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SubjectRe: sched_{set,get}attr() manpage
Hi Peter,

On 04/28/2014 10:18 AM, Peter Zijlstra wrote:
> Hi Michael,
> find below an updated manpage, I did not apply the comments on parts
> that are identical to SCHED_SETSCHEDULER(2) in order to keep these texts
> in alignment. I feel that if we change one we should also change the
> other, and such a 'patch' is best done separate from the new manpage
> itself.
> I did add the missing EBUSY error, and amended the text where it said
> we'd return EINVAL in that case.
> I added a paragraph stating that SCHED_DEADLINE preempted anything else
> userspace can do (with the explicit mention of userspace to leave me
> wriggle room for the kernel's stop task :-).
> I also did a short paragraph on the deadline sched_yield(). For further
> deadline yield details we should maybe add to the SCHED_YIELD(2)
> manpage.
> Re juri/claudio; no I think sched_yield() as implemented for deadline
> makes sense, no other yield semantics other than NOP makes sense for it,
> and since we have the syscall already might as well make it do something
> useful.

Thanks for the updated page. Would you be willing
to revise as per the comments below.

> sched_setattr, sched_getattr - set and get scheduling policy/attributes
> #include <sched.h>
> struct sched_attr {
> u32 size;
> u32 sched_policy;
> u64 sched_flags;
> s32 sched_nice;
> u32 sched_priority;
> u64 sched_runtime;
> u64 sched_deadline;
> u64 sched_period;
> };
> int sched_setattr(pid_t pid, const struct sched_attr *attr, unsigned int flags);
> int sched_getattr(pid_t pid, const struct sched_attr *attr, unsigned int size, unsigned int flags);
> sched_setattr() sets both the scheduling policy and the
> associated attributes for the process whose ID is specified in
> pid.

Around about here, I think there needs to be a sentence explaining
that sched_setattr() provides a superset of the functionality of
sched_setscheduler(2) and setpritority(2). I mean, it can do all that
those two calls can do, right?

> If pid equals zero, the scheduling policy and attributes
> of the calling process will be set. The interpretation of the
> argument attr depends on the selected policy. Currently, Linux
> supports the following "normal" (i.e., non-real-time) scheduling
> policies:
> SCHED_OTHER the standard "fair" time-sharing policy;
> SCHED_BATCH for "batch" style execution of processes; and
> SCHED_IDLE for running very low priority background jobs.
> The following "real-time" policies are also supported, for
> special time-critical applications that need precise control
> over the way in which runnable processes are selected for
> execution:
> SCHED_FIFO a first-in, first-out policy;
> SCHED_RR a round-robin policy; and
> SCHED_DEADLINE a deadline policy.
> The semantics of each of these policies are detailed below.

The semantics of each of these policies are detailed in sched(7).

[See my comments below]

> sched_attr::size must be set to the size of the structure, as in
> sizeof(struct sched_attr), if the provided structure is smaller
> than the kernel structure, any additional fields are assumed
> '0'. If the provided structure is larger than the kernel
> structure, the kernel verifies all additional fields are '0' if
> not the syscall will fail with -E2BIG.
> sched_attr::sched_policy the desired scheduling policy.
> sched_attr::sched_flags additional flags that can influence
> scheduling behaviour. Currently as per Linux kernel 3.14:
> SCHED_FLAG_RESET_ON_FORK - resets the scheduling policy
> to: (struct sched_attr){ .sched_policy = SCHED_OTHER, }
> on fork().
> is the only supported flag.
> sched_attr::sched_nice should only be set for SCHED_OTHER,
> SCHED_BATCH, the desired nice value [-20,19], see NICE(2).
> sched_attr::sched_priority should only be set for SCHED_FIFO,
> SCHED_RR, the desired static priority [1,99].
> sched_attr::sched_runtime
> sched_attr::sched_deadline
> sched_attr::sched_period should only be set for SCHED_DEADLINE
> and are the traditional sporadic task model parameters.

Could you add (a lot ;-)) more detail on these three fields? Assume the
reader does not know about this traditional sporadic task model, and
then give some explanation of what these three fields do. Probably, at
this point you can work in some statement about the admission control

[but, see my comment below. It may be that sched(7) is a better
place for this detail.

