Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933757AbaD3NJt (ORCPT ); Wed, 30 Apr 2014 09:09:49 -0400 Received: from casper.infradead.org ([85.118.1.10]:35756 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933195AbaD3NJr (ORCPT ); Wed, 30 Apr 2014 09:09:47 -0400 Date: Wed, 30 Apr 2014 15:09:37 +0200 From: Peter Zijlstra To: "Michael Kerrisk (man-pages)" Cc: Dario Faggioli , Thomas Gleixner , Ingo Molnar , rostedt@goodmis.org, Oleg Nesterov , fweisbec@gmail.com, darren@dvhart.com, johan.eker@ericsson.com, p.faure@akatech.ch, Linux Kernel , claudio@evidence.eu.com, michael@amarulasolutions.com, fchecconi@gmail.com, tommaso.cucinotta@sssup.it, juri.lelli@gmail.com, nicola.manica@disi.unitn.it, luca.abeni@unitn.it, dhaval.giani@gmail.com, hgu1972@gmail.com, Paul McKenney , insop.song@gmail.com, liming.wang@windriver.com, jkacur@redhat.com, linux-man@vger.kernel.org Subject: Re: sched_{set,get}attr() manpage Message-ID: <20140430130937.GH30445@twins.programming.kicks-ass.net> References: <20131217123352.692059839@infradead.org> <20140121153851.GZ31570@twins.programming.kicks-ass.net> <20140214161929.GL27965@twins.programming.kicks-ass.net> <53020C9D.1050208@gmail.com> <20140428081858.GX13658@twins.programming.kicks-ass.net> <535FA467.2070403@gmail.com> <20140429160441.GU11096@twins.programming.kicks-ass.net> <5360D9E5.9080206@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5360D9E5.9080206@gmail.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 30, 2014 at 01:09:25PM +0200, Michael Kerrisk (man-pages) wrote: > Hi Peter, > > Thanks for the revision. More comments below. Could you revise in > the light of those comments, and hopefully also after feedback from > Juri and Dario? New text below; hopefully a little clearer. If not, do holler. --- > [1] A page describing the sched_setattr() and sched_getattr() APIs NAME sched_setattr, sched_getattr - set and get scheduling policy/attributes SYNOPSIS #include 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 on that has a sequence of jobs, where each job is activated at most once per period [ns]. Each job will have an absolute deadline relative to its activation before which it must finish its execution, and it shall at no time run longer than runtime [ns] after its release. activation/wakeup absolute deadline | release | v v v -------x--------x--------------x--------x------- |<- Runtime -->| |<---------- Deadline ->| |<---------- Period ----------->| This gives: runtime <= (rel) 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. Therefore 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 [us] per period [us], 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/