6.3 KiB
Notes for lock optimization
Idea
We are reading BASEPRI independently if and only if we are actually changing BASEPRI. On restoring BASEPRI chose to restore read value if at the outmost nesting level (initial priority of the task). In this way, we can avoid unnecessary BASEPRI accesses, and reduce register pressure.
If you want to play around checkout the lockopt branch and use:
> arm-none-eabi-objdump target/thumbv7m-none-eabi/release/examples/lockopt -d > lockopt.asm
We extend cortex-m-rtfm/src/export::Priority with an additional fields to store init_logic (priority of the task) and old_basepri_hw. The latter field is initially None on creation.
// Newtype over `Cell` that forbids mutation through a shared reference
pub struct Priority {
init_logic: u8,
current_logic: Cell<u8>,
#[cfg(armv7m)]
old_basepri_hw: Cell<Option<u8>>,
}
impl Priority {
#[inline(always)]
pub unsafe fn new(value: u8) -> Self {
Priority {
init_logic: value,
current_logic: Cell::new(value),
old_basepri_hw: Cell::new(None),
}
}
#[inline(always)]
fn set_logic(&self, value: u8) {
self.current_logic.set(value)
}
#[inline(always)]
fn get_logic(&self) -> u8 {
self.current_logic.get()
}
#[inline(always)]
fn get_init_logic(&self) -> u8 {
self.init_logic
}
#[cfg(armv7m)]
#[inline(always)]
fn get_old_basepri_hw(&self) -> Option<u8> {
self.old_basepri_hw.get()
}
#[cfg(armv7m)]
#[inline(always)]
fn set_old_basepri_hw(&self, value: u8) {
self.old_basepri_hw.set(Some(value));
}
}
The corresponding lock is implemented as follows:
#[cfg(armv7m)]
#[inline(always)]
pub unsafe fn lock<T, R>(
ptr: *mut T,
priority: &Priority,
ceiling: u8,
nvic_prio_bits: u8,
f: impl FnOnce(&mut T) -> R,
) -> R {
let current = priority.get_logic();
if current < ceiling {
if ceiling == (1 << nvic_prio_bits) {
priority.set_logic(u8::max_value());
let r = interrupt::free(|_| f(&mut *ptr));
priority.set_logic(current);
r
} else {
match priority.get_old_basepri_hw() {
None => priority.set_old_basepri_hw(basepri::read()),
_ => (),
};
priority.set_logic(ceiling);
basepri::write(logical2hw(ceiling, nvic_prio_bits));
let r = f(&mut *ptr);
if current == priority.get_init_logic() {
basepri::write(priority.get_old_basepri_hw().unwrap());
} else {
basepri::write(logical2hw(priority.get_logic(), nvic_prio_bits));
}
priority.set_logic(current);
r
}
} else {
f(&mut *ptr)
}
}
The highest priority is achieved through an interrupt_free and does not at all affect the BASEPRI.
For the normal case, on enter we check if the BASEPRI register has been read, if not we read it and update priority. On exit we check if are to restore a logical priority (inside a nested lock) or to restore the BASEPRI (previously read).
Safety
We can safely unwrap the get_old_basepri_hw: Option<u8> as the path leading up to the unwrap passes an update to Some or was already Some. Updating get_old_basepri_hw is monotonic, the API offers no way of making get_old_basepri_hw into None (besides new).
Moreover new is the only public function of Priority, thus we are exposing nothing dangerous to the user.
Implementation
Implementation mainly regards two files, the rtfm/src/export.rs (discussed above) and macros/src/codegen/hardware_tasks.rs. For the latter the task dispatcher is updated as follows:
...
const_app.push(quote!(
#[allow(non_snake_case)]
#[no_mangle]
#section
#cfg_core
unsafe fn #symbol() {
const PRIORITY: u8 = #priority;
#let_instant
crate::#name(
#locals_new
#name::Context::new(&rtfm::export::Priority::new(PRIORITY) #instant)
);
}
));
...
Basically we create Priority (on stack) and use that to create a Context. The beauty is that LLVM is completely optimizing out the data structure (and related code), but taking into account its implications to control flow. Thus, the locks AND initial reading of BASEPRI will be optimized at compile time at Zero cost.
Overall, using this approach, we don't need a trampoline (run). We reduce the overhead by at least two machine instructions (additional reading/writing of BASEPRI) for each interrupt. It also reduces the register pressure (as less information needs to be stored).
Evaluation
The examples/lockopt.rs shows that locks are effectively optimized out.
00000132 <GPIOB>:
132: b510 push {r4, lr}
134: f000 f893 bl 25e <__basepri_r>
138: 4604 mov r4, r0
13a: 20a0 movs r0, #160 ; 0xa0
13c: f000 f892 bl 264 <__basepri_w>
140: f240 0000 movw r0, #0
144: f2c2 0000 movt r0, #8192 ; 0x2000
148: 6801 ldr r1, [r0, #0]
14a: 3101 adds r1, #1
14c: 6001 str r1, [r0, #0]
14e: 4620 mov r0, r4
150: e8bd 4010 ldmia.w sp!, {r4, lr}
154: f000 b886 b.w 264 <__basepri_w>
00000158 <GPIOC>:
158: f240 0000 movw r0, #0
15c: f2c2 0000 movt r0, #8192 ; 0x2000
160: 6801 ldr r1, [r0, #0]
162: 3102 adds r1, #2
164: 6001 str r1, [r0, #0]
166: 4770 bx lr
GPIOB/C are sharing a resource (C higher prio). Notice, there is no BASEPRI manipulation at all.
For GPIOB, there is a single read of BASEPRI (stored in old_basepri_hw) and just two writes, one for entering critical section, one for exiting. On exit we detect that we are indeed at the initial priority for the task, thus we restore the old_basepri_hw instead of a logic priority.
Limitations and Drawbacks
None spotted so far.
Observations
> llvm-objdump target/thumbv7m-none-eabi/release/examples/lockopt -d > lockopt.asm
> cargo objdump --example lockopt --release -- -d > lockopt.asm
Neither give assembly dump with symbols (very annoying to rely on arm-none-eabi-objdump for proper objdumps), maybe just an option is missing?