From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.8 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1362AC433E1 for ; Thu, 14 May 2020 09:27:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CAC6C20734 for ; Thu, 14 May 2020 09:26:59 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=unikie-com.20150623.gappssmtp.com header.i=@unikie-com.20150623.gappssmtp.com header.b="rPFX+vVu" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726444AbgENJ05 (ORCPT ); Thu, 14 May 2020 05:26:57 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44676 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1726100AbgENJ04 (ORCPT ); Thu, 14 May 2020 05:26:56 -0400 Received: from mail-lj1-x243.google.com (mail-lj1-x243.google.com [IPv6:2a00:1450:4864:20::243]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C95FCC061A0F for ; Thu, 14 May 2020 02:26:55 -0700 (PDT) Received: by mail-lj1-x243.google.com with SMTP id g4so2745966ljl.2 for ; Thu, 14 May 2020 02:26:55 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=unikie-com.20150623.gappssmtp.com; s=20150623; h=from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=fnkbFpvZq2v0/e+Y6a4L7l2+uBR3sA47vR1lYmNAiN0=; b=rPFX+vVu5MNDbiP+9J7UFb4QZpWbK/ixFGArCBfUTaNiWFK2ZP83b8bRAS2HD0CoHT ClAL+Pk61R+G8f3FvKNEBe8sQ7Iwqe1xALQaIfYLKlPGeo+BHpkLML6+SXSEcTXJyCfU atdRnBfdUGBkcEA7b5QTFMSKo3Wawr6X5aJJ5iqrTdtEKHaNzkpcLngMHFuqP8RudbQO 3y718UttNjrnFLD1Jv/s7M5pFP1kKrQxEdnP+N74ZMLcQ0lMqNnw9QVW5zUyBURekht8 /URwutly+BUlOsc5gcCdjyuDAV/ExyR9fbpx6nilbK9QwYHBi0cRjZ1WrPzCvNl8RrMI kjxQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=fnkbFpvZq2v0/e+Y6a4L7l2+uBR3sA47vR1lYmNAiN0=; b=makefGkSk3N9igqq5G25DDoXysnMqSNW1cpF6R50kWE41tk52n+4Lx7vH5INCaLL/h 6GsNSS46w1eINDov79txJxHgabctG39+J/6OsSTz7QaUqrwGiinGFliJ463NCWTNXtoy y/kKeXxMEbnR3bcNhA/DaO2Obfh06duVLPnAbghU6hmap/MX/cPLi2KwpQzovjoS3VQb nR6AaFL9eF+Tm6yDC5zibN5luhb8cdzjWBRMems2/Kp3NkfdYM3JiBaHMJc3Yi3ebSo5 wfA0JrRr/rpmpFyjhtZP0WD7+F87/a8yzvXoqxxh4YRwhXB6Fu/9Jn4jXWYdRuSoMdS7 U/Xg== X-Gm-Message-State: AOAM5311uERi8q7LWXHq6EqO5ZBTYspMX2Sfe7PvRYlitBbh6Uuy7U6d m3wCQFCJdTR+X701QpwVgFyQ3Ba/63yXRQ== X-Google-Smtp-Source: ABdhPJwuXGnKPksy5Nz9RlNBQbtHyxpSsk4MPLmvxYzSwk7kz9FmNSRnFW3WtcjRGTTYaRj1Mm/enA== X-Received: by 2002:a2e:a211:: with SMTP id h17mr2261782ljm.289.1589448411939; Thu, 14 May 2020 02:26:51 -0700 (PDT) Received: from localhost.localdomain ([109.204.235.119]) by smtp.googlemail.com with ESMTPSA id e25sm1426117lfb.89.2020.05.14.02.26.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 14 May 2020 02:26:50 -0700 (PDT) From: john mathew X-Google-Original-From: john mathew To: linux-doc@vger.kernel.org Cc: linux-kernel@vger.kernel.org, corbet@lwn.net, mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, tsbogend@alpha.franken.de, lukas.bulwahn@gmail.com, x86@kernel.org, linux-mips@vger.kernel.org, tglx@linutronix.de, mostafa.chamanara@gmail.com, willy@infradead.org, valentin.schneider@arm.com, rdunlap@infradead.org, John Mathew , Mostafa Chamanara , Oleg Tsymbal Subject: [RFC PATCH v5 2/3] docs: scheduler: Add scheduler overview documentation Date: Thu, 14 May 2020 12:26:36 +0300 Message-Id: <20200514092637.15684-3-John.Mathew@unikie.com> X-Mailer: git-send-email 2.17.1 In-Reply-To: <20200514092637.15684-1-John.Mathew@unikie.com> References: <20200514092637.15684-1-John.Mathew@unikie.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org From: John Mathew Add documentation for -scheduler overview -scheduler state transtion -CFS overview -scheduler data structs Add rst for scheduler APIs and modify sched/core.c to add kernel-doc comments. Suggested-by: Lukas Bulwahn Co-developed-by: Mostafa Chamanara Signed-off-by: Mostafa Chamanara Co-developed-by: Oleg Tsymbal Signed-off-by: Oleg Tsymbal Signed-off-by: John Mathew --- Documentation/scheduler/cfs-overview.rst | 102 +++++++ Documentation/scheduler/index.rst | 2 + Documentation/scheduler/overview.rst | 288 ++++++++++++++++++ Documentation/scheduler/sched-cas.rst | 92 ++++++ .../scheduler/sched-data-structs.rst | 182 +++++++++++ Documentation/scheduler/sched-features.rst | 1 + Documentation/scheduler/scheduler-api.rst | 31 ++ kernel/sched/core.c | 28 +- kernel/sched/sched.h | 169 +++++++++- 9 files changed, 888 insertions(+), 7 deletions(-) create mode 100644 Documentation/scheduler/cfs-overview.rst create mode 100644 Documentation/scheduler/sched-cas.rst create mode 100644 Documentation/scheduler/sched-data-structs.rst create mode 100644 Documentation/scheduler/scheduler-api.rst diff --git a/Documentation/scheduler/cfs-overview.rst b/Documentation/scheduler/cfs-overview.rst new file mode 100644 index 000000000000..34f336b8ec86 --- /dev/null +++ b/Documentation/scheduler/cfs-overview.rst @@ -0,0 +1,102 @@ +.. SPDX-License-Identifier: GPL-2.0+ + +============= +CFS Overview +============= + +Linux 2.6.23 introduced a modular scheduler core and a Completely Fair +Scheduler (CFS) implemented as a scheduling module. A brief overview of the +CFS design is provided in :doc:`sched-design-CFS` + +In addition there have been many improvements to the CFS, a few of which are + +**Thermal Pressure**: +Scale CPU capacity mechanism for CFS so it knows how much CPU capacity is left +for its use after higher priority sched classes (RT, DL), IRQs and +'Thermal Pressure' have reduced the 'original' CPU capacity. +Thermal pressure on a CPU means the maximum possible capacity is +unavailable due to thermal events. + +** Optimizations to NUMA balancing**: +When gathering NUMA statistics, information about whether a core is Idle +is also cached. In case of an imbalance, instead of doing a second scan of +the node runqueues, the idle core is used as the migration target. When +doing so multiple tasks can attempt to select an idle CPU but fail, because +a NUMA balance is active on that CPU. In this case an alternative idle CPU +scanned. Another optimization is to terminate the search for swap candidate +when a reasonable one is found instead of searching all the CPUs on the +target domain. + +**Asymmetric CPU capacity wakeup scan**: +Previous assumption that CPU capacities within an SD_SHARE_PKG_RESOURCES +domain (sd_llc) are homogeneous didn't hold for newer generations of big.LITTLE +systems (DynamIQ) which can accommodate CPUs of different compute capacity +within a single LLC domain. A new idle sibling helper function was added +which took CPU capacity into account. The policy is to pick the first idle +CPU which is big enough for the task (task_util * margin < cpu_capacity). +If no idle CPU is big enough, the idle CPU with the highest capacity is +picked. + +**Optimized idle core selection**: +Skipped looping through all the threads of a core to evaluate if the +core is idle or not. If a thread of a core is not idle, evaluation of +other threads of the core can be skipped. + +**Load balance aggressively for SCHED_IDLE CPUs**: +Newly-woken task is preferred to be enqueued on a SCHED_IDLE CPU instead +of other busy or idle CPUs. Also load balancer is made to migrate tasks more +aggressively to a SCHED_IDLE CPU. Fair scheduler now does the next +load balance soon after the last non-SCHED_IDLE task is dequeued from a +runqueue, i.e. making the CPU SCHED_IDLE. Also the the busy_factor +used with the balance interval to prevent frequent load balancing +is ignored for such CPU's. + +**Load balancing algorithm Reworked**: +Some heuristics in the load balancing algorithm became meaningless because +of the rework of the scheduler's metrics like the introduction of PELT. +Those heuristics were removed. The new load balancing algorithm also fixes +several pending wrong tasks placement + + * the 1 task per CPU case with asymmetric system + * the case of CFS task preempted by other class + * the case of tasks not evenly spread on groups with spare capacity + +Also the load balance decisions have been consolidated in the 3 separate +functions. +* update_sd_pick_busiest() select the busiest sched_group. +* find_busiest_group() checks if there is an imbalance between local and +busiest group. +* calculate_imbalance() decides what have to be moved. + +**Energy-aware wake-ups speeded up**: +Algorithmic complexity of the EAS was reduced from O(n^2) to O(n). +Previous algorithm resulted in prohibitively high wake-up latencies on +systems with complex energy models, such as systems with per-CPU DVFS. +The EAS wake-up path was re-factored to compute the energy 'delta' on a +per-performance domain basis, rather than the whole system. + +**Selection of an energy-efficient CPU on task wake-up**: +An Energy efficient CPU is found by estimating the impact on system-level +active energy resulting from the placement of the task on the CPU with the +highest spare capacity in each performance domain. Energy Model (EM) is +used for this. This strategy spreads tasks in a performance domain and avoids overly +aggressive task packing. The best CPU energy-wise is then selected if it +saves a large enough amount of energy with respect to prev_cpu. + +**Consider misfit tasks when load-balancing**: +A task which ends up on a CPU which doesn't suit its compute demand is +identified as a misfit task in asymmetric CPU capacity systems. These +'misfit' tasks are migrated to CPUs with higher compute capacity to ensure +better throughput. A new group_type: group_misfit_task is added and indicates this +scenario. Tweaks to the load-balance code are done to make the migrations +happen. Misfit balancing is done between a source group of lower per-CPU +capacity and destination group of higher compute capacity. Otherwise, misfit +balancing is ignored. + + +**Make schedstats a runtime tunable that is disabled by default**: +A kernel command-line and sysctl tunable was added to enable or disable +schedstats on demand (when it's built in). It is disabled by default. +The benefits are dependent on how scheduler-intensive the workload is. + + diff --git a/Documentation/scheduler/index.rst b/Documentation/scheduler/index.rst index 9bdccea74af9..f311abe5b711 100644 --- a/Documentation/scheduler/index.rst +++ b/Documentation/scheduler/index.rst @@ -17,6 +17,8 @@ specific implementation differences. :maxdepth: 2 overview + sched-data-structs + cfs-overview sched-design-CFS sched-features arch-specific diff --git a/Documentation/scheduler/overview.rst b/Documentation/scheduler/overview.rst index aee16feefc61..7536bec6afce 100644 --- a/Documentation/scheduler/overview.rst +++ b/Documentation/scheduler/overview.rst @@ -3,3 +3,291 @@ ==================== Scheduler overview ==================== + +Linux kernel implements priority-based scheduling. More than one process are +allowed to run at any given time and each process is allowed to run as if it +were the only process on the system. The process scheduler coordinates which +process runs when. In that context, it has the following tasks: + +* share CPU cores equally among all currently running processes. +* pick appropriate process to run next if required, considering scheduling + class/policy and process priorities. +* balance processes between multiple cores in SMP systems. + +The scheduler attempts to be responsive for I/O bound processes and efficient +for CPU bound processes. The scheduler also applies different scheduling +policies for real time and normal processes based on their respective +priorities. Higher priorities in the kernel have a numerical smaller +value. Real time priorities range from 1 (highest) – 99 whereas normal +priorities range from 100 – 139 (lowest). Scheduler implements many scheduling +classes which encapsulate a particular scheduling policy. Each scheduling +policy implements scheduler handling of tasks that belong to a particular +priority. SCHED_FIFO and SCHED_RR policies handle real time priorities tasks +while SCHED_NORMAL and SCHED_BATCH policies handle tasks with normal priorities. +SCHED_IDLE is also a normal scheduling policy when means its priority can +be set between 100 – 139 range too but they are treated as priority 139. +Their priority doesn't matter since they get minimal weight WEIGHT_IDLEPRI=3. +SCHED_DEADLINE policy tasks have negative priorities, reflecting +the fact that any of them has higher priority than RT and NORMAL/BATCH tasks. + +And then there are the maintenance scheduler classes: idle sched class and +stop sched class. Idle class doesn't manage any user tasks and so doesn't +implement a policy. Its idle tasks 'swapper/X' has priority 120 and and aren't +visible to user space. Idle tasks are responsible for by putting the CPUs +into deep idle states when there is no work to do. +Stop sched class is also used internally by the kernel doesn't implement any +scheduling policy. Stopper tasks 'migration/X' disguise as as a SCHED_FIFO +task with priority 139. Stopper tasks are a mechanism to force a CPU to stop +running everything else and perform a specific task. As this is the +highest-priority class, it can preempt everything else and nothing ever +preempts it. It is used by one CPU to stop another in order to run a specific +function, so it is only available on SMP systems. This class is used by the +kernel for task migration. + + +Process Management +================== + +Each process in the system is represented by struct task_struct. When a +process/thread is created, the kernel allocates a new task_struct for it. +The kernel then stores this task_struct in an RCU list. Macro next_task() +allows a process to obtain its next task and for_each_process() macro enables +traversal of the list. + +Frequently used fields of the task struct are: + +*state:* The running state of the task. The possible states are: + +* TASK_RUNNING: The task is currently running or in a run queue waiting + to run. +* TASK_INTERRUPTIBLE: The task is sleeping waiting for some event to occur. + This task can be interrupted by signals. On waking up the task transitions + to TASK_RUNNING. +* TASK_UNINTERRUPTIBLE: Similar to TASK_INTERRUPTIBLE but does not wake + up on signals. Needs an explicit wake-up call to be woken up. Contributes + to loadavg. +* __TASK_TRACED: Task is being traced by another task like a debugger. +* __TASK_STOPPED: Task execution has stopped and not eligible to run. + SIGSTOP, SIGTSTP etc causes this state. The task can be continued by + the signal SIGCONT. +* TASK_PARKED: State to support kthread parking/unparking. +* TASK_DEAD: If a task dies, then it sets TASK_DEAD in tsk->state and calls + schedule one last time. The schedule call will never return. +* TASK_WAKEKILL: It works like TASK_UNINTERRUPTIBLE with the bonus that it + can respond to fatal signals. +* TASK_WAKING: To handle concurrent waking of the same task for SMP. + Indicates that someone is already waking the task. +* TASK_NOLOAD: To be used along with TASK_UNINTERRUPTIBLE to indicate + an idle task which does not contribute to loadavg. +* TASK_NEW: Set during fork(), to guarantee that no one will run the task, + a signal or any other wake event cannot wake it up and insert it on + the runqueue. + +*exit_state* : The exiting state of the task. The possible states are: + +* EXIT_ZOMBIE: The task is terminated and waiting for parent to collect + the exit information of the task. +* EXIT_DEAD: After collecting the exit information the task is put to + this state and removed from the system. + +*static_prio:* Nice value of a task. The value of this field does + not change. Value ranges from -20 to 19. This value is mapped to nice + value and used in the scheduler. + +*prio:* Dynamic priority of a task. Previously a function of static + priority and tasks interactivity. Value not used by CFS scheduler but used + by the RT scheduler. Might be boosted by interactivity modifiers. Changes + upon fork, setprio syscalls, and whenever the interactivity estimator + recalculates. + +*normal_prio:* Expected priority of a task. The value of static_prio + and normal_prio are the same for non-real-time processes. For real time + processes value of prio is used. + +*rt_priority:* Field used by real time tasks. Real time tasks are + prioritized based on this value. + +*sched_class:* Pointer to sched_class CFS structure. + +*sched_entity:* Pointer to sched_entity CFS structure. + +*policy:* Value for scheduling policy. The possible values are: + +* SCHED_NORMAL: Regular tasks use this policy. +* SCHED_BATCH: Tasks which need to run longer without preemption + use this policy. Suitable for batch jobs. +* SCHED_IDLE: Policy used by background tasks. +* SCHED_FIFO & SCHED_RR: These policies for real time tasks. Handled by + real time scheduler. +* SCHED_DEADLINE: Tasks which are activated on a periodic or sporadic fashion + use this policy. This policy implements the Earliest Deadline First (EDF) + scheduling algorithm. This policy is explained in detail in the + :doc:`sched-deadline` documentation. + +*nr_cpus_allowed:* Bit field containing tasks affinity towards a set of + CPU cores. Set using sched_setaffinity() system call. + +New processes are created using the fork() system call which is described +at manpage :manpage:`FORK(2)` or the clone system call described at +:manpage:`CLONE(2)`. +Users can create threads within a process to achieve parallelism. Threads +share address space, open files and other resources of the process. Threads +are created like normal tasks with their unique task_struct, but clone() +is provided with flags that enable the sharing of resources such as address +space :: + + clone(CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND, 0); + +The scheduler schedules