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Tue, 7 Nov 2023 21:59:47 +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 41/86] sched: handle resched policy in resched_curr() Date: Tue, 7 Nov 2023 13:57:27 -0800 Message-Id: <20231107215742.363031-42-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: MW4PR04CA0309.namprd04.prod.outlook.com (2603:10b6:303:82::14) 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: 57b602de-ef27-49fa-2b7e-08dbdfdcdb63 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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Tue, 07 Nov 2023 14:04:18 -0800 (PST) X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1781944454993347711 X-GMAIL-MSGID: 1781944454993347711 One of the last ports of call before rescheduling is triggered is resched_curr(). It's task is to set TIF_NEED_RESCHED and, if running locally, either fold it in the preempt_count, or send a resched-IPI so the target CPU folds it in. To handle TIF_NEED_RESCHED_LAZY -- since the reschedule is not imminent -- it only needs to set the appropriate bit. Move all of underlying mechanism in __resched_curr(). And, define resched_curr() which handles the policy on when we want to set which need-resched variant. For now the approach is to run to completion (TIF_NEED_RESCHED_LAZY) with the following exceptions where we always want to reschedule at the next preemptible point (TIF_NEED_RESCHED): - idle: if we are polling in idle, then set_nr_if_polling() will do the right thing. When not polling, we force TIF_NEED_RESCHED and send a resched-IPI if needed. - the target CPU is in userspace: run to completion semantics are only for kernel tasks - running under the full preemption model Originally-by: Thomas Gleixner Signed-off-by: Ankur Arora --- kernel/sched/core.c | 80 +++++++++++++++++++++++++++++++++++++++------ 1 file changed, 70 insertions(+), 10 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 01df5ac2982c..f65bf3ce0e9d 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1027,13 +1027,13 @@ void wake_up_q(struct wake_q_head *head) } /* - * resched_curr - mark rq's current task 'to be rescheduled now'. + * __resched_curr - mark rq's current task 'to be rescheduled'. * - * On UP this means the setting of the need_resched flag, on SMP it - * might also involve a cross-CPU call to trigger the scheduler on - * the target CPU. + * On UP this means the setting of the need_resched flag, on SMP, for + * eager resched it might also involve a cross-CPU call to trigger + * the scheduler on the target CPU. */ -void resched_curr(struct rq *rq) +void __resched_curr(struct rq *rq, resched_t rs) { struct task_struct *curr = rq->curr; int cpu; @@ -1046,17 +1046,77 @@ void resched_curr(struct rq *rq) cpu = cpu_of(rq); if (cpu == smp_processor_id()) { - set_tsk_need_resched(curr, RESCHED_eager); - set_preempt_need_resched(); + set_tsk_need_resched(curr, rs); + if (rs == RESCHED_eager) + set_preempt_need_resched(); return; } - if (set_nr_and_not_polling(curr, RESCHED_eager)) - smp_send_reschedule(cpu); - else + if (set_nr_and_not_polling(curr, rs)) { + if (rs == RESCHED_eager) + smp_send_reschedule(cpu); + } else if (rs == RESCHED_eager) trace_sched_wake_idle_without_ipi(cpu); } +/* + * resched_curr - mark rq's current task 'to be rescheduled' eagerly + * or lazily according to the current policy. + * + * Always schedule eagerly, if: + * + * - running under full preemption + * + * - 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. + * (the polling case has already set TIF_NEED_RESCHED via + * set_nr_if_polling()). + * + * - in userspace: run to completion semantics are only for kernel tasks + * + * Otherwise (regardless of priority), run to completion. + */ +void resched_curr(struct rq *rq) +{ + resched_t rs = RESCHED_lazy; + int context; + + if (IS_ENABLED(CONFIG_PREEMPT) || + (rq->curr->sched_class == &idle_sched_class)) { + rs = RESCHED_eager; + goto resched; + } + + /* + * We might race with the target CPU while checking its ct_state: + * + * 1. The task might have just entered the kernel, but has not yet + * called user_exit(). We will see stale state (CONTEXT_USER) and + * send an unnecessary resched-IPI. + * + * 2. The user task is through with exit_to_user_mode_loop() but has + * not yet called user_enter(). + * + * We'll see the thread's state as CONTEXT_KERNEL and will try to + * schedule it lazily. There's obviously nothing that will handle + * this need-resched bit until the thread enters the kernel next. + * + * The scheduler will still do tick accounting, but a potentially + * higher priority task waited to be scheduled for a user tick, + * instead of execution time in the kernel. + */ + context = ct_state_cpu(cpu_of(rq)); + if ((context == CONTEXT_USER) || + (context == CONTEXT_GUEST)) { + + rs = RESCHED_eager; + goto resched; + } + +resched: + __resched_curr(rq, rs); +} + void resched_cpu(int cpu) { struct rq *rq = cpu_rq(cpu);