The competition window was 7.5 msec, half of the targeted latency.
However, it is too wide for some workloads, so unrelated tasks may
compete with each other. Hence, it is tightened to about 1 msec with
LAVD_LAT_WEIGHT_SHIFT to avoid unnecessary competition.
Also, when a system is overloaded, now the time space is stretched more
aggressively (i.e., lat_prio^2) when a task's latency priority is low
(high value).
Signed-off-by: Changwoo Min <changwoo@igalia.com>
The old approach was too conservative in running a new task, so when a
fork-heavy workload competes with a CPU-bound workload, the fork-heavy
one is starved. The new approach solves the starvation problem by
inheriting parent's statistics. It seems a good (at least better than
old) guess how a new task will behave.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
When the system is highly loaded with compute-intensive tasks, the old
setting chokes latensive-intensive tasks, so loosen the dealine when the
system is overloaded (> 100% utilization).
Signed-off-by: Changwoo Min <changwoo@igalia.com>
When the lavd is loaded, it prints out its build id. It helps to easily
identify what version it is when testing.
```
01:56:54 [INFO] scx_lavd scheduler is initialized (build ID: 0.8.1-g98a5fa8595430414115c504857cea1a458393838-dirty x86_64-unknown-linux-gnu)
```
Signed-off-by: Changwoo Min <changwoo@igalia.com>
This is a second attempt to optimize tunables for a wider range of
games.
1) LAVD_BOOST_RANGE increased from 14 (35%) to 40 (100% of nice range).
Now the latency priority (biased by nice value) will decide which
task should run first . The nice value will decide the time slice.
2) The first change will give higher priority to latency-critical task
compared to before. For compensation, the slice boost also increased
(2x -> 3x).
Signed-off-by: Changwoo Min <changwoo@igalia.com>
In some games (e.g., Elden Ring), it was observed that preemption
happens much less frequently. The reason is that tasks' runtime per
schedule is similar, so it does not meet the existing criteria. To
alleviate the problem, the following three tunables are revised:
1) Smaller LAVD_PREEMPT_KICK_MARGIN and LAVD_PREEMPT_TICK_MARGIN help to
trigger more preemption.
2) Smaller LAVD_SLICE_MAX_NS works better especially 250 or 300Hz
kernels.
3) Longer LAVD_ELIGIBLE_TIME_MAX purturbes time lines less frequently.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
Use the function can_task1_kick_task2() to replace places which also
checking the comp_preemption_info between two cpus for better
consistency.
Signed-off-by: I Hsin Cheng <richard120310@gmail.com>
It seems that we are not updating `is_idle` when we find an idle CPU
with pick_cpu(), causing unnecessary rescheduling events when
select_cpu() is called.
To resolve this, ensure that the is_idle state is correctly set.
Additionally, always ensure that the task is dispatched to the local DSQ
immediately upon finding (and reserving) an idle CPU.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
- clean up u63 and u32 usages in structures to reduce struct size
- refactoring pick_cpu() for readability
Signed-off-by: Changwoo Min <changwoo@igalia.com>
The required CPU performance (cpuperf) was set to 1024 (100%) when the
CPU utilization was 100%. When a sudden load spike happens, it makes the
system adapt slowly in the next interval.
The new scheme always reserves some headroom in advance, so it sets
cpuperf to 1024 when the CPU utilization reaches to 85%. This gives some
room to adapt in advance.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
In preparation of upstreaming, let's set the min version requirement at the
released v6.9 kernels. Drop __COMPAT_SCX_KICK_IDLE. The open helper macros
now check the existence of SCX_KICK_IDLE and abort if not.
In preparation of upstreaming, let's set the min version requirement at the
released v6.9 kernels. Drop __COMPAT_scx_bpf_switch_call(). The open helper
macros now check the existence of SCX_OPS_SWITCH_PARTIAL and abort if not.
The bpf_ prefix is used for BPF API. Rename bpf_log2() to u32_log2() and
bpf_log2l() to u64_log2(). While at it, relocate them below compiler
directive helpers.
The old logic for CPU frequency scaling is that the task's CPU
performance target (i.e., target CPU frequency) is checked every tick
interval and updated immediately. Indeed, it samples and updates a
performance target every tick interval. Ultimately, it fluctuates CPU
frequency every tick interval, resulting in less steady performance.
