If there is a higher priority task when running ops.tick(),
ops.select_cpu(), and ops.enqueue() callbacks, the current running tasks
yields its CPU by shrinking time slice to zero and a higher priority
task can run on the current CPU.
As low-cost, fine-grained preemption becomes available, default
parameters are adjusted as follows:
- Raise the bar for remote CPU preemption to avoid IPIs.
- Increase the maximum time slice.
- Gradually enforce the fair use of CPU time (i.e., ineligible duration)
Lastly, using CAS, we ensure that a remote CPU is preempted by only one
CPU. This removes unnecessary remote preemptions (and IPIs).
Signed-off-by: Changwoo Min <changwoo@igalia.com>
scx_lavd implemented 32 and 64 bit versions of a base-2 logarithm
function. This is now also used in rusty. To avoid code duplication,
let's pull it into a shared header.
Note that there is technically a functional change here as we remove the
always inline compiler directive. We instead assume that the compiler
will know best whether or not to inline the function.
Signed-off-by: David Vernet <void@manifault.com>
In user space in rusty, the tuner detects system utilization, and uses
it to inform how we do load balancing, our greedy / direct cpumasks,
etc. Something else we could be doing but currently aren't, is using
system utilization to inform how we dispatch tasks. We currently have a
static, unchanging slice length for the runtime of the program, but this
is inefficient for all scenarios.
Giving a task a long slice length does have advantages, such as
decreasing the number of involuntary context switches, decreasing the
overhead of preemption by doing it less frequently, possibly getting
better cache locality due to a task running on a CPU for a longer amount
of time, etc. On the other hand, long slices can be problematic as well.
When a system is highly utilized, a CPU-hogging task running for too
long can harm interactive tasks. When the system is under-utilized,
those interactive tasks can likely find an idle, or under-utilized core
to run on. When the system is over-utilized, however, they're likely to
have to park in a runqueue.
Thus, in order to better accommodate such scenarios, this patch
implements a rudimentary slice scaling mechanism in scx_rusty. Rather
than having one global, static slice length, we instead have a dynamic,
global slice length that can be changed depending on system utilization.
When over-utilized, we go with a longer slice length, and vice versa for
when the system is under-utilized. With Terraria, this results in
roughly a 50% improvement in mean FPS when playing on an AMD Ryzen 9
7950X, while running Spotify, and stress-ng -c $((4 * $(nproc))).
Signed-off-by: David Vernet <void@manifault.com>
scx_rusty doesn't do terribly well with interactive workloads. In order
to improve the situation, this patch adds support for basic deadline
scheduling in rusty. This approach doesn't incorporate eligibility, and
simply uses a crude avg_runtime tracking approach to scaling a task's
deadline.
In a series of follow-on changes, we'll update the scheduler to use more
indicators for interactivity that affect both slice length, and deadline
calculation.
Signed-off-by: David Vernet <void@manifault.com>
To know the required CPU performance (e.g., frequency) demand, we keep
track of 1) utilization of each CPU and 2) _performance criticality_ of
each task. The performance criticality of a task denotes how critical it
is to CPU performance (frequency). Like the notion of latency
criticality, we use three factors: the task's average runtime, wake-up
frequency, and waken-up frequency. A task's runtime is longer, and its
two frequencies are higher; the task is more performance-critical
because it would be a bottleneck in the middle of the task chain.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
Let's remove the extraneous copy pasting and use a lookup helper like we
do for task and pcpu context.
Signed-off-by: David Vernet <void@manifault.com>
A LoadEntity gets the load to transfer between two entities by taking
the minimum of their imbalances and reducing its abs value by
xfer_ratio.
In practice self.imbal(), the push node or domain, always has positive
imbalance and other.imbal(), the pull node or domain, always has
negative imbalance, so other.imbal() is always the minimum even though
the abs value of its imbalance might be greater than the abs value of
self.imbal(). It seems like the intent is to take the minimum of the
two absolute values instead to avoid overbalancing at the puller, so
make both values abs.
Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Rusty's load balancer calculates load differently based on average
system CPU utilization in create_domain_hierarchy(). At >= 99.999%
utilization, load is the product of a task's weight and duty cycle;
below that, load is the same as the task's duty cycle.
populate_tasks_by_load(), however, always uses the product when
calculating per-task load so that in the sub-99.999% util case, load is
inflated, typically by a factor of 100 with a normal priority task.