task_structs so from scheduler perspective there is +no difference between threads and processes. Threads are created using +the system call pthread_create described at :manpage:`PTHREAD_CREATE(3)` +POSIX threads creation is described at :manpage:`PTHREADS(7)` + +The Scheduler Entry Point +========================= + +The main scheduler entry point is an architecture independent schedule() +function defined in kernel/sched/core.c. Its objective is to find a process in +the runqueue list and then assign the CPU to it. It is invoked, directly +or in a lazy (deferred) way from many different places in the kernel. A lazy +invocation does not call the function by its name, but gives the kernel a +hint by setting a flag TIF_NEED_RESCHED. The flag is a message to the kernel +that the scheduler should be invoked as soon as possible because another +process deserves to run. + +Following are some places that notify the kernel to schedule: + +* scheduler_tick() + +* Running task goes to sleep state : Right before a task goes to sleep, + schedule() will be called to pick the next task to run and the change + its state to either TASK_INTERRUPTIBLE or TASK_UNINTERRUPTIBLE. For + instance, prepare_to_wait() is one of the functions that makes the + task go to the sleep state. + +* try_to_wake_up() + +* yield() + +* wait_event() + +* cond_resched() : It gives the scheduler a chance to run a higher-priority + process. + +* cond_resched_lock() : If a reschedule is pending, drop the given lock, + call schedule, and on return reacquire the lock. + +* do_task_dead() + +* preempt_schedule() : The function checks whether local interrupts are + enabled and the preempt_count field of current is zero; if both + conditions are true, it invokes schedule() to select another process + to run. + +* preempt_schedule_irq() + +Calling functions mentioned above leads to a call to __schedule(). Note +that preemption must be disabled before it is called and enabled after +the call using preempt_disable and preempt_enable functions family. + + +The steps during invocation are: +-------------------------------- +1. Disable preemption to avoid another task preempting the scheduling + thread itself. +2. Retrieve the runqueue of current processor and its lock is obtained to + allow only one thread to modify the runqueue at a time. +3. The state of the previously executed task when the schedule() + was called is examined. If it is not runnable and has not been + preempted in kernel mode, it is removed from the runqueue. If the + previous task has non-blocked pending signals, its state is set to + TASK_RUNNING and left in the runqueue. +4. Scheduler classes are iterated and the corresponding class hook to + pick the next suitable task to be scheduled on the CPU is called. + Since most tasks are handled by the sched_fair class, a shortcut to this + class is implemented in the beginning of the function. +5. TIF_NEED_RESCHED and architecture specific need_resched flags are cleared. +6. If the scheduler class picks a different task from what was running + before, a context switch is performed by calling context_switch(). + Internally, context_switch() switches to the new task's memory map and + swaps the register state and stack. If scheduler class picked the same + task as the previous task, no task switch is performed and the current + task keeps running. +7. Balance callback list is processed. Each scheduling class can migrate tasks + between CPUs to balance load. These load balancing operations are queued + on a Balance callback list which get executed when balance_callback() is + called. +8. The runqueue is unlocked and preemption is re-enabled. In case + preemption was requested during the time in which it was disabled, + schedule() is run again right away. + +Scheduler State Transition +========================== + +A very high level scheduler state transition flow with a few states can +be depicted as follows. :: + + * + | + | task + | forks + v + +------------------------------+ + | TASK_NEW | + | (Ready to run) | + +------------------------------+ + | + | + v + +------------------------------------+ + | TASK_RUNNING | + +---------------> | (Ready to run) | <--+ + | +------------------------------------+ | + | | | + | | schedule() calls context_switch() | task is preempted + | v | + | +------------------------------------+ | + | | TASK_RUNNING | | + | | (Running) | ---+ + | event occurred +------------------------------------+ + | | + | | task needs to wait for event + | v + | +------------------------------------+ + | | TASK_INTERRUPTIBLE | + | | TASK_UNINTERRUPTIBLE | + +-----------------| TASK_WAKEKILL | + +------------------------------------+ + | + | task exits via do_exit() + v + +------------------------------+ + | TASK_DEAD | + | EXIT_ZOMBIE | + +------------------------------+ + + +Scheduler provides trace events tracing all major events of the scheduler. +The trace events are defined in :: + + include/trace/events/sched.h + +Using these trace events it is possible to model the scheduler state transition +in an automata model. The following journal paper discusses such modeling: + +Daniel B. de Oliveira, Rômulo S. de Oliveira, Tommaso Cucinotta, **A thread +synchronization model for the PREEMPT_RT Linux kernel**, *Journal of Systems +Architecture*, Volume 107, 2020, 101729, ISSN 1383-7621, +https://doi.org/10.1016/j.sysarc.2020.101729. + +To model the scheduler efficiently the system was divided in to generators +and specifications. Some of the generators used were "need_resched", +"sleepable" and "runnable", "thread_context" and "scheduling context". +The specifications are the necessary and sufficient conditions to call +the scheduler. New trace events were added to specify the generators +and specifications. In case a kernel event referred to more than one +event, extra