The user-space scheduler dispatches tasks in batches, with the batch
size matching the number of idle CPUs.
Commit 791bdbe ("scx_rustland: introduce SMT support") changed the order
of idle CPUs, prioritizing dispatching tasks on the least busy cores
(those with the most idle CPUs) before moving on to busier cores (those
with the least idle CPUs).
While this approach works well for a small number of tasks, it can lead
to uneven performance as the number of tasks increases and all cores are
saturated. Such uneven performance can be attributed to SMT interactions
causing potential short lags and erratic system performance. In some
cases, disabling SMT entirely results in better system responsiveness.
To address this issue, instruct the scheduler to implicitly disable SMT
and consistently dispatch tasks only on the first (or last) CPU of each
core. This approach ensures an equal distribution of tasks among the
available cores, preventing SMT disturbances and aligning with non-SMT
performance, also when a significant amount of tasks are running.
Additionally, the unused sibling CPUs within each core can be used as
"spare" CPUs for the BPF dispatcher. This is particularly beneficial for
tasks that cannot be dispatched on the target CPU selected by the
scheduler, due to cpumask restrictions or congestion conditions.
Therefore, this new approach allows to enhance system responsiveness on
SMT systems, while simultaneously improving scheduler stability.
Some preliminary results on an AMD Ryzen 7 5800X 8-Cores (SMT enabled):
running my usual benchmark of measuring the fps of a videogame
(Counter-Strike 2) during a parallel kernel build-induced system
overload, shows an improvement of approximately 2x (from 8-10fps to
15-25fps vs 1-2fps with EEVDF).
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Prior to commit 676bd88 ("bpf_rustland: do not dispatch the scheduler to
the global DSQ"), the user-space scheduler was dispatched using
SCX_DSQ_GLOBAL and we needed to explicitly kick idle CPUs from
update_idle() to ensure that at least one CPU was available to run the
user-space scheduler.
Now that we are using SCX_DSQ_LOCAL_ON|cpu to dispatch the user-space
scheduler, the target CPU is implicitly kicked. Therefore, the call to
scx_bpf_kick_cpu() within .update_idle() becomes redundant and we can
get rid of it.
Fixes: 676bd88 ("bpf_rustland: do not dispatch the scheduler to the global DSQ")
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Now that scheduling BPF timers works correctly, we don't need this extra
logic to eagerly compact if a scheduling for compaction has happened a
few times in a row. Let's remove it.
Signed-off-by: David Vernet <void@manifault.com>
In scx_nest, we use a per-cpu BPF timer to schedule compaction for a
primary core before it goes idle. If a task comes along that could use
that core, we cancel the callback with bpf_timer_cancel().
bpf_timer_cancel() drops a refcnt on the prog and nullifies the
callback, so if we want to schedule the callback again, we must use
bpf_timer_set_callback() to reset the prog. This patch does that.
Reported-by: Julia Lawall <julia.lawall@inria.fr>
Signed-off-by: David Vernet <void@manifault.com>
Update the slice boost dynamically, as a function of the amount of CPUs
in the system and the amount of tasks currently waiting to be
dispatched: as the amount of waiting tasks in the task_pool increases,
reduce the slice boost.
This adjustment ensures that the scheduler adheres more closely to a
pure vruntime-based policy as the amount of tasks contending the
available CPUs increases and it allows to sustain stress tests that are
spawning a massive amount of tasks.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Introduce a basic support of CPU topology awareness. With this change,
the scheduler will prioritize dispatching tasks to idle CPUs with fewer
busy SMT siblings, then, it will proceed to CPUs with more busy SMT
siblings, in ascending order.
To implement this, introduce a new CoreMapping abstraction, that
provides a mapping of the available core IDs in the system along with
their corresponding lists of CPU IDs. This, coupled with the
get_cpu_pid() method from the BpfScheduler abstraction, allows the
user-space scheduler to enforce the policy outlined above and improve
performance on SMT systems.
Keep in mind that this improvement is relevent only when the amount of
tasks running in the system is less than the amount of CPUs. As soon as
the amount of running tasks increases, they will be distributed across
all available CPUs and cores, thereby negating the advantages of SMT
isolation.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Even if the current implementation of the user-space scheduler doesn't
require to allocate aligned memory, add a simple support to aligned
allocations in RustLandAllocator, in order to make it more generic and
potentially usable by other schedulers / components.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Periodically report a page fault counter in the scheduler output. The
user-space scheduler should never trigger page faults, otherwise we may
experience deadlocks (that would trigger the sched-ext watchdog,
unloading the scheduler).
Reporting a page fault counter periodically to stdout can be really
helpful to debug potential issues with the custom allocator.
Moreover, group together also nr_sched_congested and
nr_failed_dispatches with nr_page_faults and use the sum of all these
counters to determine the healthy status of the user-space scheduler
(reporting it to stdout as well).
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
To prevent potential deadlock conditions under heavy loads, any
scheduler that delegates scheduling decisions to user-space should avoid
triggering page faults.
