Do not always assign the maximum time slice to interactive tasks, but
use the same value of the dynamic time slice for everyone.
This seems to prevent potential audio cracking when the system is over
commissioned.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
The option --full-user is provided to delegate *all* scheduling
decisions to the user-space scheduler with no exception, including the
idle selection logic.
Therefore, make this option incompatible with --builtin-idle and
completely bypass the built-in idle selection logic when running in
full-user mode.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Provide a knob in scx_rustland_core to automatically turn the scheduler
into a simple FIFO when the system is underutilized.
This choice is based on the assumption that, in the case of system
underutilization (less tasks running than the amount of available CPUs),
the best scheduling policy is FIFO.
With this option enabled the scheduler starts in FIFO mode. If most of
the CPUs are busy (nr_running >= num_cpus - 1), the scheduler
immediately exits from FIFO mode and starts to apply the logic
implemented by the user-space component. Then the scheduler can switch
back to FIFO if there are no tasks waiting to be scheduled (evaluated
using a moving average).
This option can be enabled/disabled by the user-space scheduler using
the fifo_sched parameter in BpfScheduler: if set, the BPF component will
periodically check for system utilization and switch back and forth to
FIFO mode based on that.
This allows to improve performance of workloads that are using a small
amount of the available CPUs in the system, while still maintaining the
same good level of performance for interactive tasks when the system is
over commissioned.
In certain video games, such as Baldur's Gate 3 or Counter-Strike 2,
running in "normal" system conditions, we can experience a boost in fps
of approximately 4-8% with this change applied.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
This merge included additional commits that were supposed to be included
in a separate pull request and have nothing to do with the fifo-mode
changes.
Therefore, revert the whole pull request and create a separate one with
the correct list of commits required to implement this feature.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Dispatch non-interactive tasks on the CPU selected by the built-in idle
selection logic and allow interactive tasks to be dispatched on any CPU.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Do not always assign the maximum time slice to interactive tasks, but
use the same value of the dynamic time slice for everyone.
This seems to prevent potential audio cracking when the system is over
commissioned.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Provide a knob in scx_rustland_core to automatically turn the scheduler
into a simple FIFO when the system is underutilized.
This choice is based on the assumption that, in the case of system
underutilization (less tasks running than the amount of available CPUs),
the best scheduling policy is FIFO.
With this option enabled the scheduler starts in FIFO mode. If most of
the CPUs are busy (nr_running >= num_cpus - 1), the scheduler
immediately exits from FIFO mode and starts to apply the logic
implemented by the user-space component. Then the scheduler can switch
back to FIFO if there are no tasks waiting to be scheduled (evaluated
using a moving average).
This option can be enabled/disabled by the user-space scheduler using
the fifo_sched parameter in BpfScheduler: if set, the BPF component will
periodically check for system utilization and switch back and forth to
FIFO mode based on that.
This allows to improve performance of workloads that are using a small
amount of the available CPUs in the system, while still maintaining the
same good level of performance for interactive tasks when the system is
over commissioned.
In certain video games, such as Baldur's Gate 3 or Counter-Strike 2,
running in "normal" system conditions, we can experience a boost in fps
of approximately 4-8% with this change applied.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Report the amount of running tasks to stdout. This value also represents
the amount of active CPUs that are currently executing a task.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
The dynamic slice boost is not used anymore in the code, so there is no
reason to keep evaluating it.
Moreover, using it instead of the static slice boost seems to make
things worse, so let's just get rid of it.
Fixes: 0b3c399 ("scx_rustland: introduce dynamic slice boost")
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Introduce a low-power mode to force the scheduler to operate in a very
non-work conserving way, causing a significant saving in terms of power
consumption, while still providing a good level of responsiveness in the
system.
This option can be enabled in scx_rustland via the --low_power / -l
option.
The idea is to not immediately re-kick a CPU when it enters an idle
state, but do that only if there are no other tasks running in the
system.
In this way, latency-critical tasks can be still dispatched immediately
on the other active CPUs, while CPU-bound tasks will be forced to spend
more time waiting to be scheduled, basically enforcing a special CPU
throttling mechanism that affects only the tasks that are not latency
critical.
The consequence is a reduction in the overall system throughput, but
also a significant reduction of power consumption, that can be useful
for mobile / battery-powered devices.
Test case (using `scx_rustland -l`):
- play a video game (Terraria) while recompiling the kernel
- measure game performance (fps) and core power consumption (W)
- compare the result of normal mode vs low-power mode
Result:
Game performance | Power consumption |
------------+-----------------+-------------------+
normal mode | 60 fps | 6W |
low-power mode | 60 fps | 3W |
As we can see from the result the reduction of power consumption is
quite significant (50%), while the responsiveness of the game (fps)
remains the same, that means battery life can be potentially doubled
without significantly affecting system responsiveness.
The overall throughput of the system is, of course, affected in a
negative way (kernel build is approximately 50% slower during this
test), but the goal here is to save power while still maintaining a good
level of responsiveness in the system.
