scx/OVERVIEW.md

Overview

sched_ext is a Linux kernel feature which enables implementing and dynamically loading safe kernel thread schedulers in BPF.

The benefits of such a framework are multifaceted, with there being three main axes where sched_ext is specifically designed to provide significant value:

1. Ease of experimentation and exploration: Enabling rapid iteration of new scheduling policies.

2. Customization: Building application-specific schedulers which implement policies that are not applicable to general-purpose schedulers.

3. Rapid scheduler deployments: Non-disruptive swap outs of scheduling policies in production environments.

We'll begin by doing a deeper dive into the motivation of sched_ext in the following Motivation section. Following that, we'll provide some deatils on the overall architecture of sched_ext in the How section below.

Motivation

1. Ease of experimentation and exploration

Why is exploration important?

Scheduling is a challenging problem space. Small changes in scheduling behavior can have a significant impact on various components of a system, with the corresponding effects varying widely across different platforms, architectures, and workloads.

While complexities have always existed in scheduling, they have increased dramatically over the past 10-15 years. In the mid-late 2000s, cores were typically homogeneous and further apart from each other, with the criteria for scheduling being roughly the same across the entire die.

Systems in the modern age are by comparison much more complex. Modern CPU designs, where the total power budget of all CPU cores often far exceeds the power budget of the socket, with dynamic frequency scaling, and with or without chiplets, have significantly expanded the scheduling problem space. Cache hierarchies have become less uniform, with Core Complex (CCX) designs such as recent AMD processors having multiple shared L3 caches within a single socket. Such topologies resemble NUMA sans persistent NUMA node stickiness.

Use-cases have become increasingly complex and diverse as well. Applications such as mobile and VR have strict latency requirements to avoid missing deadlines that impact user experience. Stacking workloads in servers is constantly pushing the demands on the scheduler in terms of workload isolation and resource distribution.

Experimentation and exploration are important for any non-trivial problem space. However, given the recent hardware and software developments, we believe that experimentation and exploration are not just important, but critical in the scheduling problem space.

Indeed, other approaches in industry are already being explored. AMD has proposed an experimental patch set which enables userspace to provide hints to the scheduler via "Userspace Hinting". The approach adds a prctl() API which allows callers to set a numerical "hint" value on a struct task_struct. This hint is then optionally read by the scheduler to adjust the cost calculus for various scheduling decisions.

Huawei have also expressed interest in enabling some form of programmable scheduling. While we're unaware of any patch sets which have been sent to the upstream list for this proposal, it similarly illustrates the need for more flexibility in the scheduler.

Additionally, Google has developed ghOSt with the goal of enabling custom, userspace driven scheduling policies. Prior presentations at LPC have discussed ghOSt and how BPF can be used to accelerate scheduling.

Why can't we just explore directly with CFS?

Experimenting with CFS directly or implementing a new sched_class from scratch is of course possible, but is often difficult and time consuming. Newcomers to the scheduler often require years to understand the codebase and become productive contributors. Even for seasoned kernel engineers, experimenting with and upstreaming features can take a very long time. The iteration process itself is also time consuming, as testing scheduler changes on real hardware requires reinstalling the kernel and rebooting the host.

Core scheduling is an example of a feature that took a significant amount of time and effort to integrate into the kernel. Part of the difficulty with core scheduling was the inherent mismatch in abstraction between the desire to perform core-wide scheduling, and the per-cpu design of the kernel scheduler. This caused issues, for example ensuring proper fairness between the independent runqueues of SMT siblings.

The high barrier to entry for working on the scheduler is an impediment to academia as well. Master's/PhD candidates who are interested in improving the scheduler will spend years ramping-up, only to complete their degrees just as they're finally ready to make significant changes. A lower entrance barrier would allow researchers to more quickly ramp up, test out hypotheses, and iterate on novel ideas. Research methodology is also severely hampered by the high barrier of entry to make modifications; for example, the Shenango and Shinjuku scheduling policies used sched affinity to replicate the desired policy semantics, due to the difficulty of incorporating these policies into the kernel directly.

The iterative process itself also imposes a significant cost to working on the scheduler. Testing changes requires developers to recompile and reinstall the kernel, reboot their machines, rewarm their workloads, and then finally rerun their benchmarks. Though some of this overhead could potentially be mitigated by enabling schedulers to be implemented as kernel modules, a machine crash or subtle system state corruption is always only one innocuous mistake away. These problems are exacerbated when testing production workloads in a datacenter environment as well, where multiple hosts may be involved in an experiment; requiring a significantly longer ramp up time. Warming up memcache instances in the Meta production environment takes hours, for example.

