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Tue, 7 Nov 2023 21:59:54 +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 43/86] sched: enable PREEMPT_COUNT, PREEMPTION for all preemption models Date: Tue, 7 Nov 2023 13:57:29 -0800 Message-Id: <20231107215742.363031-44-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: MW4PR03CA0250.namprd03.prod.outlook.com (2603:10b6:303:b4::15) 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: e23fc83e-3569-4276-4692-08dbdfdcdfd9 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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Tue, 07 Nov 2023 14:05:05 -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: 1781944598387449876 X-GMAIL-MSGID: 1781944598387449876 The scheduler uses PREEMPT_COUNT and PREEMPTION to drive preemption: the first to demarcate non-preemptible sections and the second for the actual mechanics of preemption. Enable both for voluntary preemption models. In addition, define a new scheduler feature FORCE_PREEMPT which can now be used to distinguish between voluntary and full preemption models at runtime. Originally-by: Thomas Gleixner Signed-off-by: Ankur Arora --- init/Makefile | 2 +- kernel/Kconfig.preempt | 12 ++++++++---- kernel/entry/common.c | 3 +-- kernel/sched/core.c | 26 +++++++++++--------------- kernel/sched/features.h | 6 ++++++ 5 files changed, 27 insertions(+), 22 deletions(-) diff --git a/init/Makefile b/init/Makefile index 385fd80fa2ef..99e480f24cf3 100644 --- a/init/Makefile +++ b/init/Makefile @@ -24,7 +24,7 @@ mounts-$(CONFIG_BLK_DEV_INITRD) += do_mounts_initrd.o # smp-flag-$(CONFIG_SMP) := SMP -preempt-flag-$(CONFIG_PREEMPT) := PREEMPT +preempt-flag-$(CONFIG_PREEMPTION) := PREEMPT_DYNAMIC preempt-flag-$(CONFIG_PREEMPT_RT) := PREEMPT_RT build-version = $(or $(KBUILD_BUILD_VERSION), $(build-version-auto)) diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt index aa87b5cd3ecc..074fe5e253b5 100644 --- a/kernel/Kconfig.preempt +++ b/kernel/Kconfig.preempt @@ -6,20 +6,23 @@ choice config PREEMPT_NONE bool "No Forced Preemption (Server)" + select PREEMPTION help This is the traditional Linux preemption model, geared towards throughput. It will still provide good latencies most of the - time, but there are no guarantees and occasional longer delays - are possible. + time, but occasional delays are possible. Select this option if you are building a kernel for a server or scientific/computation system, or if you want to maximize the raw processing power of the kernel, irrespective of scheduling - latencies. + latencies. Unless your architecture actively disables preemption, + you can always switch to one of the other preemption models + at runtime. config PREEMPT_VOLUNTARY bool "Voluntary Kernel Preemption (Desktop)" depends on !ARCH_NO_PREEMPT + select PREEMPTION help This option reduces the latency of the kernel by adding more "explicit preemption points" to the kernel code. These new @@ -53,7 +56,8 @@ config PREEMPT Select this if you are building a kernel for a desktop or embedded system with latency requirements in the milliseconds - range. + range. You can always switch to one of lower preemption options + at runtime. config PREEMPT_RT bool "Fully Preemptible Kernel (Real-Time)" diff --git a/kernel/entry/common.c b/kernel/entry/common.c index 6433e6c77185..f7f2efabb5b5 100644 --- a/kernel/entry/common.c +++ b/kernel/entry/common.c @@ -422,8 +422,7 @@ noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t state) } instrumentation_begin(); - if (IS_ENABLED(CONFIG_PREEMPTION)) - irqentry_exit_cond_resched(); + irqentry_exit_cond_resched(); /* Covers both tracing and lockdep */ trace_hardirqs_on(); instrumentation_end(); diff --git a/kernel/sched/core.c b/kernel/sched/core.c index f65bf3ce0e9d..2a50a64255c6 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1065,7 +1065,7 @@ void __resched_curr(struct rq *rq, resched_t rs) * * Always schedule eagerly, if: * - * - running under full preemption + * - running under full preemption (sched_feat(FORCE_PREEMPT)) * * - idle: when not polling (or if we don't have TIF_POLLING_NRFLAG) * force TIF_NEED_RESCHED to be set and send a resched IPI. @@ -1081,7 +1081,7 @@ void resched_curr(struct rq *rq) resched_t rs = RESCHED_lazy; int context; - if (IS_ENABLED(CONFIG_PREEMPT) || + if (sched_feat(FORCE_PREEMPT) || (rq->curr->sched_class == &idle_sched_class)) { rs = RESCHED_eager; goto resched; @@ -1108,7 +1108,6 @@ void resched_curr(struct rq *rq) context = ct_state_cpu(cpu_of(rq)); if ((context == CONTEXT_USER) || (context == CONTEXT_GUEST)) { - rs = RESCHED_eager; goto resched; } @@ -6597,20 +6596,18 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) * * 1. Explicit blocking: mutex, semaphore, waitqueue, etc. * - * 2. TIF_NEED_RESCHED flag is checked on interrupt and userspace return - * paths. For example, see arch/x86/entry_64.S. + * 2. TIF_NEED_RESCHED flag is checked on interrupt and TIF_NEED_RESCHED[_LAZY] + * flags on userspace return paths. For example, see arch/x86/entry_64.S. * - * To drive preemption between tasks, the scheduler sets the flag in timer - * interrupt handler scheduler_tick(). + * To drive preemption between tasks, the scheduler sets one of these + * flags in timer interrupt handler scheduler_tick(). * * 3. Wakeups don't really cause entry into schedule(). They add a * task to the run-queue and that's it. * - * Now, if the new task added to the run-queue preempts the current - * task, then the wakeup sets TIF_NEED_RESCHED and schedule() gets - * called on the nearest possible occasion: - * - * - If the kernel is preemptible (CONFIG_PREEMPTION=y): + * - Now, if the new task added to the run-queue preempts the current + * task, then the wakeup sets TIF_NEED_RESCHED and schedule() gets + * called on the nearest possible occasion: * * - in syscall or exception context, at the next outmost * preempt_enable(). (this might be as soon as the wake_up()'s @@ -6619,10 +6616,9 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) * - in IRQ context, return from interrupt-handler to * preemptible context * - * - If the kernel is not preemptible (CONFIG_PREEMPTION is not set) - * then at the next: + * - If the new task preempts the current task, but the scheduling + * policy is only preempt voluntarily, then at the next: * - * - cond_resched() call * - explicit schedule() call * - return from syscall or exception to user-space * - return from interrupt-handler to user-space diff --git a/kernel/sched/features.h b/kernel/sched/features.h index f770168230ae..9b4c2967b2b7 100644 --- a/kernel/sched/features.h +++ b/kernel/sched/features.h @@ -89,3 +89,9 @@ SCHED_FEAT(UTIL_EST_FASTUP, true) SCHED_FEAT(LATENCY_WARN, false) SCHED_FEAT(HZ_BW, true) + +#if defined(CONFIG_PREEMPT) +SCHED_FEAT(FORCE_PREEMPT, true) +#else +SCHED_FEAT(FORCE_PREEMPT, false) +#endif