To address this issue, replace the default Rust allocator with a custom
one (RustLandAllocator), designed to operate on a pre-allocated buffer.
This, coupled with the memory locking (via mlockall), prevents page
faults from happening during the execution of the user-space scheduler,
avoiding the deadlock condition.
This memory allocator is completely transparent to the user-space
scheduler code and it is applied automatically when the bpf module is
imported.
In the future we may decide to move this allocator to a more generic
place (scx_utils crate), so that also other user-space Rust schedulers
can use it.
This initial implementation of the RustLandAllocator is very simple: a
basic block-based allocator that uses an array to track the status of
each memory block (allocated or free).
This allocator can be improved in the future, but right now, despite its
simplicity, it shows a reasonable speed and efficiency in meeting memory
requests from the user-space scheduler, having to deal mostly with small
and uniformly sized allocations.
With this change in place scx_rustland survived more than 10hrs on a
heavily stressed system (with stress-ng and kernel builds running in a
loop):
$ ps -o pid,rss,etime,cmd -p `pidof scx_rustland`
PID RSS ELAPSED CMD
34966 75840 10:00:44 ./build/scheds/rust/scx_rustland/debug/scx_rustland
Without this change it is possible to trigger the sched-ext watchdog
timeout in less than 5min, under the same system load conditions.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Entries from TaskInfoMap associated to exiting tasks are already removed
via the BPF .exit_task() callback, so drop the obsolete TODO note and
replace it with a proper comment.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Improve priority boosting using voluntary context switches metric.
Overview
========
The current criteria to apply the time slice boost (option `-b`) is to
distinguish between newly created tasks and tasks that are already
running: in order to prioritize interactive applications (games,
multimedia, etc.) we apply a time slice usage penalty on newly created
tasks, indirectly boosting the priority of tasks that are already
running, which are likely to be the interactive applications that we
aim to prioritize.
Problem
=======
This approach works well when the background workload forks a bunch of
short-lived tasks (e.g., a parallel kernel build), but it fails to
properly classify CPU-intensive background tasks (i.e., video/3D
rendering, encryption, large data analysis, etc.), because these
applications, typically, do not generate many short-lived processes.
In presence of such workloads the time slice penalty is not enforced,
resulting in a lack of any boost for interactive applications.
Solution
========
A more effective critiria for distinguishing between interactive
applications and background CPU-intensive applications is to examine the
voluntary context switches: an application that periodically releases
the CPU voluntarily is very likely to be interactive.
Therefore, change the time slice boost logic to apply a bonus (scale down
the accounted used time slice) to tasks that show an increase in their
voluntary context switches counter over a time frame of 10 sec.
Based on experimental results, this simple heurstic appears to be quite
effective in classifying interactive tasks and prioritize them over
potential background CPU-intensive tasks.
Additionally, having a better criteria to identify interactive tasks
allow to prioritize also newly created tasks, thereby enhancing the
responsiveness of interactive shell sessions.
This always ensures the prompt execution of system commands, even when
the system is massively overloaded, unlike the previous time slice boost
logic, which made interactive shell sessions less responsive by
deprioritizing newly created tasks.
Results
=======
With this new logic in place it is possible to play a video game (e.g.,
Terraria) without experiencing any frame rate drop (60 fps), while a
parallel CPU stress test (`stress-ng -c 32`) is running in the
background. The same result can also be obtained with a parallel kernel
build (`make -j 32`). Thus, there is no regression compared to the
previous "ideal" test case.
Even when mixing both workloads (`make -j 16` + `stress-ng -c 16`),
Terraria can still be played without noticeable lag in the audio or
video, maintaining a consistent 60 fps.
In addition to that, shell commands are also very responsive.
Following, the results (average and standard deviation of 10 runs) of
two simple interactive shell commands, while both the `make -j 16` and
`stress-ng -c 16` workloads are running in background:
avg time "uname -r" "ps axuw > /dev/null"
=========================================================
EEVDF 11.1ms 231.8ms
scx_rustland 2.6ms 212.0ms
stdev "uname -r" "ps axuw > /dev/null"
=========================================================
EEVDF 2.28 23.41
scx_rustland 0.70 9.11
Tests conducted on a 8-cores laptop (11th Gen Intel i7-1195G7 @
4.800GHz) with 16GB of RAM.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Provide the number of voluntary context switches (nvcsw) for each task
to the user-space scheduler.
This extra information can then be used by the scheduler to enhance its
decision-making process when scheduling tasks.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
552b75a9c7 ("scx: Build fix after kernel update") updated scx_flatcg along
with other schedulers to use the new direct dispatching from
ops.select_cpu() mechanism. However, this was buggy for flatcg.
flatcg uses direct dispatch for two purposes - as an optimization when there
are idle cpus and to avoid dealing with custom CPU affinities in the
dispatch logic. While the former can be moved to ops.select_cpu(), the
latter can't as it should also apply to tasks which get enqueued without
preceding ops.select_cpu(), e.g., when the task gets requeued after an
attribute change or runs out of time slice. The API update incorrectly moved
both to ops.select_cpu() leading to futile retries of try_pick_next_cgroup()
and scheduling misbheaviors.
