preempt_rt/
sched.rs

1use crate::sched::PreemptRtError::{PriorityAboveMax, PriorityBelowMin};
2use libc::{c_int, pid_t};
3use std::fmt;
4use thiserror::Error;
5
6/// PreemptRt result type
7pub type RtResult<T> = Result<T, PreemptRtError>;
8
9#[derive(Debug, Error)]
10/// PreemptRt error type. When libc functions return -1, errno will be fetched via libc.
11pub enum PreemptRtError {
12    #[error("c function returned errno: {0}")]
13    Errno(c_int),
14    #[error("unknown scheduler for value {0}")]
15    UnknownScheduler(c_int),
16    #[error("priority {0} is higher than max priority {1}")]
17    PriorityAboveMax(c_int, c_int),
18    #[error("priority {0} is lower than min priority {1}")]
19    PriorityBelowMin(c_int, c_int),
20    #[cfg(feature = "non-linux-stubs")]
21    #[error("current platform {0} does not support preempt-rt")]
22    NonLinuxPlatform(&'static str),
23}
24
25#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
26/// Wrapper around pid_t.
27pub struct Pid(pid_t);
28
29impl Pid {
30    /// The current thread is represented by `0` when using sched_* functions.
31    /// Note that this is different from the value of calling `getpid()`, which returns the
32    /// actual pid for the current _process_.
33    pub const fn current_thread() -> Self {
34        Pid(0)
35    }
36}
37
38impl From<Pid> for pid_t {
39    fn from(pid: Pid) -> Self {
40        pid.0
41    }
42}
43
44impl fmt::Display for Pid {
45    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
46        fmt::Display::fmt(&self.0, f)
47    }
48}
49
50#[repr(i32)]
51#[allow(non_camel_case_types)] // intentionally matching with libc / linux docs
52#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
53/// The type of scheduler for use with [`sched_getscheduler`](https://man7.org/linux/man-pages/man3/sched_getscheduler.3p.html)
54/// and [`sched_setscheduler`](https://man7.org/linux/man-pages/man2/sched_setscheduler.2.html).
55/// See [man_sched(7)](https://man7.org/linux/man-pages/man7/sched.7.html) for more details
56/// on the differences in behavior.
57///
58/// This type is a wrapper around the libc::SCHED_* enum.
59pub enum Scheduler {
60    /// The default scheduler on non-realtime linux - also known as SCHED_OTHER.
61    #[cfg(target_os = "linux")]
62    SCHED_NORMAL = libc::SCHED_NORMAL,
63    /// The default scheduler on non-realtime linux - also known as SCHED_OTHER.
64    #[cfg(target_os = "macos")]
65    SCHED_NORMAL = libc::SCHED_OTHER,
66    /// The realtime FIFO scheduler. All FIFO threads have priority higher than 0 and
67    /// preempt SCHED_NORMAL threads. Threads are executed in priority order, using
68    /// first-in-first-out lists to handle two threads with the same priority.
69    SCHED_FIFO = libc::SCHED_FIFO,
70    /// Round-robin scheduler, similar to SCHED_FIFO but with a time quantum.
71    SCHED_RR = libc::SCHED_RR,
72    /// Batch scheduler, similar to SCHED_OTHER but assumes the thread is CPU intensive.
73    /// The kernel applies a mild penalty to switching to this thread.
74    /// As of Linux 2.6.16, the only valid priority is 0.
75    #[cfg(target_os = "linux")]
76    SCHED_BATCH = libc::SCHED_BATCH,
77    /// The idle scheduler only executes the thread when there are idle CPUs. SCHED_IDLE
78    /// threads have no progress guarantees.
79    #[cfg(target_os = "linux")]
80    SCHED_IDLE = libc::SCHED_IDLE,
81    /// Deadline scheduler, attempting to provide guaranteed latency for requests.
82    /// See the [linux kernel docs](https://docs.kernel.org/scheduler/sched-deadline.html)
83    /// for details.
