linux-kernel - Re: [RFC][PATCH 02/22] sched: add extended scheduling interface

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Message-ID: <4CDAC587.7090904@evidence.eu.com>
Date:	Wed, 10 Nov 2010 17:17:11 +0100
From:	Claudio Scordino <claudio@...dence.eu.com>
To:	Dhaval Giani <dhaval@...is.sssup.it>
CC:	Raistlin <raistlin@...ux.it>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Chris Friesen <cfriesen@...tel.com>, oleg@...hat.com,
	Frederic Weisbecker <fweisbec@...il.com>,
	Darren Hart <darren@...art.com>,
	Johan Eker <johan.eker@...csson.com>,
	"p.faure" <p.faure@...tech.ch>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	michael trimarchi <trimarchi@...is.sssup.it>,
	Fabio Checconi <fabio@...dalf.sssup.it>,
	Tommaso Cucinotta <cucinotta@...up.it>,
	Juri Lelli <juri.lelli@...il.com>,
	Nicola Manica <nicola.manica@...i.unitn.it>,
	Luca Abeni <luca.abeni@...tn.it>,
	Harald Gustafsson <hgu1972@...il.com>,
	paulmck <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC][PATCH 02/22] sched: add extended scheduling interface

Dhaval Giani ha scritto:
>> +/*
>> + * Extended scheduling parameters data structure.
>> + *
>> + * This is needed because the original struct sched_param can not be
>> + * altered without introducing ABI issues with legacy applications
>> + * (e.g., in sched_getparam()).
>> + *
>> + * However, the possibility of specifying more than just a priority for
>> + * the tasks may be useful for a wide variety of application fields, e.g.,
>> + * multimedia, streaming, automation and control, and many others.
>> + *
>> + * This variant (sched_param_ex) is meant at describing a so-called
>> + * sporadic time-constrained task. In such model a task is specified by:
>> + *  - the activation period or minimum instance inter-arrival time;
>> + *  - the maximum (or average, depending on the actual scheduling
>> + *    discipline) computation time of all instances, a.k.a. runtime;
>> + *  - the deadline (relative to the actual activation time) of each
>> + *    instance.
>> + * Very briefly, a periodic (sporadic) task asks for the execution of
>> + * some specific computation --which is typically called an instance--
>> + * (at most) every period. Moreover, each instance typically lasts no more
>> + * than the runtime and must be completed by time instant t equal to
>> + * the instance activation time + the deadline.
>> + *
>> + * This is reflected by the actual fields of the sched_param_ex structure:
>> + *
>> + *  @sched_priority     task's priority (might still be useful)
>> + *  @sched_deadline     representative of the task's deadline
>> + *  @sched_runtime      representative of the task's runtime
>> + *  @sched_period       representative of the task's period
>> + *  @sched_flags        for customizing the scheduler behaviour
>> + *
>> + * There are other fields, which may be useful for implementing (in
>> + * user-space) advanced scheduling behaviours, e.g., feedback scheduling:
>> + *
>> + *  @curr_runtime       task's currently available runtime
>> + *  @used_runtime       task's totally used runtime
>> + *  @curr_deadline      task's current absolute deadline
>> + *
>> + * Given this task model, there are a multiplicity of scheduling algorithms
>> + * and policies, that can be used to ensure all the tasks will make their
>> + * timing constraints.
>> + */
>> +struct sched_param_ex {
>> +	int sched_priority;
>> +	struct timespec sched_runtime;
>> +	struct timespec sched_deadline;
>> +	struct timespec sched_period;
>> +	unsigned int sched_flags;
>> +
>> +	struct timespec curr_runtime;
>> +	struct timespec used_runtime;
>> +	struct timespec curr_deadline;
>> +};
>> +
> 
> So, how extensible is this. What about when real time theory develops a
> new algorithm which actually works practically, but needs additional
> parameters? :-). (I am guessing that this interface should handle most
> of the algorithms used these days). Any ideas?

AFAIK, these parameters already address most real-time algorithms.

It could happen that in the future a new algorithm will need some
further parameter (like jitter). However, IMHO, this is too unlikely
to justify the addition of some padding in this data structure.

Best regards,

	Claudio
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