From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753957Ab0J2Gpu (ORCPT ); Fri, 29 Oct 2010 02:45:50 -0400 Received: from rt-pi1-ru-sssup.pi1.garr.net ([193.206.136.46]:26584 "EHLO sssup.it" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1754323Ab0J2Gpp (ORCPT ); Fri, 29 Oct 2010 02:45:45 -0400 Subject: [RFC][PATCH 22/22] sched: add sched_dl documentation From: Raistlin To: Peter Zijlstra Cc: Ingo Molnar , Thomas Gleixner , Steven Rostedt , Chris Friesen , oleg@redhat.com, Frederic Weisbecker , Darren Hart , Johan Eker , "p.faure" , linux-kernel , Claudio Scordino , michael trimarchi , Fabio Checconi , Tommaso Cucinotta , Juri Lelli , Nicola Manica , Luca Abeni , Dhaval Giani , Harald Gustafsson , paulmck In-Reply-To: <1288333128.8661.137.camel@Palantir> References: <1288333128.8661.137.camel@Palantir> Content-Type: multipart/signed; micalg="pgp-sha1"; protocol="application/pgp-signature"; boundary="=-BsKmMhJledqtAduNlEgG" Date: Fri, 29 Oct 2010 08:45:34 +0200 Message-ID: <1288334734.8661.164.camel@Palantir> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --=-BsKmMhJledqtAduNlEgG Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Add in Documentation/scheduler/ some hints about the design choices, the usage and the future possible developments of the sched_dl scheduling class and of the SCHED_DEADLINE policy. Signed-off-by: Dario Faggioli --- Documentation/scheduler/sched-deadline.txt | 147 ++++++++++++++++++++++++= ++++ 1 files changed, 147 insertions(+), 0 deletions(-) create mode 100644 Documentation/scheduler/sched-deadline.txt diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/sch= eduler/sched-deadline.txt new file mode 100644 index 0000000..e795968 --- /dev/null +++ b/Documentation/scheduler/sched-deadline.txt @@ -0,0 +1,147 @@ + Deadline Task and Group Scheduling + ---------------------------------- + +CONTENTS +=3D=3D=3D=3D=3D=3D=3D=3D + +0. WARNING +1. Overview +2. Task scheduling +2. The interface +3. Bandwidth management + 3.1 System wide settings + 2.2 Task interface + 2.4 Default behavior +3. Future plans + + +0. WARNING +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + + Fiddling with these settings can result in an unpredictable or even unsta= ble + system behavior. As for -rt (group) scheduling, it is assumed that root + knows what he is doing. + + +1. Overview +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + + The SCHED_DEADLINE policy contained inside the sched_dl scheduling class = is + basically an implementation of the Earliest Deadline First (EDF) scheduli= ng + algorithm, augmented with a mechanism (called Constant Bandwidth Server, = CBS) + that make it possible to isolate the behaviour of tasks between each othe= r. + + +2. Task scheduling +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + + The typical -deadline task will be made up of a computation phase (instan= ce) + which is activated on a periodic or sporadic fashion. The expected (maxim= um) + duration of such computation is called the task's runtime; the time inter= val + by which each instance need to be completed is called the task's relative + deadline. The task's absolute deadline is dynamically calculated as the + time instant a task (better, an instance) activates plus the relative + deadline. + + The EDF algorithms selects the task with the smallest absolute deadline a= s + the one to be executed first, while the CBS ensures each task to run for + at most the its runtime every (relative) deadline length time interval, + avoiding any interference between different tasks (bandwidth isolation). + Thanks to this feature, also tasks that do not strictly comply with the + computational model sketched above can effectively use the new policy. + IOW, there are no limitations on what kind of task can exploit this new + scheduling discipline, even if it must be said that it is particularly + suited for periodic or sporadic tasks that need guarantees on their + timing behaviour, e.g., multimedia, streaming, control applications, etc. + + +3. Bandwidth management +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + + In order of -deaadline scheduling to be effective and useful, it is impor= tant + that some method of having the allocation of the available CPU bandwidth = to + the tasks under control. + This is usually called "admission control" and if it is not performed at = all, + no guarantee can be given on the actual scheduling of the -deadline tasks= . + + Since when RT-throttling has been introduced each task group have a bandw= idth + associated to itself, calculated as a certain amount of runtime over a pe= riod. + Moreover, to make it possible to manipulate such bandwidth, readable/writ= able + controls have been added to both procfs (for system wide settings) and cg= roupfs + (for per-group settings). + Therefore, the same interface is being used for controlling the bandwidth + distrubution to -deadline tasks and task groups, i.e., new controls but w= ith + similar names, equivalent meaning and with the same usage paradigm are ad= ded. + + However, more discussion is needed in order to figure out how we want to = manage + SCHED_DEADLINE bandwidth at the task group level. Therefore, SCHED_DEADLI= NE uses + (for now) a less sophisticated, but actually very sensible, machanism to = ensure + that a certain utilization cap is not overcome per each root_domain. + + Another main difference between deadline bandwidth management and RT-thro= ttling + is that -deadline tasks have bandwidth on their own (while -rt ones doesn= 't!), + and thus we don't need an higher level throttling mechanism to enforce th= e + desired bandwidth. + +3.1 System wide settings +------------------------ + +The system wide settings are configured under the /proc virtual file syste= m: + + The per-group controls that are added to the cgroupfs virtual file system= are: + * /proc/sys/kernel/sched_dl_runtime_us, + * /proc/sys/kernel/sched_dl_period_us, + + They accepts (if written) and provides (if read) the new runtime and peri= od, + respectively, for each CPU in each root_domain. + + This means that, for a root_domain comprising M CPUs, -deadline tasks + can be created until the sum of their bandwidths stay below: + + M * (sched_dl_runtime_us / sched_dl_period_us) + + It is also possible to disable this bandwidth management logic, and + be thus free of oversubscribing the system up to any arbitrary level. + This is done by writing -1 in /proc/sys/kernel/sched_dl_runtime_us. + + +2.2 Task interface +------------------ + + Specifying a periodic/sporadic task that executes for a given amount of + runtime at each instance, and that is scheduled according to the usrgency= of + their own timing constraints needs, in general, a way of declaring: + - a (maximum/typical) instance execution time, + - a minimum interval between consecutive instances, + - a time constraint by which each instance must be completed. + + Therefore: + * a new struct sched_param_ex, containing all the necessary fields is + provided; + * the new scheduling related syscalls that manipulate it, i.e., + sched_setscheduler_ex(), sched_setparam_ex() and sched_getparam_ex() + are implemented. + + +2.4 Default behavior +--------------------- + +The default values for SCHED_DEADLINE bandwidth is to have dl_runtime and +dl_period equal to 500000 and 1000000, respectively. This means -deadline +tasks can use at most 5%, multiplied by the number of CPUs that compose th= e +root_domain, for each root_domain. + +When a -deadline task fork a child, its dl_runtime is set to 0, which mean= s +someone must call sched_setscheduler_ex() on it, or it won't even start. + + +3. Future plans +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +Still Missing parts: + + - refinements in deadline inheritance, especially regarding the possibili= ty + of retaining bandwidth isolation among non-interacting tasks. This is + being studied from both theoretical and practical point of views, and + hopefully we can have some demonstrative code soon. + --=20 1.7.2.3 --=20 <> (Raistlin Majere) ---------------------------------------------------------------------- Dario Faggioli, ReTiS Lab, Scuola Superiore Sant'Anna, Pisa (Italy) http://blog.linux.it/raistlin / raistlin@ekiga.net / dario.faggioli@jabber.org --=-BsKmMhJledqtAduNlEgG Content-Type: application/pgp-signature; name="signature.asc" Content-Description: This is a digitally signed message part -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) iEYEABECAAYFAkzKbY4ACgkQk4XaBE3IOsSqkwCggxJcVWHMntHwNTwN56qaxdmY DtMAoIDH+VCgeQdI0HhMXFexTz66VIOz =nFKm -----END PGP SIGNATURE----- --=-BsKmMhJledqtAduNlEgG--