> The flags argument should be 0.
> sched_getattr() queries the scheduling policy currently applied
> to the process identified by pid. If pid equals zero, the
> policy of the calling process will be retrieved.
> The size argument should reflect the size of struct sched_attr
> as known to userspace. The kernel fills out sched_attr::size to
> the size of its sched_attr structure. If the user provided
> structure is larger, additional fields are not touched. If the
> user provided structure is smaller, but the kernel needs to
> return values outside the provided space, the syscall will fail
> with -E2BIG.
> The flags argument should be 0.
> The other sched_attr fields are filled out as described in
> sched_setattr().

I assume that everything between my [[[ and ]]] blocks below is taken straight
from sched_setscheduler(2). (If that is not true, please let me know.)
This reminds me that there is a structural fault in this part of man-pages ;-).
The problem is sched_setscheduler(2) currently tries to do two things:

[a] Document the sched_setscheduler() and sched_scheduler system calls
[b] Provide and overview od scheduling policies and parameters.

It should really only do the former. I have now gone through the task of
separating [b] out into a separate page, sched(7), which other pages,
such as sched_setscheduler(2) and sched_setattr(2) can refer to. You
can see the current versions of sched_setscheduelr.2 and sched.7 in Git
( )

So, what I would ideally like to see

[1] A page describing the sched_setattr() and sched_getattr() APIs
[2] A piece of text describing the SCHED_DEADLINE policy, which I can
drop into sched(7).

Could you revise like that?

> Scheduling Policies
> The scheduler is the kernel component that decides which runnable
> process will be executed by the CPU next. Each process has an associ‐
> ated scheduling policy and a static scheduling priority, sched_prior‐
> ity; these are the settings that are modified by sched_setscheduler().
> The scheduler makes it decisions based on knowledge of the scheduling
> policy and static priority of all processes on the system.
> For processes scheduled under one of the normal scheduling policies
> (SCHED_OTHER, SCHED_IDLE, SCHED_BATCH), sched_priority is not used in
> scheduling decisions (it must be specified as 0).
> Processes scheduled under one of the real-time policies (SCHED_FIFO,
> SCHED_RR) have a sched_priority value in the range 1 (low) to 99
> (high). (As the numbers imply, real-time processes always have higher
> priority than normal processes.) Note well: POSIX.1-2001 only requires
> an implementation to support a minimum 32 distinct priority levels for
> the real-time policies, and some systems supply just this minimum.
> Portable programs should use sched_get_priority_min(2) and
> sched_get_priority_max(2) to find the range of priorities supported for
> a particular policy.
> Conceptually, the scheduler maintains a list of runnable processes for
> each possible sched_priority value. In order to determine which
> process runs next, the scheduler looks for the nonempty list with the
> highest static priority and selects the process at the head of this
> list.
> A process's scheduling policy determines where it will be inserted into
> the list of processes with equal static priority and how it will move
> inside this list.
> All scheduling is preemptive: if a process with a higher static prior‐
> ity becomes ready to run, the currently running process will be pre‐
> empted and returned to the wait list for its static priority level.
> The scheduling policy only determines the ordering within the list of
> runnable processes with equal static priority.

> SCHED_DEADLINE: Sporadic task model deadline scheduling
> SCHED_DEADLINE is an implementation of GEDF (Global Earliest
> Deadline First) with additional CBS (Constant Bandwidth Server).
> The CBS guarantees that tasks that over-run their specified
> budget are throttled and do not affect the correct performance
> of other SCHED_DEADLINE tasks.
> SCHED_DEADLINE tasks will fail FORK(2) with -EAGAIN
> Setting SCHED_DEADLINE can fail with -EBUSY when admission
> control tests fail.
> Because of the nature of (G)EDF, SCHED_DEADLINE tasks are the
> highest priority (user controllable) tasks in the system, if any
> SCHED_DEADLINE task is runnable it will preempt anything
> FIFO/RR/OTHER/BATCH/IDLE task out there.
> A SCHED_DEADLINE task calling sched_yield() will 'yield' the
> current job and wait for a new period to begin.

This is the piece that could go into sched(7), but I'd like it to include
a discussion of deadline, period, and runtime.


