Using SCHED DEADLINE in Linux. Luca Abeni

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Archibald Campbell
6 years ago
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1 in Linux Luca Abeni

2 Using Fixed Priorities in Linux SCHED FIFO and SCHED RR use fixed priorities They can be used for real-time tasks, to implement RM and DM Real-time tasks have priority over non real-time (SCHED OTHER) tasks The difference between the two policies is visible when more tasks have the same priority In real-time applications, try to avoid multiple tasks with the same priority

3 Setting the Scheduling Policy int sched_get_priority_max(int policy); int sched_get_priority_min(int policy); int sched_setscheduler(pid_t pid, int policy, const struct sched_param *param); int sched_setparam(pid_t pid, const struct sched_param *param); Ifpid == 0, then the parameters of the running task are changed The only meaningful field ofstruct sched param issched priority See the code: rt utils.c and the examples: priority1.c andpriority2.c

4 Problems with Real-Time Priorities In general, regular (SCHED OTHER) tasks are scheduled in background respect to real-time ones Real-time tasks can / starve other applications Example: the following task scheduled at high priority can make a CPU / core unusable 1 void bad_bad_task() 2 { 3 while(1); 4 } Real-time computation have to be limited (use real-time priorities only when really needed!) Using real-time priorities requires root privileges (or part of them!)

5 Real-Time Throttling A bad rt task can make a CPU / core unusable......linux provides the real-time throttling mechanism to address this problem By limiting the execution time of real-time tasks per period. # echo -1 > /proc/sys/kernel/sched rt runtime us Very useful idea, but something more theoretically founded might be needed...

6 EDF in the Linux kernel EDF assigns dynamic priorities based on absolute deadlines So, a more advanced API for the scheduler is needed... Assign relative deadlines D i to the tasks... A runtime and a period are also needed Moreover, d i,j = r i,j +D i......however, how can the scheduler know r i,j? The scheduler is not aware of jobs...

7 Tasks and Jobs... EDF need to know when a job starts / finishes Applications must be modified to signal the beginning / end of a job (some kind of startjob() /endjob() system call)......or the scheduler can assume that a new job arrives each time a task wakes up! Or, some other algorithm can be used to assign dynamic scheduling deadlines to tasks

8 ... And Scheduling Deadlines! The scheduler does EDF on scheduling deadlines Scheduling deadline d s i: assigned by the kernel But the task cares about its absolute deadlines If the scheduling deadline d s i matches the absolute deadline d i,j of a job, then the scheduler can respect d i,j!!!

9 CBS: The Basic Idea Constant Bandwidth Server (CBS): algorithm to assign a dynamic scheduling deadline d s i to a task τ i Based on the Resource Reservation paradigm Task τ i is periodically reserved a maximum runtime Q i every reservation period P i Temporal isolation between tasks The worst case finishing time for a task does not depend on the other tasks......because the task is guaranteed to receive its reserved time Solves the issue with bad tasks trying to consume too much execution time

10 CBS: Some More Details Based on CPU reservations (Q i,p i ) If τ i tries to execute for more than Q i every P i, the algorithm decreases its priority, or throttles it τ i has the same CPU utilisation of a task with WCET Q i and period P i Q i /P i : fraction of CPU time reserved to τ i EDF on the scheduling deadlines τ i is guaranteed to receive Q i time units every P i if jq j /P j 1!!!

11 SCHED DEADLINE NewSCHED DEADLINE scheduling policy Foreground respect to all of the other policies Uses the CBS to assign scheduling deadline to SCHED DEADLINE tasks Assign a (maximum) runtime Q i and a (reservation) period P i to every SCHED DEADLINE task Additional parameter: relative deadline D i The check if the current scheduling deadline can be used rule is used at task wake-up...use EDF (based on scheduling deadlines) to schedule them

12 Linux provides a (non standard) API for using SCHED DEADLINE, but......how to dimension the scheduling parameters? (Maximum) runtime Q i (Reservation) period P i Obviously, it must be Q i M i P i The kernel can do this admission control Better to use a limit smaller than M (so that other tasks are not starved!) Setting the policy might fail if admission control fails

13 Setting the Scheduling Policy Nosched setsched() new syscalls (and data structures added to be extensible) int sched_setattr(pid_t pid, const struct sched_attr *attr, unsigned int flags); int sched_getattr(pid_t pid, struct sched_attr *attr, unsigned int size, unsigned int flags); struct sched_attr { u32 size; u32 sched_policy; u64 sched_flags; /* SCHED_NORMAL, SCHED_BATCH */ s32 sched_nice; /* SCHED_FIFO, SCHED_RR */ u32 sched_priority; /* SCHED_DEADLINE */ u64 sched_runtime; u64 sched_deadline; u64 sched_period; };

14 Usingsched setattr() pid: as forsched setscheduler() flags: currently unused (for future extensions!) attr: scheduling parameters for the task size: must be set tosizeof(struct sched attr) sched policy: set to SCHED DEADLINE! sched runtime: Q sched deadline: D sched period: P sched flags: will see later (set to 0 for now)

15 Example Typical use of SCHED DEADLINE in a program #include <sched.h>... struct sched_attr attr; attr.size = sizeof(struct attr); attr.sched_policy = SCHED_DEADLINE; attr.sched_runtime = ; attr.sched_period = ; attr.sched_deadline = attr.sched_period;... if (sched_setattr(gettid(), &attr, 0)) perror("sched_setattr()");... sched runtime, sched period and sched deadline defined in ns Try the code: resource reservation.c

16 libdl So, can we use SCHED DEADLINE in our user programs? sched setattr() & friends are in the kernel since But the user-space side of things is still missing in many Linux distributions No support in glibc, no definition ofstruct sched attr, etc... Solution: small user-space library providing the sched *attr() system calls and related data structures libdl, released by Juri Lelli under GPL

SCHED DEADLINE: a real-time CPU scheduler for Linux

SCHED DEADLINE: a real-time CPU scheduler for Linux : a real-time CPU scheduler for Luca Abeni luca.abeni@santannapisa.it TuToR 2017 Luca Abeni 1 / 33 Scheduling Real-Time Tasks c j f j c j+1 r d j j r f j+1 d j+1 j+1 Consider a set ofn real-time tasks