How does sched_ext help with exploration?

sched_ext attempts to address all of the problems described above. In this section, we'll describe the benefits to experimentation and exploration that are afforded by sched_ext, provide real-world examples of those benefits, and discuss some of the trade-offs and considerations in our design choices.

One of our main goals was to lower the barrier to entry for experimenting with the scheduler. sched_ext provides ergonomic callbacks and helpers to ease common operations such as managing idle CPUs, scheduling tasks on arbitrary CPUs, handling preemptions from other scheduling classes, and more. While sched_ext does require some ramp-up, the complexity is self-contained, and the learning curve gradual. Developers can ramp up by first implementing simple policies such as global weighted vtime scheduling in only tens of lines of code, and then continue to learn the APIs and building blocks available with sched_ext as they build more featureful and complex schedulers.

Another critical advantage provided by sched_ext is the use of BPF. BPF provides strong safety guarantees by statically analyzing programs at load time to ensure that they cannot corrupt or crash the system. sched_ext guarantees system integrity no matter what BPF scheduler is loaded, and provides mechanisms to safely disable the current BPF scheduler and migrate tasks back to a trusted scheduler. For example, we also implement in-kernel safety mechanisms to guarantee that a misbehaving scheduler cannot indefinitely starve tasks. BPF also enables sched_ext to significantly improve iteration speed for running experiments. Loading and unloading a BPF scheduler is simply a matter of running and terminating a sched_ext binary.

BPF also provides programs with a rich set of APIs, such as maps, kfuncs, and BPF helpers. In addition to providing useful building blocks to programs that run entirely in kernel space (such as many of our example schedulers), these APIs also allow programs to leverage user space in making scheduling decisions. Specifically, the Atropos sample scheduler has a relatively simple weighted vtime or FIFO scheduling layer in BPF, paired with a load balancing component in userspace written in Rust. As described in more detail below, we also built a more general user-space scheduling framework called "rhone" by leveraging various BPF features.

On the other hand, BPF does have shortcomings, as can be plainly seen from the complexity in some of the example schedulers. scx_pair.bpf.c illustrates this point well. To start, it requires a good amount of code to emulate cgroup-local-storage. In the kernel proper, this would simply be a matter of adding another pointer to the struct cgroup, but in BPF, it requires a complex juggling of data amongst multiple different maps, a good amount of boilerplate code, and some unwieldy bpf_loop()'s and atomics. The code is also littered with explicit and often unnecessary sanity checks to appease the verifier.

That being said, BPF is being rapidly improved. For example, Yonghong Song recently upstreamed a patch set to add a cgroup local storage map type, allowing scx_pair.bpf.c to be simplified. There are plans to address other issues as well, such as providing statically-verified locking, and avoiding the need for unnecessary sanity checks. Addressing these shortcomings is a high priority for BPF, and as progress continues to be made, we expect most deficiencies to be addressed in the not-too-distant future.

Yet another exploration advantage of sched_ext is helping widening the scope of experiments. For example, sched_ext makes it easy to defer CPU assignment until a task starts executing, allowing schedulers to share scheduling queues at any granularity (hyper-twin, CCX and so on). Additionally, higher level frameworks can be built on top to further widen the scope. For example, the aforementioned rhone library allows implementing scheduling policies in user-space by encapsulating the complexity around communicating scheduling decisions with the kernel. This allows taking advantage of a richer programming environment in user-space, enabling experimenting with, for instance, more complex mathematical models.

sched_ext also allows developers to leverage machine learning. At Meta, we experimented with using machine learning to predict whether a running task would soon yield its CPU. These predictions can be used to aid the scheduler in deciding whether to keep a runnable task on its current CPU rather than migrating it to an idle CPU, with the hope of avoiding unnecessary cache misses. Using a tiny neural net model with only one hidden layer of size 16, and a decaying count of 64 syscalls as a feature, we were able to achieve a 15% throughput improvement on an Nginx benchmark, with an 87% inference accuracy.

2. Customization

This section discusses how sched_ext can enable users to run workloads on application-specific schedulers.

Why deploy custom schedulers rather than improving CFS?