Tasks look too heavy to migrate as a result because a single task would
transfer more load than the domain imbalance allows, leading to
significant imbalance in some cases.
Make populate_tasks_by_load() calculate task load the same way as
domain load, checking lb_apply_weight.
Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
The current code replenishes the task's time slice whenever the task
becomes ops.running(). However, there is a case where such behavior can
starve the other tasks, causing the watchdog timeout error. One (if not
all) such case is when a task is preempted while running by the higher
scheduler class (e.g., RT, DL). In such a case, the task will be transit
in a cycle of ops.running() -> ops.stopping() -> ops.running() -> etc.
Whenever it becomes re-running, it will be placed at the head of local
DSQ and ops.running() will renew its time slice. Hence, in the worst
case, the task can run forever since its time slice is never exhausted.
The fix is assigning the time slice only once by checking if the time
slice is calculated before.
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Changwoo Min <changwoo@igalia.com>
Provide a command line option to print the version of the scheduler and
the scx_rustland_core crate.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Given that rustland_core now supports task preemption and it has been
tested successfully, it's worhtwhile to cut a new version of the crate.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
In Rust c_char can be aliased to i8 or u8, depending on the particular
target architecture.
For example, trying to build scx_lavd on ppc64 triggers the following
error:
error[E0308]: mismatched types
--> src/main.rs:200:38
|
200 | let c_tx_cm: *const c_char = (&tx.comm as *const [i8; 17]) as *const i8;
| ------------- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `*const u8`, found `*const i8`
| |
| expected due to this
|
= note: expected raw pointer `*const u8`
found raw pointer `*const i8`
To fix this, consistently use c_char instead of assuming it corresponds
to i8.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
In _some_ kernel versions, loading scx_lavd fails with an error of
"bpf_rcu_read_unlock is missing". The usage of
bpf_rcu_read_lock/unlock() in proc_dump_all_tasks() is correct but the
bpf verifier still think bpf_rcu_read_unlock() is missing. The most
plausible reason so far is that the problematic kernel does not have a
commit 6fceea0fa59f ("bpf: Transfer RCU lock state between subprog
calls"), failing inter-procedural analysis between proc_dump_all_tasks()
and submit_task_ctx(). Thus, we force inline submit_task_ctx() (no
inter-procedural analysis by the verifier is necessary) for the time
being.
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Changwoo Min <changwoo@igalia.com>
Only the very newest kernels support scx_bpf_cpuperf_set(). Let's update
scx_layered to accommodate older kernels as well.
Signed-off-by: David Vernet <void@manifault.com>
Looking at perf top it seems that the scheduler can spend a significant
amount of time iterating over the CPU topology/cpumask information,
especially when the system is running a significant amount of tasks:
2.57% scx_rustland [.] <scx_utils::cpumask::CpumaskIntoIterator as core::iter::traits::iterator::Iterator>::next
Considering that scx_rustland doesn't support CPU hotplugging yet (it
requires a full restart to properly handle CPU hotplug events), we can
completely avoid this overhead by caching a TopologyMap object at the
beginning, when the scheduler starts, instead of constantly
re-evaluating the CPU topology information.
This allows to reduce the scheduler overhead by ~5% CPU utilization
under heavy load conditions (from ~65% -> ~60%, according to top).
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
This change adds `scx_bpf_cpuperf_cap`, `scx_bpf_cpuperf_cur` and
`scx_bpf_cpuperf_set` definitions that were recently introduced into
[`sched_ext`](https://github.com/sched-ext/sched_ext/pull/180). It adds
a `perf` field to `scx_layered` to allow for controlling performance per
layer.
Signed-off-by: Daniel Hodges <hodges.daniel.scott@gmail.com>
If a library creates threads, those threads will often have the same
name. If two different processes of different priority both use a
library, it may be that we want the library's threads in each process to
be put into different layers.
To support this, let's add the ability to filter not only by task name,
but also by process name via the task thread group leader's comm.
Tested by creating two executables named "foo" and "bar", which both
spawn a bunch of tasks named "exp_worker" that spin until being
interrupted. With this config: https://pastebin.com/Uz2phzxQ, the tasks
were correctly matched to the expected layers.
