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>
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>
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>
* scx-lavd: preemption of a lower-priority task using kick cpu
When a task is enqueued to the global queue, the scheduler checks if
there is a lower priority task than the enqueued task. If so, it kicks
out the lower-priority task, hoping the newly enqueued task or another
higher-priority task runs on the kicked CPU. Kicking another CPU is
expensive as an IPI is involved, so the scheduler judiciously kicks the
CPU when its benefit (i.e., priority gap) is clear enough.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
The scx_rusty scheduler does not support hotplug, and expects a static
host topology throughout its runtime. Though the kernel does have
support for detecting hotplug events, we currently don't detect this in
the kernel, nor surface it to user space when it happens. Now that we
have scx_bpf_exit(), we can gracefully exit the kernel in the event of a
hotplug, and communicate to user space that it should restart the
scheduler.
This patch adds that support to scx_rusty. Note that this assumes that
we're running on a recent enough kernel that has scx_bpf_exit(). If it
doesn't, then we instead just error out of the kernel scheduler and exit
the application.
Signed-off-by: David Vernet <void@manifault.com>
In rusty_select_cpu(), if a task is WAKE_SYNC, we'll currently migrate
the task to that CPU if there are any idle cores on the system. As in
[0], this condition is insufficient, as there could be idle cores
elsewhere on the system, but still tasks piled up on a single local DSQ.
Let's add a condition that the local DSQ has to be empty in order to
apply the WAKE_SYNC migration.
Before patch:
[void@maniforge src]$ hackbench
Running in process mode with 10 groups using 40 file descriptors each (== 400 tasks)
Each sender will pass 100 messages of 100 bytes
Time: 0.433
With patch:
[void@maniforge src]$ hackbench
Running in process mode with 10 groups using 40 file descriptors each (== 400 tasks)
Each sender will pass 100 messages of 100 bytes
Time: 0.035
Signed-off-by: David Vernet <void@manifault.com>
Change the upper bound of ineligible duration (LAVD_ELIGIBLE_TIME_MAX).
The updated (2x increased) upper bound reflects the distribution of
tasks' eligible_delta_ns better.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
Change the calculation of the run_frequence using the wait_period from
the last time the task yielded CPU to this time when the task is
running. The old implementation measures the time interval between the
last stopping and the current running and increases run_freq without
reason.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
Change the last_{start/stop/wait/wake}_clk in task_ctx to
last_{running/stopping/quiescent/runnable}_clk, matching with state
transition names. In addition, add comments and reorder fields in
task_ctx for readability.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
When a task runs more than once (running <->stopping) within one
runnable-quiescent transition, accumulate runtime of multiple runnings
for statistics. This helps to get the task's runtime per schedule when
supposing that a huge time slice is given, which is what we want to
collect for scheduling decisions.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
Remove runtime_boost using slice_boost_prio. Without slice_boost_prio,
the scheduler collects the exact time slice.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
Let's change the function names of update_stat_for_*() as follow their
callers for consistency and less confusion.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
The run_time_boosted_ns calculation requires updated slice_boost_prio,
so updating slice_boost_prio should be done before updating
run_time_boosted_ns.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
In scx_layered, we're using a BPF_MAP_TYPE_HASH map (indexed by pid)
rather than a BPF_MAP_TYPE_TASK_STORAGE, to track local storage for a
task. As far as I can tell, there's no reason we need to be doing this.
We never access the map from user space, and we're even passing a
struct task_struct * to a helper subprog to look up the task context
rather than only doing it by pid.
Using a hashmap is error prone for this because we end up having to
manually track lifecycles for entries in the map rather than relying on
BPF to do it for us. For example, BPF will automatically free a task's
entry from the map when it exits. Let's just use TLS here rather than a
hashmap to avoid issues from this (e.g. we've observed the scheduler
getting evicted because we're accessing a stale map entry after a task
has been destroyed).