For this reason the low-power mode should be considered only in
emergency conditions, for example when the system is close to completely
run out of power or simply to extend the battery life of a mobile device
without compromising its responsiveness.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
During the initialization phase the scheduler needs to be aware of all
the available CPUs in the system (also those that are offline), in order
to create a proper per-CPU DSQ for all of them.
Otherwise, if some cores are offline, we may get errors like the
following:
swapper/7[0] triggered exit kind 1024:
runtime error (invalid DSQ ID 0x0000000000000007)
Backtrace:
scx_bpf_consume+0xaa/0xd0
bpf_prog_42ff1b9d1ac5b184_rustland_dispatch+0x12b/0x187
Change the code to configure the BpfScheduler object with the total
amount of CPUs available in the system and prevent such failure.
This fixes#280.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Always dispatch at least one task, even if all the CPUs are busy.
This small overcommitment allows to maximize the CPU utilization without
introducing bubbles in the scheduling and also without introducing
regressions in terms of resposiveness.
Before this change the average CPU utilization of a `stress-ng -c 8` on
an 8-cores system is around 95%. With this change applied the CPU
utilization goes up to a consistent 100%.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Add a method to TopologyMap to get the amount of online CPUs.
Considering that most of the schedulers are not handling CPU hotplugging
it can be useful to expose also this metric in addition to the amount of
available CPUs in the system.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Drop the global effective time-slice and use the more fine-grained
per-task time-slice to implement the dynamic time-slice capability.
This allows to reduce the scheduler's overhead (dropping the global time
slice volatile variable shared between user-space and BPF) and it
provides a more fine-grained control on the per-task time slice.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Replace the BPF_MAP_TYPE_QUEUE with a BPF_MAP_TYPE_USER_RINGBUF to store
the tasks dispatched from the user-space scheduler to the BPF component.
This eliminates the need of the bpf() syscalls, significantly reducing
the overhead of the user-space->kernel communication and delivering a
notable performance boost in the overall system throughput.
Based on experimental results, this change allows to reduces the scheduling
overhead by approximately 30-35% when the system is overcommitted.
This improvement has the potential to make user-space schedulers based
on scx_rustland_core viable options for real production systems.
Link: https://github.com/libbpf/libbpf-rs/pull/776
Signed-off-by: Andrea Righi <andrea.righi@canonical.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>
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>
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>
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>
As described in https://bugzilla.kernel.org/show_bug.cgi?id=218109,
https://github.com/sched-ext/scx/issues/147 and
https://github.com/sched-ext/sched_ext/issues/69, AMD chips can
sometimes report fully disabled CPUs as offline, which causes us to
count them when looking at /sys/devices/system/cpu/possible.
Additionally, systems can have holes in their active CPU maps. For
example, a system with CPUs 0, 1, 2, 3 possible, may have only 0 and 2
active. To address this, we need to do a few things:
1. Update topology.rs to be clear that it's returning the number of
_possible_ CPUs in the system. Also update Topology to only record
online CPUs when creating its span and iterating over sysfs when
creating domains. It was previously trying to record when a CPU was
online, but this was actually broken as the topology directory isn't
present in sysfs when the CPU is offline.
2. Schedulers should not be relying on nr_possible_cpus for anything
other than interacting with per-CPU data (e.g. for stats extraction),
or e.g. verifying maximum sizes of statically sized arrays in BPF. It
should _not_ be used for e.g. performing load calculations, etc. With
that said, we'll also need to update schedulers to not rely on the
nr_possible_cpus figure being exported by the topology crate. We do
that for rusty in this patch, but don't fix any of the others other
than updating how they call topology.rs.
3. Account for the fact that LLC IDs may be non-contiguous. For example,
if there is a single core in an LLC, then if we assign LLC IDs to
domains, then the domain IDs won't be contiguous. This doesn't fit
our current model which is used by e.g. infeasible_weights.rs. We'll
update some of the code in rusty to accomodate this, but we'll need
to do more.
4. Update schedulers to properly reset themselves in the event of a
hotplug event. We'll take care of that in a follow-on change.
Signed-off-by: David Vernet <void@manifault.com>
We're iterating from min..max cpu in cpus_online(), but that's not
inclusive of the max CPU. Let's also include that so we don't think that
last CPU is offline.
Signed-off-by: David Vernet <void@manifault.com>
Most of the schedulers assume that the amount of possible CPUs in the
system represents the actual number of CPUs available.
This is not always true: some CPUs may be offline or certain CPU models
(AMD CPUs for example) may include unavailable CPUs in this number.
This can lead to sub-optimal performance or even errors in the scheduler
(see for example [1][2]).
Ideally, we need to attack this issue in a more generic way, such as
having a proper API provided by a C library, that can be used by all
schedulers and the topology Rust module (scx_utils crate).
But for now, let's try to mitigate most of the common sub-optimal cases
separately inside each scheduler.
