linux-kernel - Re: [RFC 8/12][PATCH] SCHED_DEADLINE: wait next instance syscall added.

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Date:	Mon, 28 Dec 2009 15:30:01 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Raistlin <raistlin@...ux.it>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	michael trimarchi <michael@...dence.eu.com>,
	Fabio Checconi <fabio@...dalf.sssup.it>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dhaval Giani <dhaval.giani@...il.com>,
	Johan Eker <johan.eker@...csson.com>,
	"p.faure" <p.faure@...tech.ch>,
	Chris Friesen <cfriesen@...tel.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Henrik Austad <henrik@...tad.us>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Darren Hart <darren@...art.com>,
	Sven-Thorsten Dietrich <sven@...bigcorporation.com>,
	Bjoern Brandenburg <bbb@...unc.edu>,
	Tommaso Cucinotta <tommaso.cucinotta@...up.it>,
	"giuseppe.lipari" <giuseppe.lipari@...up.it>,
	Juri Lelli <juri.lelli@...il.com>
Subject: Re: [RFC 8/12][PATCH] SCHED_DEADLINE: wait next instance syscall
 added.

On Fri, 2009-10-16 at 17:44 +0200, Raistlin wrote:
> This commit introduces another new SCHED_DEADLINE related syscall. It is
> called sched_wait_interval() and it has close-to-clock_nanosleep semantic.
> 
> However, for SCHED_DEADLINE tasks, it should be the call with which each
> job closes its current instance. In fact, in this case, the task is put to
> sleep and, when it wakes up, the scheduler is informed that a new job
> arrived, saving the overhead that usually comes with a task activation
> to enforce maximum task bandwidth.

The changelog suggests (and a very brief looks seems to confirm) that
this code could be much smaller by using hrtimer_nanosleep().

The implementation as presented seems to only call ->wait_interval()
when the timer arms, which seems like a bug, we should always call it,
regardless of whether we're on a period boundary.

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