Fix it by separating out the two cases and only keeping the idle
optimization case in ops.select_cpu().
Signed-off-by: Tejun Heo <tj@kernel.org>
We may end up stalling for too long in fcg_dispatch() if
try_pick_next_cgroup() doesn't find another valid cgroup to pick. This
can be quite risky, considering that we are holding the rq lock in
dispatch().
This condition can be reproduced easily in our CI, where we can trigger
stalling softirq works:
[ 4.972926] NOHZ tick-stop error: local softirq work is pending, handler #200!!!
Or rcu stalls:
[ 47.731900] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
[ 47.731900] rcu: 1-...!: (0 ticks this GP) idle=b29c/1/0x4000000000000000 softirq=2204/2204 fqs=0
[ 47.731900] rcu: 3-...!: (0 ticks this GP) idle=db74/1/0x4000000000000000 softirq=2286/2286 fqs=0
[ 47.731900] rcu: (detected by 0, t=26002 jiffies, g=6029, q=54 ncpus=4)
[ 47.731900] Sending NMI from CPU 0 to CPUs 1:
To mitigate this issue reduce the amount of try_pick_next_cgroup()
retries from BPF_MAX_LOOPS (8M) to CGROUP_MAX_RETRIES (1024).
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Introduce a parameter to prioritize active running tasks over newly
created tasks.
This option can be used to enhance interactive applications (e.g.,
games, audio/video, GUIs, etc.) that are concurrently running with
fork-intensive background workloads (such as a large parallel build for
example).
The boost value (which functions as a penalty) is applied to the time
slice attributed to newly generated tasks, increasing their vruntime
and, in an indirect manner, "boosting" the priority of all the other
concurrent active tasks.
The time slice boost parameter was applied in the live demo video [1] to
enhance the frames per second (fps) of a video game (Terraria), running
simultaneously with a parallel kernel build (`make -j 32`) on an 8-core
laptop (the value used in the video matches the existing setting of
running `scx_rustland -b 200`).
[1] https://www.youtube.com/watch?v=oCfVbz9jvVQ
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
With the introduction of a the dynamic time slice that scales down based
on the number of tasks in the system, there is no obvious benefit in
utilizing SCX_ENQ_PREEMPT to dispatch the user-space scheduler.
The reduced time slice as the task count increases already enhances the
user-space scheduler's opportunities to run and efficiently manage
scheduling tasks, even when the system is massively overloaded.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Temporarily switch to the unstable sched-ext ppa, so that we can resume
testing with the new kernel API.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Move scaling after tasks are sent to the dispatcher: tasks are
dispatched based on the amount of idle CPUs, so checking for any
remaining tasks still sitting in the scheduler after dispatch gives a
better idea how busy the system is.
Moreover, do not scale the time slice based on nr_cpus (otherwise,
systems with a large amount of CPUs would rarely get any scaling at
all).
Instead, apply a scaling factor as a function of how many tasks are
still waiting in the scheduler: nr_scheduled / 2. This method scales
better as the number of CPUs increases.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Now that we can dispatch directly from select_cpu() we can make the code
more compact and readable by removing the force_local logic.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
After updates to reflect the updated init and direct dispatch API, the
schedulers aren't compatible with older kernels. Bump versions and publish
releases.
In the latest kernel, sched_ext API has changed in two areas:
- ops.prep_enable/cancel_enable/enable/disable() replaced with
ops.init_task/enable/disable/exit_task().
- scx_bpf_dispatch() can now be called from ops.select_cpu(). Also,
SCX_ENQ_LOCAL flag is removed. Instead, users can call
scx_bpf_select_cpu_dfl() from ops.select_cpu() and use the @is_idle out
param value to determine whether to dispatch directly.
This commit updates all schedules so that they build.
- Init functions renamed / merged / split.
- ops.select_cpu() is added to several schedulers and local direct
disptching logic is moved there.
This is the minimum update which is need to make the schedulers build and
work. It needs further update to e.g. move vtime udpates to ops.enable().
With the introduction of a the dynamic time slice that scales down based
on the number of tasks in the system, there is no need anymore to apply
a constant scaling factor to time slice to extend the range of the
allowed time slices.
Therefore, get rid of the static scaling and use slice_ns as the upper
limit for the time slice accounted to the tasks.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
libbpf_rs::num_possible_cpus() may take into account multi-threads
multi-cores information, that are not used efficiently by the scheduler
at the moment.
For simplicity rely on /proc/stat to determine the amount of CPUs that
can be used by the scheduler and provide a proper abstraction to access
this information from the bpf Rust module.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Fix the ternary operator expression to return the CPU id, instead of the
boolean result of the condition.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
