Support the deadline CPUSchedulingPolicy

[bristot]

systemd supports setting the scheduling policy and priority of a service via the directives:

CPUSchedulingPolicy=fifo
CPUSchedulingPriority=10

This works fine! But as SCHED_DEADLINE is also supported on Linux, it would be nice to have the ability to set the deadline policy and its parameters.

The deadline policy is set using the sched_setattr() (Linux specific) system call:

int sched_setattr(pid_t pid, struct sched_attr *attr, unsigned int flags);

The first argument is the pid of the thread and the last is for future usage (always 0 now). The scheduling information is set in the second argument, the struct sched_attr.

The sched_attr struct is defined as:

       struct sched_attr {
           u32 size;              /* Size of this structure */
           u32 sched_policy;      /* Policy (SCHED_*) */
           u64 sched_flags;       /* Flags */
           s32 sched_nice;        /* Nice value (SCHED_OTHER,
                                     SCHED_BATCH) */
           u32 sched_priority;    /* Static priority (SCHED_FIFO,
                                     SCHED_RR) */
           /* Remaining fields are for SCHED_DEADLINE */
           u64 sched_runtime;
           u64 sched_deadline;
           u64 sched_period;
       };

The fields of this structure are as follows:

• The size field should be set to the size of the structure in bytes, as in sizeof(struct sched_attr).

• The sched_policy specifies the scheduling policy, sched deadline is SCHED_DEADLINE

• The sched_flags contains flags controlling scheduling behavior, for example, RESET_ON_FORK and SCHED_FLAG_RECLAIM. The last is specific for the deadline scheduler.

• sched_nice sched_priority should be zero for sched deadline.

The main fields for the sched deadline are (from man page):

• sched_runtime: Specifies the "Runtime" in nanoseconds

• sched_deadline: Specifies the "Deadline" in nanoseconds.

• sched_period: Specifies the "Period" parameter in nanoseconds.

The runtime/deadline/and period should be at least 1024 (nanosconds) and must be: runtime <= deadline <=period.

These articles might be useful:

Deadline scheduling part 1 — overview and theory: https://lwn.net/Articles/743740/
Deadline scheduler part 2 — details and usage: https://lwn.net/Articles/743946/

Feel free to ping me to make questions about the parameters and to test a possible implementation (unfortunately, I don't have time to implement this now).

[kakra]

Then please also support SCHED_ISO.

[SudoNova]

Is there any progress/milestone on this issue?

[oakwhiz]

This would be a helpful feature. It seems logical to have the option to set up SCHED_DEADLINE inside systemd rather than inside the applications that use SCHED_DEADLINE.