For rustland we can apply some mitigations both in select_cpu() (for the
BPF part) and in the user-space part:
- the former is fixed in the sched-ext kernel by commit 94dc0c01b957
("scx: Use cpu_online_mask when resetting idle masks"). However,
adding an extra check `cpu < num_possible_cpus` in select_cpu(),
allows to properly support AMD CPUs, even with kernels that don't
have the cpu_online_mask fix yet (this doesn't always guarantee the
validity of cpu, but it should be enough to mitigate the majority of
the potential sub-optimal cases, without introducing any significant
overhead)
- the latter can be fixed relying on topology.span(), instead of
topology.nr_cpus(), to count the amount of available CPUs in the
system.
[1] https://github.com/sched-ext/sched_ext/issues/69
[2] https://github.com/sched-ext/scx/issues/147
Link: 94dc0c01b9
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
In order to use the new consume_raw() API we need to depend on a version
of libbpf-rs that is not released yet.
Apparently adding such dependency may introduce a potential dependency
conflict with libbpf-sys.
Therefore, revert this change and go back to the previous consume() API.
One a new version of libbpf-rs will be out we can update all our
dependencies to use the new libbpf-rs and re-apply this patch to
scx_rustland_core.
Fixes: 7c8c5fd ("scx_rustland_core: use new consume_raw() libbpf-rs API")
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
In line with rustland's focus on prioritizing interactive tasks, set the
default base time slice to 5ms.
This allows to mitigate potential audio craking issues or system lags
when the system is overloaded or under memory pressure condition (i.e.,
https://github.com/sched-ext/scx/issues/96#issuecomment-1978154324).
A downside of this change is to introduce potential regressions in the
throughput of CPU-intensive workloads, but in such scenarios rustland
may not be the optimal choice and alternative schedulers may be
preferred.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Some high-priority tasks may have a weight too high, that can
potentially disrupt the slice boost optimization logic, causing
interactive tasks to be less responsive.
In line with rustland's focus on prioritizing interactive tasks, prevent
giving too much CPU bandwidth to such high-priority tasks by limiting
the maximum task weight to 1000.
This allows to maintain a good level of system responsiveness even in
presence of tasks with a really high priority.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Use the new consume_raw() API provided by libbpf-rs with
https://github.com/libbpf/libbpf-rs/pull/680.
This allows to be more precise and efficient at processing tasks
consumed from the BPF ring buffer.
NOTE: the new consume_raw() API is not available yet in any official
release of the libbpf-rs crate, but cargo allows to pick versions
directly from git. This slightly increases the build time of
scx_rustland_core and the schedulers based on this crate (since we need
to recompile libbpf-rs from source), but we can re-add a proper
versioned dependency once the libbpf-rs is out.
TODO: this new API also offers the possibility to consume multiple items
from the BPF ring buffer with a single call to consume_raw(). This could
be investigated and implemented as a potential future enhancement.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
The current topology.rs crate assumes that all cores have unique core
IDs in a system. This need not be the case, such as in certain Intel
Xeon processors which reuse core IDs in different NUMA nodes. Let's
update the crate to assume unique core IDs only per socket.
Signed-off-by: David Vernet <void@manifault.com>
Provide distinct methods to set the target CPU and the per-task time
slice to dispatched tasks.
Moreover, also provide a constructor to create a DispatchedTask from a
QueuedTask (this allows to automatically bounce a task from the
scheduler to the BPF dispatcher without having to take care of setting
the individual task's attributes).
This also allows to make most of the attributes of DispatchedTask
private, especially it allows to hide cpumask_cnt, that should be only
used internally between the BPF and the user-space component.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Provide a way to set a different time slice per-task, by adding a new
attribute slice_ns to the DispatchedTask struct.
This attribute determines the time slice assigned to the task, if it is
set to 0 then the global time slice (either the default one or the
effective one, if set) will be used.
At the same time, remove the payload attribute, that is basically unused
(scx_rustland uses it to send the task's vruntime to the BPF dispatcher
for debugging purposes, but it's not very useful anymore at this point).
In the future we may introduce a proper interface to attach a custom
payload to each task with a proper interface.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
This is to potentinally reduce issues with folks
using different versions of libbpf at runtime.
This also:
- makes static linking of libbpf the default
- adds steps in `meson setup` to fetch libbpf and make it
There is no need to generate source code in a temporary directory with
RustLandBuilder(), we can simply generate code in-tree and exclude the
generated source files from .gitignore.
Having the generated source files in-tree can help to debug potential
build issues (and it also allows to drop the the tempfile crate
dependency).
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Introduce a wrapper to scx_utils::BpfBuilder that can be used to build
the BPF component provided by scx_rustland_core.
The source of the BPF components (main.bpf.c) is included in the crate
as an array of bytes, the content is then unpacked in a temporary file
to perform the build.
The RustLandBuilder() helper is also used to generate bpf.rs (that
implements the low-level user-space Rust connector to the BPF
commponent).
Schedulers based on scx_rustland_core can simply use RustLandBuilder(),
to build the backend provided by scx_rustland_core.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
