Message-ID: <4CDAC587.7090904@evidence.eu.com>
Date: Wed, 10 Nov 2010 17:17:11 +0100
From: Claudio Scordino <claudio@evidence.eu.com>
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To: Dhaval Giani <dhaval@retis.sssup.it>
CC: Raistlin <raistlin@linux.it>, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Chris Friesen <cfriesen@nortel.com>, oleg@redhat.com,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Darren Hart <darren@dvhart.com>, Johan Eker <johan.eker@ericsson.com>,
        "p.faure" <p.faure@akatech.ch>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        michael trimarchi <trimarchi@retis.sssup.it>,
        Fabio Checconi <fabio@gandalf.sssup.it>,
        Tommaso Cucinotta <cucinotta@sssup.it>,
        Juri Lelli <juri.lelli@gmail.com>,
        Nicola Manica <nicola.manica@disi.unitn.it>,
        Luca Abeni <luca.abeni@unitn.it>,
        Harald Gustafsson <hgu1972@gmail.com>,
        paulmck <paulmck@linux.vnet.ibm.com>
Subject: Re: [RFC][PATCH 02/22] sched: add extended scheduling interface
References: <1288333128.8661.137.camel@Palantir> <1288333622.8661.141.camel@Palantir> <20101110160004.GA9092@gondor.retis>
In-Reply-To: <20101110160004.GA9092@gondor.retis>
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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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