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>
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>
* 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>
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>