fields of the kernel event was used to distinguish between +automation events. The final model was generated from parallel composition +of all generators and specifications which composed of 34 events, +12 generators and 33 specifications. This resulted in 9017 states, and +20103 transitions. diff --git a/Documentation/scheduler/sched-cas.rst b/Documentation/scheduler/sched-cas.rst new file mode 100644 index 000000000000..fcebc5770803 --- /dev/null +++ b/Documentation/scheduler/sched-cas.rst @@ -0,0 +1,92 @@ +.. SPDX-License-Identifier: GPL-2.0+ + +========================= +Capacity-Aware Scheduling +========================= + +Scheduling load balancing on Asymmetric Multiprocessor systems was improved +through the introduction of Capacity-Aware Scheduling. It identifies the +most efficient CPU to assign a task based on its capacity. This capacity +may be asymmetric due to heterogeneous computing architecture such +as ARM big.LITTLE. Scheduler gets information about asymmetric capacities +when the scheduler domain hierarchy is built using build_sched_domains(). +CPU capacities are provided to the scheduler topology code through the +architecture specific implementation of the arch_scale_cpu_capacity(). +The SD_ASYM_CPUCAPACITY flag is set by the scheduler topology for a domain +in the hierarchy where all CPU capacities are visible for any cpu's point +of view on asymmetric CPU capacity systems. The scheduler can then take +capacity asymmetry into account when load balancing. + +Initial CPU capacities are derived from the Device Tree and CPU frequency. +For RISC-V & ARM64 it is done in drivers/base/arch_topology.c. A cpu-map +device tree is parsed to obtain the cpu topology and the initial CPU capacity +is set using the CPUFreq subsystem. A callback is registered to the CPUFreq +subsystem to rebuild sched_domains once the CPUFreq is loaded, which is when +a complete view of the capacities of the CPUs (which is a mix of µarch and +frequencies) is available. + +Asymmetric CPU capacity information is used in + +* Energy Aware Scheduling: The scheduler is able to predict the impact of + its decisions on the energy consumed by CPUs. Described in :doc:`sched-energy` . +* Optimized task wakeup load balancing by finding idle CPU with enough capacity. + +The different scheduler classes asymmetric use the Asymmetric CPU capacity +information differently. + +CFS Capacity Awareness +====================== + +Used to identify misfit tasks: +A load intensive task on a CPU which doesn't meet its compute demand is +identified as a misfit task. 'Misfit' tasks are migrated to CPUs with +higher compute capacity to ensure better throughput. CFS frequently updates +the misfit status of the current task by comparing its utilization vs the +CPU capacity using task_fits_capacity(). If the utilization is more than the +CPU capacity the calculated misfit load is updated to the runqueue +rq->misfit_task_load. This misfit load is then checked by the load +balancing operations to migrate the task to a CPU of higher capacity. + +Modified wakeup logic to support DynamIQ systems: +When the scheduler class calls select_task_rq_fair to select a runqueue for +a waking task, load balancing is performed by selecting the idlest CPU in +the idlest group, or under certain conditions an idle sibling CPU if the +domain has SD_WAKE_AFFINE set. In DynamIQ systems Last Level Cache (LLC) +domain of a CPU spans all CPUs in the system. This may include CPU's of +different capacities. So in select_idle_sibling() an idle sibling is picked +based on CPU capacity for asymmetric CPU capacity systems and for symmetric +systems use LLC domain is used. The policy is to pick the first idle CPU +which is big enough for the task (task_util * margin < cpu_capacity). +If no idle CPU is big enough, the idle CPU with the highest capacity is +picked. For asymmetric CPU capacity systems select_idle_sibling() operates +on the sd_asym_cpucapacity sched_domain pointer, which is guaranteed to span +all known CPU capacities in the system. This works for both "legacy" +big.LITTLE (LITTLEs & bigs split at MC, joined at DIE) and for newer +DynamIQ systems (e.g. LITTLEs and bigs in the same MC domain). + + +RT Capacity Awareness +===================== + +Since RT tasks doesn't have a per task utilization signal RT tasks uses uclamp +to guarantee a minimum performance point. Utilization clamping is a mechanism +which allows to "clamp" (i.e. filter) the utilization generated by RT and +FAIR tasks within a range defined by user-space. It exposes to user-space a +new set of per-task attributes the scheduler can use as hints about the +expected/required utilization for a task. RT is made capacity aware +by ensuring that the capacity of the CPU is >= uclamp_min value. This check +is done in the rt_task_fits_capacity() + +DL Capacity Awareness +===================== + +TBD + + + + + + + + + diff --git a/Documentation/scheduler/sched-data-structs.rst b/Documentation/scheduler/sched-data-structs.rst new file mode 100644 index 000000000000..a16408676b71 --- /dev/null +++ b/Documentation/scheduler/sched-data-structs.rst @@ -0,0 +1,182 @@ +.. SPDX-License-Identifier: GPL-2.0+ + +========================= +Scheduler Data Structures +========================= + +The main parts of the Linux scheduler are: + +Runqueue +~~~~~~~~ + +:c:type:`struct rq ` is the central data structure of process +scheduling. It keeps track of tasks that are in a runnable state assigned +for a particular processor. Each CPU has its own run queue and stored in a +per CPU array:: + + DEFINE_PER_CPU(struct rq, runqueues); + +Access to the queue requires locking and lock acquire operations must be +ordered by ascending runqueue. Macros for accessing and locking the runqueue +are provided in:: + + kernel/sched/sched.h + +The runqueue contains