Now, we take a different strategy. The key idea is to increase the
frequency as soon as possible when a task starts running for quick
adoption to load spikes. However, if necessary, it decreases gradually
every tick interval to avoid frequency fluctuations.
In my testing, it shows more stable performance in many workloads
(games, compilation).
Signed-off-by: Changwoo Min <changwoo@igalia.com>
Originally, do_update_sys_stat() simply calculated the system-wide CPU
utilization. Over time, it has evolved to collect all kinds of
system-wide, periodic statistics for decision-making, so it has become
bulky. Now, it is time to refactor it for readability. This commit does
not contain functional changes other than refactoring.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
The periodic CPU utilization routine does a lot of other work now. So we
rename LAVD_CPU_UTIL_INTERVAL_NS to LAVD_SYS_STAT_INTERVAL_NS.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
When a device is suspended and resumed, the suspended duration is added
up to a task's runtime if the task was running on the CPU. After the
resume, the task's runtime is incorrectly long and the scheduler starts
to recognize the system is under heavy load. To avoid such problem, the
suspended duration is measured and substracted from the task's runtime.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
scx_lavd: core compaction for low power consumption
When system-wide CPU utilization is low, it is very likely all the CPUs
are running with very low utilization. That means all CPUs run with low
clock frequency thanks to dynamic frequency scaling and very frequently
go in and out from/to C-state. That results in low performance (i.e.,
low clock frequency) and high power consumption (i.e., frequent
P-/C-state transition).
The idea of *core compaction* is using less number of CPUs when
system-wide CPU utilization is low. The chosen cores (called "active
cores") will run in higher utilization and higher clock frequency, and
the rest of the cores (called "idle cores") will be in a C-state for a
much longer duration. Thus, the core compaction can achieve higher
performance with lower power consumption.
One potential problem of core compaction is latency spikes when all the
active cores are overloaded. A few techniques are incorporated to solve
this problem.
1) Limit the active CPU core's utilization below a certain limit (say 50%).
2) Do not use the core compaction when the system-wide utilization is
moderate (say 50%).
3) Do not enforce the core compaction for kernel and pinned user-space
tasks since they are manually optimized for performance.
In my experiments, under a wide range of system-wide CPU utilization
(5%—80%), the core compaction reduces 7-30% power consumption without
sacrificing average and 99p tail latency.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
Make restart handling with user_exit_info simpler and consistently use the
load and report macros consistently across the rust schedulers. This makes
all schedulers automatically handle auto restarts from CPU hotplug events.
Note that this is necessary even for scx_lavd which has CPU hotplug
operations as CPU hotplug operations which took place between skel open and
scheduler init can still trigger restart.
In order to prevent compiler from merging or refetching load/store
operations or unwanted reordering, we take the implemetation of
READ_ONCE()/WRITE_ONCE() from kernel sources under
"/include/asm-generic/rwonce.h".
Use WRITE_ONCE() in function flip_sys_cpu_util() to ensure the compiler
doesn't perform unnecessary optimization so the compiler won't make
incorrect assumptions when performing the operation of modifying of bit
flipping.
Signed-off-by: I Hsin Cheng <richard120310@gmail.com>
Use the GNU built-in __sync_fetch_and_xor() to perform the XOR operation
on global variable "__sys_cpu_util_idx" to ensure the operations
visibility.
The built-in function "__sync_fetch_and_xor()" can provide both atomic
operation and full memory barrier which is needed by every operation
(especially store operation) on global variables.
Signed-off-by: I Hsin Cheng <richard120310@gmail.com>
C SCX_OPS_ATTACH() and rust scx_ops_attach() macros were not calling
.attach() and were only attaching the struct_ops. This meant that all
non-struct_ops BPF programs contained in the skels were never attached which
breaks e.g. scx_layered.
Let's fix it by adding .attach() invocation the the attach macros.
Originally the implementation of function rsigmoid_u64 will
perform substraction even when the value of "v" equals to the value
of "max" , in which the result is certainly zero.
We can avoid this redundant substration by changing the condition from
">" to ">=" since we know when the value of "v" and "max" are equal
we can return 0 without any substract operation.
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>
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>
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>