Reported-by: Valentin Andrei <vandrei@meta.com>
Signed-off-by: David Vernet <void@manifault.com>
transit_task_stat() is now tracking the same runnable, running, stopping,
quiescent transitions that sched_ext core already tracks and always returns
%true. Let's remove it.
LAVD_TASK_STAT_ENQ is tracking a subset of runnable task state transitions -
the ones which end up calling ops.enqueue(). However, what it is trying to
track is a task becoming runnable so that its load can be added to the cpu's
load sum.
Move the LAVD_TASK_STAT_ENQ state transition and update_stat_for_enq()
invocation to ops.runnable() which is called for all runnable transitions.
Note that when all the methods are invoked, the invocation order would be
ops.select_cpu(), runnable() and then enqueue(). So, this change moves
update_stat_for_enq() invocation before calc_when_to_run() for
put_global_rq(). update_stat_for_enq() updates taskc->load_actual which is
consumed by calc_greedy_ratio() and thus affects calc_when_to_run().
Before this patch, calc_greedy_ratio() would use load_actual which doesn't
reflect the last running period. After this patch, the latest running period
will be reflected when the task gets queued to the global queue.
The difference is unlikely to matter but it'd probably make sense to make it
more consistent (e.g. do it at the end of quiescent transition).
After this change, transit_task_stat() doesn't detect any invalid
transitions.
scx_lavd tracks task state transitions and updates statistics on each valid
transition. However, there's an asymmetry between the runnable/running and
stopping/quiescent transitions. In the former, the runnable and running
transitions are accounted separately in update_stat_for_enq() and
update_stat_for_run(), respectively. However, in the latter, the two
transitions are combined together in update_stat_for_stop().
This asymmetry leads to incorrect accounting. For example, a task's load
should be added to the cpu's load sum when the task gets enqueued and
subtracted when the task is no longer runnable (quiescent). The former is
accounted correctly from update_stat_for_enq() but the latter is done
whenever the task stops. A task can transit between running and stopping
multiple times before becoming quiescent, so the asymmetry can end up
subtracting the load of a task which is still running from the cpu's load
sum.
This patch:
- introduces LAVD_TASK_STAT_QUIESCENT and updates transit_task_stat() so
that it can handle all valid state transitions including the multiple back
and forth transitions between two pairs - QUIESCENT <-> ENQ and RUNNING
<-> STOPPING.
- restores the symmetry by moving load adjustments part from
update_stat_for_stop() to new update_stat_for_quiescent().
This removes a good chunk of ignored transitions. The next patch will take
care of the rest.
lookup_task_ctx(), lookup_task_ctx_may_fail(), and lookup_layer()
currently don't have the static keyword, so BPF may treat them as a
global function. We don't actually want these to be global, so let's
make them static to avoid confusing the verifier.
Signed-off-by: David Vernet <void@manifault.com>
The old approach is mapping [0, maximum latency criticliy] to [-boost
range, boost range). This approach is easily affected by one outlier
maximum value and suffers from the integer truncation error. The new
approach divides the range into two -- [minimum latency criticality,
average latency criticality) and [average latency criticality, maximum
latency criticality] -- and maps them into [boost range/2, 0) and [0,
-boost range/2), respectively,
Signed-off-by: Changwoo Min <changwoo@igalia.com>
Replace a latency weight arrary to more skewed one, which is the
inverse of sched_prio_to_slice_weight. It turns out more skewed one
works better under highly CPU-overloaded cases since it gives a longer
deadline to non-latency-critical tasks.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
As the calculated runtime increases by considering the number of
full-time slice consumption, increase the upper bound
(LAVD_LC_RUNTIME_MAX) of runtime to be considered in latency
calculation. Also, add LAVD_SLICE_BOOST_MAX_PRIO to avoid
slice_boost_prio dropping to zero suddenly.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
Take slice_boost_prio -- how many times a full time slice was consumed
-- into consideration in calculating run_time_ns (runtime per schedule).
This improve the accuracy especially when a task is overscheduled and
its time slice is reduced for enforcing fairness.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
