 2.43.5 >> -- ankur