Implementing application-specific schedulers and improving CFS are not conflicting goals. Scheduling features explored with sched_ext which yield beneficial results, and which are sufficiently generalizable, can and should be integrated into CFS. However, CFS is fundamentally designed to be a general purpose scheduler, and thus is not conducive to being extended with some highly targeted application or hardware specific changes.

Targeted, bespoke scheduling has many potential use cases. For example, VM scheduling can make certain optimizations that are infeasible in CFS due to the constrained problem space (scheduling a static number of long-running VCPUs versus an arbitrary number of threads). Additionally, certain applications might want to make targeted policy decisions based on hints directly from the application (for example, a service that knows the different deadlines of incoming RPCs).

Google has also experimented with some promising, novel scheduling policies. One example is "central" scheduling, wherein a single CPU makes all scheduling decisions for the entire system. This allows most cores on the system to be fully dedicated to running workloads, and can have significant performance improvements for certain use cases. For example, central scheduling with VCPUs can avoid expensive vmexits and cache flushes, by instead delegating the responsibility of preemption checks from the tick to a single CPU. See scx_central.bpf.c for a simple example of a central scheduling policy built in sched_ext.

Some workloads also have non-generalizable constraints which enable optimizations in a scheduling policy which would otherwise not be feasible. For example,VM workloads at Google typically have a low overcommit ratio compared to the number of physical CPUs. This allows the scheduler to support bounded tail latencies, as well as longer blocks of uninterrupted time.

Yet another interesting use case is the scx_flatcg scheduler, which provides a flattened hierarchical vtree for cgroups. This scheduler does not account for thundering herd problems among cgroups, and therefore may not be suitable for inclusion in CFS. However, in a simple benchmark using wrk on apache serving a CGI script calculating sha1sum of a small file, it outperformed CFS by ~3% with CPU controller disabled and by ~10% with two apache instances competing with 2:1 weight ratio nested four level deep.

Certain industries require specific scheduling behaviors that do not apply broadly. For example, ARINC 653 defines scheduling behavior that is widely used by avionic software, and some out-of-tree implementations (https://ieeexplore.ieee.org/document/7005306) have been built. While the upstream community may decide to merge one such implementation in the future, it would also be entirely reasonable to not do so given the narrowness of use-case, and non-generalizable, strict requirements. Such cases can be well served by sched_ext in all stages of the software development lifecycle -- development, testing, deployment and maintenance.

There are also classes of policy exploration, such as machine learning, or responding in real-time to application hints, that are significantly harder (and not necessarily appropriate) to integrate within the kernel itself.

Won't this increase fragmentation?

We acknowledge that to some degree, sched_ext does run the risk of increasing the fragmentation of scheduler implementations. As a result of exploration, however, we believe that enabling the larger ecosystem to innovate will ultimately accelerate the overall development and performance of Linux.

BPF programs are required to be GPLv2, which is enforced by the verifier on program loads. With regards to API stability, just as with other semi-internal interfaces such as BPF kfuncs, we won't be providing any API stability guarantees to BPF schedulers. While we intend to make an effort to provide compatibility when possible, we will not provide any explicit, strong guarantees as the kernel typically does with e.g. UAPI headers. For users who decide to keep their schedulers out-of-tree,the licensing and maintenance overheads will be fundamentally the same as for carrying out-of-tree patches.

With regards to the schedulers included in this patch set, and any other schedulers we implement in the future, both Meta and Google will open-source all of the schedulers we implement which have any relevance to the broader upstream community. We expect that some of these, such as the simple example schedulers and scx_rusty scheduler, will be upstreamed as part of the kernel tree. Distros will be able to package and release these schedulers with the kernel, allowing users to utilize these schedulers out-of-the-box without requiring any additional work or dependencies such as clang or building the scheduler programs themselves. Other schedulers and scheduling frameworks such as rhone may be open-sourced through separate per-project repos.

3. Rapid scheduler deployments

Rolling out kernel upgrades is a slow and iterative process. At a large scale it can take months to roll a new kernel out to a fleet of servers. While this latency is expected and inevitable for normal kernel upgrades, it can become highly problematic when kernel changes are required to fix bugs. Livepatch is available to quickly roll out critical security fixes to large fleets, but the scope of changes that can be applied with livepatching is fairly limited, and would likely not be usable for patching scheduling policies. With sched_ext, new scheduling policies can be rapidly rolled out to production environments.

