On Wed, Apr 09, 2014 at 05:19:11PM +0200, Henrik Austad wrote:
> > The following "real-time" policies are also supported, for
>
> why the "'s?
I borrowed those from SCHED_SETSCHEDULER(2).
> > 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.
...
> > The flags argument should be 0.
>
> What about SCHED_FLAG_RESET_ON_FOR?
Different flags. The one is sched_attr::flags the other is
sched_setattr(.flags).
> > The other sched_attr fields are filled out as described in
> > sched_setattr().
> >
> > 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.
>
> Isn't this last sentence redundant/sliglhtly repetitive?
I borrowed that from SCHED_SETSCHEDULER(2) again.
> > 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 -EINVAL when admission
> > control tests fail.
>
> Perhaps add a note about the deadline-class having higher priority than the
> other classes; i.e. if a deadline-task is runnable, it will preempt any
> other SCHED_(RR|FIFO) regardless of priority?
Yes, good point, will do.
> > 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.
>
> How about the recent discussion regarding sched_yield(). Is this correct?
>
> lkml.kernel.org/r/alpine.DEB.2.02.1403312333100.14882@xxxxxxxxxxxxxxxxxxxxxxx
>
> Is this the correct place to add a note explaining te potentional pitfalls
> using sched_yield?
I'm not sure; there's a SCHED_YIELD(2) manpage to fill with that
nonsense.
Also; I realized I have not described the DEADLINE sched_yield()
behaviour.
> > 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).
>
> -> Default is 0.1HZ ms
>
> This is a question I get form time to time, having this in the manpage
> would be helpful.
Again, brazenly stolen from SCHED_SETSCHEDULER(2); but yes. Also I'm not
sure I'd call RR an enhancement of anything much at all ;-)
> > ERRORS
> > 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().
>
> Where's the EBUSY? It can throw this from __sched_setscheduler() when it
> checks if there's enough bandwidth to run the task.
Uhhm.. it got lost :-) /me quickly adds.
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