84    #[cfg(target_os = "linux")]
85    SCHED_DEADLINE = libc::SCHED_DEADLINE,
86}
87
88impl TryFrom<c_int> for Scheduler {
89    type Error = PreemptRtError;
90
91    fn try_from(value: c_int) -> RtResult<Self> {
92        match value {
93            #[cfg(target_os = "linux")]
94            libc::SCHED_NORMAL => Ok(Scheduler::SCHED_NORMAL),
95            #[cfg(target_os = "macos")]
96            libc::SCHED_OTHER => Ok(Scheduler::SCHED_NORMAL),
97            libc::SCHED_FIFO => Ok(Scheduler::SCHED_FIFO),
98            libc::SCHED_RR => Ok(Scheduler::SCHED_RR),
99            #[cfg(target_os = "linux")]
100            libc::SCHED_BATCH => Ok(Scheduler::SCHED_BATCH),
101            #[cfg(target_os = "linux")]
102            libc::SCHED_IDLE => Ok(Scheduler::SCHED_IDLE),
103            #[cfg(target_os = "linux")]
104            libc::SCHED_DEADLINE => Ok(Scheduler::SCHED_DEADLINE),
105            _ => Err(PreemptRtError::UnknownScheduler(value)),
106        }
107    }
108}
109
110fn handle_errno(result: c_int) -> RtResult<c_int> {
111    if result == -1 {
112        #[cfg(target_os = "linux")]
113        return Err(PreemptRtError::Errno(unsafe { *libc::__errno_location() }));
114        #[cfg(target_os = "macos")]
115        return Err(PreemptRtError::Errno(unsafe { *libc::__error() }));
116    } else {
117        Ok(result)
118    }
119}
120
121impl Scheduler {
122    /// Get the highest priority value for a given scheduler.
123    pub fn priority_max(&self) -> RtResult<c_int> {
124        let res = unsafe { libc::sched_get_priority_max(*self as c_int) };
125        handle_errno(res)
126    }
127
128    /// Get the lowest priority value for a given scheduler.
129    pub fn priority_min(&self) -> RtResult<c_int> {
130        let res = unsafe { libc::sched_get_priority_min(*self as c_int) };
131        handle_errno(res)
132    }
133
134    /// Create a ParameterizedScheduler with a numeric priority. Validates that the priority
135    /// is within the bounds of priority_min and priority_max for the chosen scheduler.
136    pub fn with_params(self, params: SchedulerParams) -> RtResult<ParameterizedScheduler> {
137        let priority = params.priority;
138        let max = self.priority_max()?;
139        let min = self.priority_min()?;
140        if priority > max {
141            Err(PriorityAboveMax(priority, max))
142        } else if priority < min {
143            Err(PriorityBelowMin(priority, min))
144        } else {
145            Ok(ParameterizedScheduler {
146                scheduler: self,
147                params,
148            })
149        }
150    }
151}
152
153#[repr(C)]
154#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
155/// Schedule parameters for a thread. Priority is the only supported parameter by the kernel
156/// at the moment. This is a wrapper around `libc::sched_param`
157pub struct SchedulerParams {
158    /// Priority of the current schedule.
159    pub priority: c_int,
160}
161
162#[cfg(target_os = "linux")]
163impl From<SchedulerParams> for libc::sched_param {
164    #[cfg(not(any(target_env = "musl", target_env = "ohos")))]
165    fn from(param: SchedulerParams) -> Self {
166        libc::sched_param {
167            sched_priority: param.priority,
168        }
169    }
170
171    #[cfg(any(target_env = "musl", target_env = "ohos"))]
172    fn from(param: SchedulerParams) -> Self {
173        let ts_zero = libc::timespec {
174            tv_sec: 0,
175            tv_nsec: 0,
176        };
177        // musl and ohos have additional fields for SCHED_DEADLINE - this is not abstracted
178        // in this library yet.