As an example, one of the variants of the L1 Terminal Fault (L1TF) vulnerability allows a VCPU running a VM to read arbitrary host kernel memory for pages in L1 data cache. The solution was to implement core scheduling, which ensures that tasks running as hypertwins have the same "cookie".

While core scheduling works well, it took a long time to finalize and land upstream. This long rollout period was painful, and required organizations to make difficult choices amongst a bad set of options. Some companies such as Google chose to implement and use their own custom L1TF-safe scheduler, others chose to run without hyper-threading enabled, and yet others left hyper-threading enabled and crossed their fingers.

Once core scheduling was upstream, organizations had to upgrade the kernels on their entire fleets. As downtime is not an option for many, these upgrades had to be gradually rolled out, which can take a very long time for large fleets.

An example of an sched_ext scheduler that illustrates core scheduling semantics is scx_pair.bpf.c, which co-schedules pairs of tasks from the same cgroup, and is resilient to L1TF vulnerabilities. While this example scheduler is certainly not suitable for production in its current form, a similar scheduler that is more performant and featureful could be written and deployed if necessary.

Rapid scheduling deployments can similarly be useful to quickly roll-out new scheduling features without requiring kernel upgrades. At Google, for example, it was observed that some low-priority workloads were causing degraded performance for higher-priority workloads due to consuming a disproportionate share of memory bandwidth. While a temporary mitigation was to use sched affinity to limit the footprint of this low-priority workload to a small subset of CPUs, a preferable solution would be to implement a more featureful task-priority mechanism which automatically throttles lower-priority tasks which are causing memory contention for the rest of the system. Implementing this in CFS and rolling it out to the fleet could take a very long time.

sched_ext would directly address these gaps. If another hardware bug or resource contention issue comes in that requires scheduler support to mitigate, sched_ext can be used to experiment with and test different policies. Once a scheduler is available, it can quickly be rolled out to as many hosts as necessary, and function as a stop-gap solution until a longer-term mitigation is upstreamed.

How

sched_ext is a new sched_class which allows scheduling policies to be implemented in BPF programs.

sched_ext leverages BPF's struct_ops feature to define a structure which exports function callbacks and flags to BPF programs that wish to implement scheduling policies. The struct_ops structure exported by sched_ext is struct sched_ext_ops, and is conceptually similar to struct sched_class. The role of sched_ext is to map the complex sched_class callbacks to the more simple and ergonomic struct sched_ext_ops callbacks.

Unlike some other BPF program types which have ABI requirements due to exporting UAPIs, struct_ops has no ABI requirements whatsoever. This provides us with the flexibility to change the APIs provided to schedulers as necessary. BPF struct_ops is also already being used successfully in other subsystems, such as in support of TCP congestion control.

The only struct_ops field that is required to be specified by a scheduler is the 'name' field. Otherwise, sched_ext will provide sane default behavior, such as automatically choosing an idle CPU on the task wakeup path if .select_cpu() is missing.

Dispatch queues

To match the impedance between the scheduler core and the BPF scheduler, sched_ext uses DSQs (dispatch queues) which can operate as both a FIFO and a priority queue. By default, there is one global FIFO (SCX_DSQ_GLOBAL), and one local dsq per CPU (SCX_DSQ_LOCAL). The BPF scheduler can manage an arbitrary number of dsq's using scx_bpf_create_dsq() and scx_bpf_destroy_dsq().

A CPU always executes a task from its local DSQ. A task is "dispatched" to a DSQ. A non-local DSQ is "consumed" to transfer a task to the consuming CPU's local DSQ.

When a CPU is looking for the next task to run, if the local DSQ is not empty, the first task is picked. Otherwise, the CPU tries to consume the global DSQ. If that doesn't yield a runnable task either, ops.dispatch() is invoked.

Scheduling cycle

The following briefly shows how a waking task is scheduled and executed.

1. When a task is waking up, ops.select_cpu() is the first operation invoked. This serves two purposes. First, CPU selection optimization hint. Second, waking up the selected CPU if idle.

   The CPU selected by ops.select_cpu() is an optimization hint and not binding. The actual decision is made at the last step of scheduling. However, there is a small performance gain if the CPU ops.select_cpu() returns matches the CPU the task eventually runs on.

