Re: sched_{set,get}attr() manpage

[Michael Kerrisk (man-pages)]

Hi Peter,

I'm working on this text. I see the following in kernel/sched/core.c:

[[
static int __sched_setscheduler(struct task_struct *p,
                                const struct sched_attr *attr,
                                bool user)
{
        ...

        int policy = attr->sched_policy;
        ...
        if (policy < 0) {
                reset_on_fork = p->sched_reset_on_fork;
                policy = oldpolicy = p->policy;
]]

What's a negative policy about? Is this something that should 
be documented?

Cheers,

Michael

On 05/06/2014 10:16 AM, Peter Zijlstra wrote:
> On Mon, May 05, 2014 at 09:21:14AM +0200, Peter Zijlstra wrote:
>> On Mon, May 05, 2014 at 08:55:28AM +0200, Michael Kerrisk (man-pages) wrote:
>>> Hi Peter,
>>>
>>> Looks like a good set of comments from Juri. Could you revise and 
>>> resubmit?
>>
>> Yeah, I'll try and get it done today, but there's a few icky bugs
>> waiting for my attention as well, I'll do me bestest :-)
> 
> OK, not quite managed it yesterday, but here goes.
> 
> So Verbatim license, for the first part to me and whoever I borrowed
> sched_setscheduler() bits from.
> 
> For the second part to me and Juri.
> 
> ---
> 
>> [1] A page describing the sched_setattr() and sched_getattr() APIs
> 
> NAME
> 	sched_setattr, sched_getattr - set and get scheduling policy/attributes
> 
> SYNOPSIS
> 	#include <sched.h>
> 
> 	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;
> 	};
> 
> 	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);
> 
> DESCRIPTION
> 	sched_setattr() sets both the scheduling policy and the
> 	associated attributes for the process whose ID is specified in
> 	pid.
> 
> 	sched_setattr() replaces sched_setscheduler(), sched_setparam(),
> 	nice() and some of setpriority().
> 
> 	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 static priority first-in, first-out policy;
> 
> 	SCHED_RR	a static priority round-robin policy; and
> 
> 	SCHED_DEADLINE	a dynamic priority deadline policy.
> 
> 	The semantics of each of these policies are detailed in
> 	sched(7).
> 
> 	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 sched(7).
> 
> 	sched_attr::sched_priority should only be set for SCHED_FIFO,
> 	SCHED_RR, the desired static priority [1,99], see sched(7).
> 
> 	sched_attr::sched_runtime in nanoseconds,
> 	sched_attr::sched_deadline in nanoseconds,
> 	sched_attr::sched_period in nanoseconds, should only be set for
> 	SCHED_DEADLINE and are the traditional sporadic task model
> 	parameters, see sched(7).
> 
> 	The flags argument should be 0.
> 
> 	sched_getattr() queries the scheduling policy currently applied
> 	to the process identified by pid.
> 
> 	Similar to sched_setattr(), sched_getattr() replaces
> 	sched_getscheduler(), sched_getparam() and some of
> 	getpriority().
> 
> 	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().
> 
> RETURN VALUE
> 	On success, sched_setattr() and sched_getattr() return 0. On
> 	error, -1 is returned, and errno is set appropriately.
> 
> ERRORS
>        EINVAL The scheduling policy is not one  of  the  recognized  policies,
>               param is NULL, or param does not make sense for the selected
> 	      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, see sched(7).
> 
> NOTES
>        While the text above (and in sched_setscheduler(2)) talks about
>        processes, in actual fact these system calls are thread specific.
> 
>        While the SCHED_DEADLINE parameters are in nanoseconds, current
>        kernels truncate the lower 10 bits and we get an effective
>        microsecond resolution.
> 
>> [2] A piece of text describing the SCHED_DEADLINE policy, which I can
>> drop into sched(7).
> 
>     SCHED_DEADLINE: Sporadic task model deadline scheduling
>        SCHED_DEADLINE is currently implemented using GEDF (Global
>        Earliest Deadline First) with additional CBS (Constant Bandwidth
>        Server).
> 
>        A sporadic task is one that has a sequence of jobs, where each
>        job is activated at most once per period. Each job has also a
>        relative deadline, before which it should finish execution, and a
>        computation time, that is the time necessary for executing the
>        job without interruption. The instant of time when a task wakes
>        up, because a new job has to be executed, is called arrival time
>        (and it is also referred to as request time or release time).
>        Start time is instead the time at which a task starts its
>        execution. The absolute deadline is thus obtained adding the
>        relative deadline to the arrival time.
> 
>        The following diagram clarifies these terms:
> 
>                arrival/wakeup           absolute deadline
>                |        start time          |
>                v        v                   v
>         -------x--------xoooooooooooo-------x--------x-----
>                         |<- comp. ->|
>                |<---------- rel. deadline ->|
>                |<---------- period ----------------->|
> 
>        SCHED_DEADLINE allows the user to specify three parameters (see
>        sched_setattr(2)): Runtime [ns], Deadline [ns] and Period [ns].
>        Such parameters has not necessarily to correspond to the
>        aforementioned terms, while usual practise is to set Runtime to
>        something bigger than the average computation time (or worst-case
>        execution time for hard real-time tasks), Deadline to the
>        relative deadline and Period to the period of the task. With such
>        a setting we would have:
> 
>                arrival/wakeup           absolute deadline
>                |        start time          |
>                v        v                   v
>         -------x--------xoooooooooooo-------x--------x-----
>                         |<- Runtime -->|
>                |<---------- Deadline ------>|
>                |<---------- Period ----------------->|
> 
>        It is checked that: Runtime <= Deadline <= Period.
> 
>        The CBS guarantees non-interference between tasks, by throttling
>        tasks that attempt to over-run their specified Runtime.
> 
>        In general the set of all SCHED_DEADLINE tasks is not
>        feasible/schedulable within the given constraints. To guarantee
>        some degree of timeliness we must do an admittance test on
>        setting/changing SCHED_DEADLINE policy/attributes.
> 
>        This admission test calculates that the task set is
>        feasible/schedulable, failing this, sched_setattr() will return
>        -EBUSY.
> 
>        For example, it is required (but not necessarily sufficient) for
>        the total utilization to be less or equal to the total amount of
>        CPUs available, where, since each task can maximally run for
>        Runtime per Period, that task's utilization is its
>        Runtime/Period.
> 
>        Because we must be able to calculate admittance SCHED_DEADLINE
>        tasks are the highest priority (user controllable) tasks in the
>        system, if any SCHED_DEADLINE task is runnable it will preempt
>        any FIFO/RR/OTHER/BATCH/IDLE task.
> 
>        SCHED_DEADLINE tasks will fail fork(2) with -EAGAIN, except when
>        the forking task has SCHED_FLAG_RESET_ON_FORK set.
> 
>        A SCHED_DEADLINE task calling sched_yield() will 'yield' the
>        current job and wait for a new period to begin.
> 


-- 
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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