179        libc::sched_param {
180            sched_priority: param.priority,
181            sched_ss_init_budget: ts_zero.clone(),
182            sched_ss_low_priority: 0,
183            sched_ss_repl_period: ts_zero.clone(),
184            sched_ss_max_repl: 0,
185        }
186    }
187}
188
189impl From<libc::sched_param> for SchedulerParams {
190    fn from(param: libc::sched_param) -> Self {
191        SchedulerParams {
192            priority: param.sched_priority,
193        }
194    }
195}
196
197pub trait IntoSchedParams {
198    fn into_sched_params(self) -> SchedulerParams;
199}
200
201impl IntoSchedParams for SchedulerParams {
202    fn into_sched_params(self) -> SchedulerParams {
203        self
204    }
205}
206
207impl IntoSchedParams for i32 {
208    fn into_sched_params(self) -> SchedulerParams {
209        SchedulerParams {
210            priority: self as c_int,
211        }
212    }
213}
214
215impl<T: IntoSchedParams> IntoSchedParams for Option<T> {
216    fn into_sched_params(self) -> SchedulerParams {
217        match self {
218            None => SchedulerParams { priority: 0 },
219            Some(param) => param.into_sched_params(),
220        }
221    }
222}
223
224#[cfg_attr(target_os = "macos", allow(unused))]
225#[derive(Debug, Clone)]
226pub struct ParameterizedScheduler {
227    scheduler: Scheduler,
228    params: SchedulerParams,
229}
230
231impl ParameterizedScheduler {
232    #[cfg_attr(target_os = "macos", allow(unused_variables))]
233    pub fn set_on(self, pid: Pid) -> RtResult<()> {
234        #[cfg(target_os = "linux")]
235        return set_scheduler(pid, self.scheduler, self.params);
236        #[cfg(target_os = "macos")]
237        return Err(PreemptRtError::NonLinuxPlatform("macos"));
238    }
239
240    pub fn set_current(self) -> RtResult<()> {
241        self.set_on(Pid::current_thread())
242    }
243}
244
245#[cfg(target_os = "linux")]
246mod linux {
247    use super::*;
248    use std::mem::MaybeUninit;
249
250    /// Get the current scheduler in use for a given process or thread.
251    /// Using `Pid::from_raw(0)` will fetch the scheduler for the calling thread.
252    pub fn get_scheduler(pid: Pid) -> RtResult<Scheduler> {
253        let res = unsafe { libc::sched_getscheduler(pid.into()) };
254        handle_errno(res).and_then(Scheduler::try_from)
255    }
256
257    /// Set the scheduler and parameters for a given process or thread.
258    /// Using `Pid::from_raw(0)` will set the scheduler for the calling thread.
259    ///
260    /// SCHED_OTHER, SCHED_IDLE and SCHED_BATCH only support a priority of `0`, and can be used
261    /// outside a Linux PREEMPT_RT context.
262    ///
263    /// SCHED_FIFO and SCHED_RR allow priorities between the min and max inclusive.
264    ///
265    /// SCHED_DEADLINE cannot be set with this function, `libc::sched_setattr` must be used instead.
266    pub fn set_scheduler(pid: Pid, scheduler: Scheduler, param: SchedulerParams) -> RtResult<()> {
267        let param: libc::sched_param = param.into();
268        let res = unsafe { libc::sched_setscheduler(pid.into(), scheduler as c_int, &param) };
269
270        handle_errno(res).map(drop)
271    }
272
273    /// Get the schedule parameters (currently only priority) for a given thread.
274    /// Using `Pid::from_raw(0)` will return the parameters for the calling thread.
275    pub fn get_scheduler_params(pid: Pid) -> RtResult<SchedulerParams> {
276        let mut param: MaybeUninit<libc::sched_param> = MaybeUninit::uninit();
277        let res = unsafe { libc::sched_getparam(pid.into(), param.as_mut_ptr()) };
278
279        handle_errno(res).map(|_| unsafe { param.assume_init() }.into())
280    }
281    /// Set the schedule parameters (currently only priority) for a given thread.
282    /// Using `Pid::from_raw(0)` will return the parameters for the calling thread.
283    ///
284    /// Changing the priority to something other than `0` requires using a SCHED_FIFO or SCHED_RR
285    /// and using a Linux kernel with PREEMPT_RT enabled.
286    pub fn set_scheduler_params(pid: Pid, param: SchedulerParams) -> RtResult<()> {
287        let param: libc::sched_param = param.into();
288        let res = unsafe { libc::sched_setparam(pid.into(), &param) };
289        handle_errno(res).map(drop)
290    }
291}
292
293#[cfg(target_os = "linux")]
294pub use linux::*;
preempt_rt/sched.rs

preempt_rt/
sched.rs
