Hi Peter,
Following the review comments that one or two people sent, are you
planning to send in a revised version of this page? Also, is there any
test code lying about somewhere that I could play with?
Thanks,
Michael
On Wed, Apr 9, 2014 at 5:42 PM, Peter Zijlstra <pet...@infradead.org> wrote:
> 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.2.02.1403312333100.14...@ionos.tec.linutronix.de
>>
>> 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.
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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