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Date: Wed, 03 Sep 2014 10:18:30 +0100 From: Juri Lelli <juri.lelli@....com> To: Luca Abeni <luca.abeni@...tn.it>, Henrik Austad <henrik@...tad.us> CC: "peterz@...radead.org" <peterz@...radead.org>, "rdunlap@...radead.org" <rdunlap@...radead.org>, "mingo@...hat.com" <mingo@...hat.com>, "raistlin@...ux.it" <raistlin@...ux.it>, "juri.lelli@...il.com" <juri.lelli@...il.com>, "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v3 3/4] Documentation/scheduler/sched-deadline.txt: improve and clarify AC bits Hi, On 03/09/14 07:49, Luca Abeni wrote: > Hi, > > On 09/02/2014 11:45 PM, Henrik Austad wrote: > [...] >>> + On multiprocessor systems with global EDF scheduling (non partitioned >>> + systems), a sufficient test for schedulability can not be based on the >>> + utilisations (it can be shown that task sets with utilisations slightly >>> + larger than 1 can miss deadlines regardless of the number of CPUs M). >>> + However, as previously stated, enforcing that the total utilisation is smaller >>> + than M is enough to guarantee that non real-time tasks are not starved and >>> + that the tardiness of real-time tasks has an upper bound. >> >> I'd _really_ appreciate a link to a paper where all of this is presented >> and proved! > Well, my original plan was to add the bibliography in the next round of patches... > Is this ok? > > [...] >>> + As already stated in Section 3, a necessary condition to be respected to >>> + correctly schedule a set of real-time tasks is that the total utilisation >>> + is smaller than M. When talking about -deadline tasks, this requires to >>> + impose that the sum of the ratio between runtime and period for all tasks >>> + is smaller than M. >> >> "This requires to impose that .." uhm, what? Drop 'to impose'. > Ok. I'll send an updated patch to Juri in few days > > >>> [...] Notice that the ratio runtime/period is equivalent to >>> + the utilisation of a "traditional" real-time task, and is also often >>> + referred to as "bandwidth". >>> + The interface used to control the CPU bandwidth that can be allocated >>> + to -deadline tasks is similar to the one already used for -rt >>> tasks with real-time group scheduling (a.k.a. RT-throttling - see >>> Documentation/scheduler/sched-rt-group.txt), and is based on readable/ >>> writable control files located in procfs (for system wide settings). >>> @@ -232,8 +285,16 @@ CONTENTS >>> 950000. With rt_period equal to 1000000, by default, it means that -deadline >>> tasks can use at most 95%, multiplied by the number of CPUs that compose the >>> root_domain, for each root_domain. >>> - >>> - A -deadline task cannot fork. >>> + This means that non -deadline tasks will receive at least 5% of the CPU time, >>> + and that -deadline tasks will receive their runtime with a guaranteed >>> + worst-case delay respect to the "deadline" parameter. If "deadline" = "period" >>> + and the cpuset mechanism is used to implement partitioned scheduling (see >>> + Section 5), then this simple setting of the bandwidth management is able to >>> + deterministically guarantee that -deadline tasks will receive their runtime >>> + in a period. >> >> The whole 950000 / 1000000, is at least 50 *consecutive* ms given to non >> rt/dl tasks every second, or is this more finegrained now? >> >> If the 50ms can be given in a single go, then I don't think you can >> guarantee that deadline-tasks will receive their runtime in a period - a >> period can be <50ms, no? > Uhmm... Maybe there is something I am missing in how the SCHED_DEADLINE admission > control is implemented, but I do not know about any "50 consecutive ms to non dl > tasks" rule. I agree that if there is such a rule then deadline tasks are screwed. > Juri? > > In SCHED_DEADLINE we use those values only at admission control time (when the user calls sched_setattr()). Then, at runtime, we use tasks' parameters to perform scheduling. So there is no consecutive 50ms time for !SCHED_DEADLINE tasks. We could probably clarify this aspect in the previous patch with something like this: [snip] + The interface used to control the fraction of CPU bandwidth that can be + allocated to -deadline tasks is similar to the one already used for -rt + tasks with real-time group scheduling (a.k.a. RT-throttling - see + Documentation/scheduler/sched-rt-group.txt), and is based on readable/ + writable control files located in procfs (for system wide settings). + Notice that per-group settings (controlled through cgroupfs) are still not + defined for -deadline tasks, because more discussion is needed in order to + figure out how we want to manage SCHED_DEADLINE bandwidth at the task group + level. + + A main difference between deadline bandwidth management and RT-throttling is that -deadline tasks have bandwidth on their own (while -rt ones don't!), - and thus we don't need an higher level throttling mechanism to enforce the - desired bandwidth. + and thus we don't need a higher level throttling mechanism to enforce the ----> + desired bandwidth. In other words, this means that interface parameters are + only used at admission control time (i.e., when the user calls + sched_setattr()). Scheduling is then performed considering actual tasks' + parameters, so that CPU bandwidth is allocated to SCHED_DEADLINE tasks + respecting their needs in terms of granularity. Therefore, using this simple <--- + interface we can put a cap on total utilization of -deadline tasks (i.e., + \Sum (runtime_i / period_i) < some_desired_value). [snip] What you think? Thanks, - Juri >>> + Finally, notice that in order not to jeopardize this admission control a >>> + -deadline task cannot fork. >> >> s/this/the >> (there aren't any other admission controls in the kernel) > Ok; this will go in my updated patch > > > > Thanks, > Luca > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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