> SCHED_FIFO: First In-First Out scheduling
> SCHED_FIFO can only be used with static priorities higher than 0, which
> means that when a SCHED_FIFO processes becomes runnable, it will always
> immediately preempt any currently running SCHED_OTHER, SCHED_BATCH, or
> SCHED_IDLE process. SCHED_FIFO is a simple scheduling algorithm with‐
> out time slicing. For processes scheduled under the SCHED_FIFO policy,
> the following rules apply:
> * A SCHED_FIFO process that has been preempted by another process of
> higher priority will stay at the head of the list for its priority
> and will resume execution as soon as all processes of higher prior‐
> ity are blocked again.
> * When a SCHED_FIFO process becomes runnable, it will be inserted at
> the end of the list for its priority.
> * A call to sched_setscheduler() or sched_setparam(2) will put the
> SCHED_FIFO (or SCHED_RR) process identified by pid at the start of
> the list if it was runnable. As a consequence, it may preempt the
> currently running process if it has the same priority.
> (POSIX.1-2001 specifies that the process should go to the end of the
> list.)
> * A process calling sched_yield(2) will be put at the end of the list.
> No other events will move a process scheduled under the SCHED_FIFO pol‐
> icy in the wait list of runnable processes with equal static priority.
> A SCHED_FIFO process runs until either it is blocked by an I/O request,
> it is preempted by a higher priority process, or it calls
> sched_yield(2).
> SCHED_RR: Round Robin scheduling
> SCHED_RR is a simple enhancement of SCHED_FIFO. Everything described
> above for SCHED_FIFO also applies to SCHED_RR, except that each process
> is only allowed to run for a maximum time quantum. If a SCHED_RR
> process has been running for a time period equal to or longer than the
> time quantum, it will be put at the end of the list for its priority.
> A SCHED_RR process that has been preempted by a higher priority process
> and subsequently resumes execution as a running process will complete
> the unexpired portion of its round robin time quantum. The length of
> the time quantum can be retrieved using sched_rr_get_interval(2).
> SCHED_OTHER: Default Linux time-sharing scheduling
> SCHED_OTHER can only be used at static priority 0. SCHED_OTHER is the
> standard Linux time-sharing scheduler that is intended for all pro‐
> cesses that do not require the special real-time mechanisms. The
> process to run is chosen from the static priority 0 list based on a
> dynamic priority that is determined only inside this list. The dynamic
> priority is based on the nice value (set by nice(2) or setpriority(2))
> and increased for each time quantum the process is ready to run, but
> denied to run by the scheduler. This ensures fair progress among all
> SCHED_OTHER processes.
> SCHED_BATCH: Scheduling batch processes
> (Since Linux 2.6.16.) SCHED_BATCH can only be used at static priority
> 0. This policy is similar to SCHED_OTHER in that it schedules the
> process according to its dynamic priority (based on the nice value).
> The difference is that this policy will cause the scheduler to always
> assume that the process is CPU-intensive. Consequently, the scheduler
> will apply a small scheduling penalty with respect to wakeup behaviour,
> so that this process is mildly disfavored in scheduling decisions.
> This policy is useful for workloads that are noninteractive, but do not
> want to lower their nice value, and for workloads that want a determin‐
> istic scheduling policy without interactivity causing extra preemptions
> (between the workload's tasks).
> SCHED_IDLE: Scheduling very low priority jobs
> (Since Linux 2.6.23.) SCHED_IDLE can only be used at static priority
> 0; the process nice value has no influence for this policy.
> This policy is intended for running jobs at extremely low priority
> (lower even than a +19 nice value with the SCHED_OTHER or SCHED_BATCH
> policies).

> On success, sched_setattr() and sched_getattr() return 0. On
> error, -1 is returned, and errno is set appropriately.
> EINVAL The scheduling policy is not one of the recognized policies,
> param is NULL, or param does not make sense for the policy.
> EPERM The calling process does not have appropriate privileges.
> ESRCH The process whose ID is pid could not be found.
> E2BIG The provided storage for struct sched_attr is either too
> big, see sched_setattr(), or too small, see sched_getattr().
> EBUSY SCHED_DEADLINE admission control failure

The above is the only place on the page that mentions admission control.
As well as the suggestions above, it would be nice to have somewhere a
summary of how admission control is calculated.

> While the text above (and in SCHED_SETSCHEDULER(2)) talks about
> processes, in actual fact these system calls are thread specific.



Michael Kerrisk
Linux man-pages maintainer;
Linux/UNIX System Programming Training:
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