scheduling class specific queues and several scheduling +statistics. + +Scheduling entity +~~~~~~~~~~~~~~~~~ +Scheduler uses scheduling entities which contain sufficient information to +actually accomplish the scheduling job of a task or a task-group. The +scheduling entity may be a group of tasks or a single task. Every task is +associated with a sched_entity structure. CFS adds support for nesting of +tasks and task groups. Each scheduling entity may be run from its parents +runqueue. The scheduler traverses the sched_entity hierarchy to pick the +next task to run on the CPU. The entity gets picked up from the cfs_rq on +which it is queued and its time slice is divided among all the tasks on its my_q. + +Scheduler classes +~~~~~~~~~~~~~~~~~ +It is an extensible hierarchy of scheduler modules. The modules encapsulate +scheduling policy details. They are called from the core code which is +independent. Scheduling classes are implemented through the sched_class +structure. dl_sched_class for deadline scheduler, fair_sched_class for CFS +and rt_sched_class for RT are implementations of this class. + +The important methods of scheduler class are: + +enqueue_task and dequeue_task + These functions are used to put and remove tasks from the runqueue + respectively to change a property of a task. This is referred to as + change pattern. Change is defined as the following sequence of calls:: + + * dequeue_task() + * put_prev_task() + * change a property + * enqueue_task() + * set_next_task() + + The enqueue_task function takes the runqueue, the task which needs to + be enqueued/dequeued and a bit mask of flags as parameters. The main + purpose of the flags is to describe why the enqueue or dequeue is being + called. The different flags used are described in :: + + kernel/sched/sched.h + + Some places where the enqueue_task and dequeue_task are called for + changing task properties are + + * When migrating a task from one CPU's runqueue to another. + * When changing a tasks CPU affinity. + * When changing the priority of a task. + * When changing the nice value of the task. + * When changing the scheduling policy and/or RT priority of a thread. + +pick_next_task + Called by the scheduler to pick the next best task to run. The scheduler + iterates through the corresponding functions of the scheduler classes + in priority order to pick up the next best task to run. Since tasks + belonging to the idle class and fair class are frequent, the scheduler + optimizes the picking of next task to call the pick_next_task_fair() + if the previous task was of the similar scheduling class. + +put_prev_task + Called by the scheduler when a running task is being taken off a CPU. + The behavior of this function depends on individual scheduling classes. + In CFS class this function is used to put the currently running task back + into the CFS RB tree. When a task is running it is dequeued from the tree. + This is to prevent redundant enqueue's and dequeue's for updating its + vruntime. vruntime of tasks on the tree needs to be updated by update_curr() + to keep the tree in sync. In SCHED_DEADLINE and RT classes additional tree + is maintained to push tasks from the current CPU to another CPU where the + task can preempt and start executing. Task will be added to this queue + if it is present on the scheduling class rq and the task has affinity + to more than one CPU. + +set_next_task + Pairs with the put_prev_task(), this function is called when the next + task is set to run on the CPU. This function is called in all the places + where put_prev_task is called to complete the 'change pattern'. In case + of CFS scheduling class, it will set current scheduling entity to the + picked task and accounts bandwidth usage on the cfs_rq. In addition it + will also remove the current entity from the CFS runqueue for the vruntime + update optimization, opposite to what was done in put_prev_task. + For the SCHED_DEADLINE and RT classes it will remove the task from the + tree of pushable tasks trigger the balance callback to push another task + which is non running on the current CPU for execution on another CPU. + + * dequeue the picked task from the tree of pushable tasks. + * update the load average in case the previous task belonged to another + class. + * queues the function to push tasks from current runqueue to other CPUs + which can preempt and start execution. Balance callback list is used. + +task_tick + Called from scheduler_tick(), hrtick() and sched_tick_remote() to update + the current task statistics and load averages. Also restarting the high + resolution tick timer is done if high resolution timers are enabled. + scheduler_tick() runs at 1/HZ and is called from the timer interrupt + handler of the Kernel internal timers. + hrtick() is called from high resolution timers to deliver an accurate + preemption tick as the regular scheduler tick that runs at 1/HZ can be + too coarse when nice levels are used. + sched_tick_remote() gets called by the offloaded residual 1Hz scheduler + tick. In order to reduce interruptions to bare metal tasks, it is possible + to outsource these scheduler ticks to the global workqueue so that a + housekeeping CPU handles those remotely. + +select_task_rq + Called by scheduler to get the CPU to assign a task to and migrating + tasks between CPUs. Flags describe the reason the function was called. + Called by try_to_wake_up() with SD_BALANCE_WAKE flag which wakes up a + sleeping task. + Called by wake_up_new_task() with SD_BALANCE_FORK flag which wakes up a + newly forked task. + Called by sched_exec() with SD_BALANCE_EXEC which is called from execv + syscall. + SCHED_DEADLINE class decides the CPU on which the task should be woken + up based on the deadline. RT class decides based on the RT priority. Fair + scheduling class balances