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Tue, 7 Nov 2023 22:00:01 +0000 From: Ankur Arora To: linux-kernel@vger.kernel.org Cc: tglx@linutronix.de, peterz@infradead.org, torvalds@linux-foundation.org, paulmck@kernel.org, linux-mm@kvack.org, x86@kernel.org, akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, willy@infradead.org, mgorman@suse.de, jon.grimm@amd.com, bharata@amd.com, raghavendra.kt@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com, jgross@suse.com, andrew.cooper3@citrix.com, mingo@kernel.org, bristot@kernel.org, mathieu.desnoyers@efficios.com, geert@linux-m68k.org, glaubitz@physik.fu-berlin.de, anton.ivanov@cambridgegreys.com, mattst88@gmail.com, krypton@ulrich-teichert.org, rostedt@goodmis.org, David.Laight@ACULAB.COM, richard@nod.at, mjguzik@gmail.com, Ankur Arora Subject: [RFC PATCH 45/86] preempt: ARCH_NO_PREEMPT only preempts lazily Date: Tue, 7 Nov 2023 13:57:31 -0800 Message-Id: <20231107215742.363031-46-ankur.a.arora@oracle.com> X-Mailer: git-send-email 2.31.1 In-Reply-To: <20231107215742.363031-1-ankur.a.arora@oracle.com> References: <20231107215742.363031-1-ankur.a.arora@oracle.com> X-ClientProxiedBy: MW4PR04CA0169.namprd04.prod.outlook.com (2603:10b6:303:85::24) To CO6PR10MB5409.namprd10.prod.outlook.com (2603:10b6:5:357::14) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CO6PR10MB5409:EE_|CY5PR10MB6141:EE_ X-MS-Office365-Filtering-Correlation-Id: b64a4c84-2a99-4663-0218-08dbdfdce3f0 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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Tue, 07 Nov 2023 14:07:07 -0800 (PST) X-Spam-Status: No, score=-0.9 required=5.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,T_SCC_BODY_TEXT_LINE autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on groat.vger.email X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1781944637513652999 X-GMAIL-MSGID: 1781944637513652999 Note: this commit is badly broken. Only here for discussion. Configurations with ARCH_NO_PREEMPT support preempt_count, but might not be tested well enough under PREEMPTION to support it might not be demarcating the necessary non-preemptible sections. One way to handle this is by limiting them to PREEMPT_NONE mode, not doing any tick enforcement and limiting preemption to happen only at user boundary. Unfortunately, this is only a partial solution because eager rescheduling could still happen (say, due to RCU wanting an expedited quiescent period.) And, because we do not trust the preempt_count accounting, this would mean preemption inside an unmarked critical section. I suppose we could disable that (say by selecting PREEMPTION=n), but then the only avenue for driving scheduling between kernel contexts (when there is no ongoing userspace work) would be explicit calls to schedule(). Signed-off-by: Ankur Arora --- kernel/sched/core.c | 12 ++++++++++-- kernel/sched/features.h | 7 +++++++ 2 files changed, 17 insertions(+), 2 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 3fa78e8afb7d..bf5df2b866df 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1059,6 +1059,14 @@ void __resched_curr(struct rq *rq, resched_t rs) trace_sched_wake_idle_without_ipi(cpu); } +#ifndef CONFIG_ARCH_NO_PREEMPT +#define force_preempt() sched_feat(FORCE_PREEMPT) +#define preempt_priority() sched_feat(PREEMPT_PRIORITY) +#else +#define force_preempt() false +#define preempt_priority() false +#endif + /* * resched_curr - mark rq's current task 'to be rescheduled' eagerly * or lazily according to the current policy. @@ -1084,7 +1092,7 @@ void resched_curr(struct rq *rq, bool above) resched_t rs = RESCHED_lazy; int context; - if (sched_feat(FORCE_PREEMPT) || + if (force_preempt() || (rq->curr->sched_class == &idle_sched_class)) { rs = RESCHED_eager; goto resched; @@ -1115,7 +1123,7 @@ void resched_curr(struct rq *rq, bool above) goto resched; } - if (sched_feat(PREEMPT_PRIORITY) && above) + if (preempt_priority() && above) rs = RESCHED_eager; resched: diff --git a/kernel/sched/features.h b/kernel/sched/features.h index 9bf30732b03f..2575d018b181 100644 --- a/kernel/sched/features.h +++ b/kernel/sched/features.h @@ -90,6 +90,12 @@ SCHED_FEAT(LATENCY_WARN, false) SCHED_FEAT(HZ_BW, true) +#ifndef CONFIG_ARCH_NO_PREEMPT +/* + * Architectures with CONFIG_ARCH_NO_PREEMPT cannot safely preempt. + * So even though they enable CONFIG_PREEMPTION, they never have the + * option to dynamically switch preemption models. + */ #if defined(CONFIG_PREEMPT) SCHED_FEAT(FORCE_PREEMPT, true) SCHED_FEAT(PREEMPT_PRIORITY, true) @@ -100,3 +106,4 @@ SCHED_FEAT(PREEMPT_PRIORITY, true) SCHED_FEAT(FORCE_PREEMPT, false) SCHED_FEAT(PREEMPT_PRIORITY, false) #endif +#endif