Signed-off-by: David Vernet <void@manifault.com>
We're currently cloning cpumasks returned by calls to {Core, Cache,
Node, Topology}::span(). If a caller needs to clone it, they can. Let's
not penalize the callers that just want to query the underlying cpumask.
Signed-off-by: David Vernet <void@manifault.com>
Some people have expressed confusion at this behavior. Let's be a bit
more explicit in the documentation.
Signed-off-by: David Vernet <void@manifault.com>
Provide a run-time option to disable task preemption.
This option can be used to improve the throughput of the CPU-intensive
tasks while still providing a good level of responsiveness in the
system.
By default preemption is enabled, to provide a higher level of
responsiveness to the interactive tasks.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Use the new scx_rustland_core dispatch flag RL_PREEMPT_CPU to allow
interactive tasks to preempt other tasks with scx_rustland.
If the built-in idle selection logic is enforced (option `-i`), the
scheduler prioritizes keeping tasks on the target CPU designated by this
logic. With preemption enabled, these tasks have a higher likelihood of
reusing their cached working set, potentially improving performance.
Alternatively, when tasks are dispatched to the first available CPU
(default behavior), interactive tasks benefit from running more promptly
by kicking out other tasks before their assigned time slice expires.
This potentially allows to increase the default time slice to higher
values in the future, to improve the overall throughput in the system
and, at the same time, still maintain a good level of responsiveness,
because interactive tasks are now able to run pretty much immediately,
independently on the remaining time slice of the other tasks that are
contending the CPUs in the system.
= Results =
Measuring the performance of the usual benchmark "playing a video game
while running a parallel kernel build in background" seems to give
around 2-10% boost in the fps with preemption enabled, depending on the
particular video game.
Results were obtained running a `make -j32` kernel build on a AMD Ryzen
7 5800X 8-Cores 16GB RAM, while testing video games such as Baldur's
Gate 3 (with a solid +10% fps), Counter Strike 2 (around +5%) and Team
Fortress 2 (+2% boost).
Moreover, some WebGL applications (such as
https://webglsamples.org/aquarium/aquarium.html) seem to benefit even
more with preemption enabled, providing up to a +15% fps boost.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Reserve some bits of the `cpu` attribute of a task to store special
dispatch flags.
Initially, let's introduce just RL_CPU_ANY to replace the special value
NO_CPU, indicating that the task can be dispatched on any CPU,
specifically the first CPU that becomes available.
This allows to keep the CPU value assigned by the builtin idle selection
logic, that can potentially be used later for further optimizations.
Moreover, having the possibility to specify dispatch flags gives more
flexibility and it allows to map new scheduling features to such flags.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
When I transitioned layered to using task local storage, I messed up
initializing the task ctx, not realizing we previously had a separate
variable that was initializing the hasmap entry. We need to initialize
the task's layer to -11, and also set refresh_layer to 1.
Signed-off-by: David Vernet <void@manifault.com>
scx_simple no longer supports running in "partial" mode, with only
certain tasks usig scx_simple. When this option was removed, we also
removed the call to scx_bpf_switch_all();
While switching-all is the default behavior for newer kernels, let's add
__COMPAT_scx_bpf_switch_all() so that scx_simple can work on older
kernels as well.
Signed-off-by: David Vernet <void@manifault.com>
We have a lot of boilerplate code where we create a cpumask, initialize
it, and then bpf_kptr_xchg() it into the map. In an effort to slightly
reduce the amount of boilerplate, let's create a helper that can
alleviate some of it.
Signed-off-by: David Vernet <void@manifault.com>
There are some random issues in the code, like unused variables, and bad
print formatters. I'm not sure why the compiler isn't consistently
complaining, but let's fix them.
Signed-off-by: David Vernet <void@manifault.com>
In scx_rusty, now that we have a complete view of the host's topology
thanks to the Topology crate, we can update our calls to
scx_bpf_create_dsq() to create the DSQ on the NUMA node of the domain.
It's unclear how much this will end up mattering for performance in the
typical case, but we might as well do the right thing given that host
topolgoy is static, and we have the information.
Signed-off-by: David Vernet <void@manifault.com>