   A side-effect of selecting a CPU is waking it up from idle. While a BPF scheduler can wake up any cpu using the scx_bpf_kick_cpu() helper, using ops.select_cpu() judiciously can be simpler and more efficient.

   A task can be immediately dispatched to a DSQ from ops.select_cpu() by calling scx_bpf_dispatch(). If the task is dispatched to SCX_DSQ_LOCAL from ops.select_cpu(), it will be dispatched to the local DSQ of whichever CPU is returned from ops.select_cpu(). Additionally, dispatching directly from ops.select_cpu() will cause the ops.enqueue() callback to be skipped.

   Note that the scheduler core will ignore an invalid CPU selection, for example, if it's outside the allowed cpumask of the task.

2. Once the target CPU is selected, ops.enqueue() is invoked (unless the task was dispatched directly from ops.select_cpu()). ops.enqueue() can make one of the following decisions:

   • Immediately dispatch the task to either the global or local DSQ by calling scx_bpf_dispatch() with SCX_DSQ_GLOBAL or SCX_DSQ_LOCAL, respectively.

   • Immediately dispatch the task to a custom DSQ by calling scx_bpf_dispatch() with a DSQ ID which is smaller than 2^63.

   • Queue the task on the BPF side.

3. When a CPU is ready to schedule, it first looks at its local DSQ. If empty, it then looks at the global DSQ. If there still isn't a task to run, ops.dispatch() is invoked which can use the following two functions to populate the local DSQ.

   • scx_bpf_dispatch() dispatches a task to a DSQ. Any target DSQ can be used - SCX_DSQ_LOCAL, SCX_DSQ_LOCAL_ON | cpu, SCX_DSQ_GLOBAL or a custom DSQ. While scx_bpf_dispatch() currently can't be called with BPF locks held, this is being worked on and will be supported. scx_bpf_dispatch() schedules dispatching rather than performing them immediately. There can be up to ops.dispatch_max_batch pending tasks.

   • scx_bpf_consume() tranfers a task from the specified non-local DSQ to the dispatching DSQ. This function cannot be called with any BPF locks held. scx_bpf_consume() flushes the pending dispatched tasks before trying to consume the specified DSQ.

4. After ops.dispatch() returns, if there are tasks in the local DSQ, the CPU runs the first one. If empty, the following steps are taken:

   • Try to consume the global DSQ. If successful, run the task.

   • If ops.dispatch() has dispatched any tasks, retry #3.

   • If the previous task is an SCX task and still runnable, keep executing it (see SCX_OPS_ENQ_LAST).

   • Go idle.

Note that the BPF scheduler can always choose to dispatch tasks immediately in ops.select_cpu() or ops.enqueue(). If only the built-in DSQs are used, there is no need to implement ops.dispatch() as a task is never queued on the BPF scheduler and both the local and global DSQs are consumed automatically.

scx_bpf_dispatch() queues the task on the FIFO of the target DSQ. Use scx_bpf_dispatch_vtime() for the priority queue. Internal DSQs such as SCX_DSQ_LOCAL and SCX_DSQ_GLOBAL do not support priority-queue dispatching, and must be dispatched to with scx_bpf_dispatch().

Verifying callback behavior

sched_ext always verifies that any value returned from a callback is valid, and will issue an error and unload the scheduler if it is not. For example, if .select_cpu() returns an invalid CPU, or if an attempt is made to invoke the scx_bpf_dispatch() with invalid enqueue flags. Furthermore, if a task remains runnable for too long without being scheduled, sched_ext will detect it and error-out the scheduler.

Closing Thoughts

Both Meta and Google have experimented quite a lot with schedulers in the last several years. Google has benchmarked various workloads using user space scheduling, and have achieved performance wins by trading off generality for application specific needs. At Meta, we are actively experimenting with multiple production workloads and seeing significant performance gains, and are in the process of deploying sched_ext schedulers on production workloads at scale. We expect to leverage it extensively to run various experiments and develop customized schedulers for a number of critical workloads.

Written By

• David Vernet dvernet@meta.com
• Josh Don joshdon@google.com
• Tejun Heo tj@kernel.org
• Barret Rhoden brho@google.com

Supported By

• Paul Turner pjt@google.com
• Neel Natu neelnatu@google.com
• Patrick Bellasi derkling@google.com
• Hao Luo haoluo@google.com
• Dimitrios Skarlatos dskarlat@cs.cmu.edu