load by selecting the idlest CPU in the + idlest group, or under certain conditions an idle sibling CPU if the + domain has SD_WAKE_AFFINE set. + +balance + Called by pick_next_task() from scheduler to enable scheduling classes + to pull tasks from runqueues of other CPUs for balancing task execution + between the CPUs. + +task_fork + Called from sched_fork() of scheduler which assigns a task to a CPU. + Fair scheduling class updates runqueue clock, runtime statistics and + vruntime for the scheduling entity. + +yield_task + Called from SYSCALL sched_yield to yield the CPU to other tasks. + SCHED_DEADLINE class forces the runtime of the task to zero using a special + flag and dequeues the task from its trees. RT class requeues the task + entities to the end of the run list. Fair scheduling class implements + the buddy mechanism. This allows skipping onto the next highest priority + scheduling entity at every level in the CFS tree, unless doing so would + introduce gross unfairness in CPU time distribution. + +check_preempt_curr + Check whether the task that woke up should preempt the currently + running task. Called by scheduler, + + * when moving queued task to new runqueue + * ttwu() + * when waking up newly created task for the first time. + + SCHED_DEADLINE class compares the deadlines of the tasks and calls + scheduler function resched_curr() if the preemption is needed. In case + the deadlines are equal, migratability of the tasks is used a criteria + for preemption. + RT class behaves the same except it uses RT priority for comparison. + Fair class sets the buddy hints before calling resched_curr() to preempt. + +Scheduler sets the scheduler class for each task based on its priority. +Tasks assigned with SCHED_NORMAL, SCHED_IDLE and SCHED_BATCH call +fair_sched_class hooks and tasks assigned with SCHED_RR and +SCHED_FIFO call rt_sched_class hooks. Tasks assigned with SCHED_DEADLINE +policy calls dl_sched_class hooks. diff --git a/Documentation/scheduler/sched-features.rst b/Documentation/scheduler/sched-features.rst index 1afbd9cc8d52..e576c7d9e556 100644 --- a/Documentation/scheduler/sched-features.rst +++ b/Documentation/scheduler/sched-features.rst @@ -17,4 +17,5 @@ Scheduler Features sched-energy sched-nice-design sched-rt-group + sched-cas completion diff --git a/Documentation/scheduler/scheduler-api.rst b/Documentation/scheduler/scheduler-api.rst new file mode 100644 index 000000000000..1fc6bd4c2908 --- /dev/null +++ b/Documentation/scheduler/scheduler-api.rst @@ -0,0 +1,31 @@ +.. SPDX-License-Identifier: GPL-2.0+ + +============================= +Scheduler related functions +============================= + + +.. kernel-doc:: kernel/sched/core.c + :functions: __schedule + +.. kernel-doc:: kernel/sched/core.c + :functions: scheduler_tick + +.. kernel-doc:: kernel/sched/core.c + :functions: try_to_wake_up + +.. kernel-doc:: kernel/sched/core.c + :functions: do_task_dead + +.. kernel-doc:: kernel/sched/core.c + :functions: preempt_schedule_irq + +.. kernel-doc:: kernel/sched/core.c + :functions: prepare_task_switch + +.. kernel-doc:: kernel/sched/core.c + :functions: finish_task_switch + +.. kernel-doc:: kernel/sched/sched.h + :functions: rq + diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 9a2fbf98fd6f..b349ed9b4d92 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3576,9 +3576,13 @@ void arch_set_thermal_pressure(struct cpumask *cpus, WRITE_ONCE(per_cpu(thermal_pressure, cpu), th_pressure); } -/* +/** + * scheduler_tick - sched tick timer handler + * * This function gets called by the timer code, with HZ frequency. * We call it with interrupts disabled. + * + * Return: 0. */ void scheduler_tick(void) { @@ -3959,8 +3963,10 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) BUG(); } -/* - * __schedule() is the main scheduler function. +/** + * __schedule() - the main scheduler function. + * + * @preempt: preemption enabled/disabled * * The main means of driving the scheduler and thus entering this function are: * @@ -4089,6 +4095,12 @@ static void __sched notrace __schedule(bool preempt) balance_callback(rq); } +/** + * do_task_dead - handle task exit + * + * Changes the the task state to TASK_DEAD and calls + * schedule to pick next task to run. + */ void __noreturn do_task_dead(void) { /* Causes final put_task_struct in finish_task_switch(): */ @@ -4320,7 +4332,8 @@ EXPORT_SYMBOL_GPL(preempt_schedule_notrace); #endif /* CONFIG_PREEMPTION */ -/* +/** + * preempt_schedule_irq - schedule from irq context * This is the entry point to schedule() from kernel preemption * off of irq context. * Note, that this is called and return with irqs disabled. This will @@ -5618,6 +5631,13 @@ SYSCALL_DEFINE0(sched_yield) } #ifndef CONFIG_PREEMPTION +/** + * _cond_resched - explicit rescheduling + * + * gives the scheduler a chance to run a higher-priority process + * + * Return: 1 if reschedule was done, 0 if reschedule not done. + */ int __sched _cond_resched(void) { if (should_resched(0)) { diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index db3a57675ccf..21f2953b72c7 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -865,12 +865,175 @@ struct uclamp_rq { }; #endif /* CONFIG_UCLAMP_TASK */ -/* - * This is the main, per-CPU runqueue data structure. +/** + * struct rq - This is the main, per-CPU runqueue data structure. * * Locking rule: those places that want to lock multiple runqueues * (such as the load balancing or the thread migration code), lock * acquire operations must be ordered by ascending &runqueue. + * + * @lock: + * lock to be acquired while modifying the runqueue + * @nr_running: + * number of runnable tasks on this queue + * @nr_numa_running: + * number of tasks running that care about their placement + * @nr_preferred_running: + * number of tasks that are optimally NUMA placed + * @numa_migrate_on: + * per run-queue variable to check if NUMA-balance is + * active on the run-queue + * @last_blocked_load_update_tick: + * tick stamp for decay of blocked load + * @has_blocked_load: + * idle CPU has blocked load + * @nohz_tick_stopped: + * CPU is going idle with tick stopped + * @nohz_flags: + * flags indicating NOHZ idle balancer actions + * @nr_load_updates: + * unused + * @nr_switches: + * number of context switches + * @uclamp: + * utilization clamp values based on CPU's RUNNABLE tasks + * @uclamp_flags: + * flags for uclamp actions, currently one flag for idle. + * @cfs: + * fair scheduling class runqueue + * @rt: + * rt scheduling class runqueue + * @dl: + * dl scheduing class runqueue + * @leaf_cfs_rq_list: + * list of leaf cfs_rq on this CPU + * @tmp_alone_branch: + * reference to add child before its parent in leaf_cfs_rq_list + * @nr_uninterruptible: + * global counter where the total sum over all CPUs matters. A task + * can increase this counter on one CPU and if it got migrated + * afterwards it may decrease it on another CPU. Always updated under + * the runqueue lock + * @curr: + * points to the currently running task of this rq. + * @idle: + * points to the idle task of this rq + * @stop: + * points to the stop task of this rq + * @next_balance: + * shortest next balance before updating nohz.next_balance + * @prev_mm: + * real address space of the previous task + * @clock_update_flags: + * RQCF clock_update_flags bits + * @clock: + * sched_clock() value for the queue + * @clock_task: + * clock value minus irq handling time + * @clock_pelt: + * clock which scales with current capacity when something is + * running on rq and synchronizes with clock_task when rq is idle + * @lost_idle_time: + * idle time lost when utilization of a rq has reached the + * maximum value + * @nr_iowait: + * account the idle time that we could have spend running if it + * were not for IO + * @membarrier_state: + * copy of membarrier_state from the mm_struct + * @rd: + * root domain, each exclusive cpuset essentially defines an island + * domain by fully partitioning the member CPUs from any other cpuset + * @sd: + * a domain heirarchy of CPU groups to balance process load among them + * @cpu_capacity: + * information about CPUs heterogeneity used for CPU performance + * scaling + * @cpu_capacity_orig: + * original capacity of a CPU before being altered by + * rt tasks and/or IRQ + * @balance_callback: + * queue to hold load balancing push and pull operations + * @idle_balance: + * flag to do the nohz idle load balance + * @misfit_task_load: + * set whenever the current running task has a utilization + * greater than 80% of rq->cpu_capacity. A non-zero value + * in this field enables misfit load balancing + * @active_balance: + * synchronizes accesses to ->active_balance_work + * @push_cpu: + * idle cpu to push the running task on to during active load + * balancing. + * @active_balance_work: + * callback scheduled to run on one or multiple cpus + * with maximum priority monopolozing those cpus. + * @cpu: + * CPU of this runqueue + * @online: + * Used by scheduling classes to support CPU hotplug + * @cfs_tasks: + * an MRU list used for load balancing, sorted (except + * woken tasks) starting from recently given CPU time tasks + * toward tasks with max wait time in a run-queue + * @avg_rt: + * track the utilization of RT tasks for a more accurate + * view of the utilization of the CPU when overloaded by CFS and + * RT tasks + * @avg_dl: + * track the utilization of DL tasks as CFS tasks can be preempted + * by DL tasks and the CFS's utilization might no longer describe + * the real utilization level + * @avg_irq: + * track the the utilization of interrupt to give a more accurate + * level of utilization of CPU taking into account the time spent + * under interrupt context when rqs' clock is updated + * @avg_thermal: + * tracks thermal pressure which is the reduction in maximum + * possible capacity due to thermal events + * @idle_stamp: + * time stamp at which idle load balance started for this rq. + * Used to find the idlest CPU, when multiple idle CPUs are in + * the same state + * @avg_idle: + * average idle time for this rq + * @max_idle_balance_cost: + * used to determine avg_idle's max value + * @prev_irq_time: + * updated to account time consumed when a previous + * update_rq_clock() happened inside a {soft,}irq region + * @prev_steal_time: + * to account how much elapsed time was spent in steal + * @prev_steal_time_rq: + * for fine granularity task steal time accounting by + * making update_rq_clock() aware of steal time + * @calc_load_update: + * sample window for global load-average calculations + * @calc_load_active: + * fold any nr_active delta into a global accumulate + * @hrtick_csd: + * call_single_data used to set hrtick timer state on a specific CPU + * @hrtick_timer: + * HR-timer to deliver an accurate preemption tick + * @rq_sched_info: + * runqueue specific latency stats + * @rq_cpu_time: + * runqueue specific accumulated per-task cpu runtime + * @yld_count: + * runqueue specific sys_sched_yield() stats + * @sched_count: + * runqueue specific __schedule() stats + * @sched_goidle: + * runqueue specific idle scheduling class stats + * @ttwu_count: + * runqueue specific idle ttwu stats , both remote and local + * @ttwu_local: + * ttwu count for the CPU of the rq + * @wake_list: + * list which stores tasks being woken up remotely by ttwu + * @idle_state: + * cpuidle state pointer of the CPU of this rq used to make a + * better decision when balancing tasks */ struct rq { /* runqueue lock: */ @@ -1136,7 +1299,7 @@ static inline u64 rq_clock_task(struct rq *rq) return rq->clock_task; } -/** +/* * By default the decay is the default pelt decay period. * The decay shift can change the decay period in * multiples of 32. -- 2.17.1