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Date: Thu, 04 Sep 2014 09:57:51 +0100 From: Juri Lelli <juri.lelli@....com> To: Henrik Austad <henrik@...tad.us> CC: "peterz@...radead.org" <peterz@...radead.org>, "luca.abeni@...tn.it" <luca.abeni@...tn.it>, "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 2/4] Documentation/scheduler/sched-deadline.txt: Rewrite section 4 intro Hi Henrik, On 02/09/14 22:14, Henrik Austad wrote: > On Thu, Aug 28, 2014 at 11:00:27AM +0100, Juri Lelli wrote: >> Section 4 intro was still describing the old interface. Rewrite it. >> >> Signed-off-by: Juri Lelli <juri.lelli@....com> >> Signed-off-by: Luca Abeni <luca.abeni@...tn.it> >> Cc: Randy Dunlap <rdunlap@...radead.org> >> Cc: Peter Zijlstra <peterz@...radead.org> >> Cc: Ingo Molnar <mingo@...hat.com> >> Cc: Henrik Austad <henrik@...tad.us> >> Cc: Dario Faggioli <raistlin@...ux.it> >> Cc: Juri Lelli <juri.lelli@...il.com> >> Cc: linux-doc@...r.kernel.org >> Cc: linux-kernel@...r.kernel.org >> --- >> Documentation/scheduler/sched-deadline.txt | 51 +++++++++++++++--------------- >> 1 file changed, 25 insertions(+), 26 deletions(-) >> >> diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt >> index dce6d63..0aff2d5 100644 >> --- a/Documentation/scheduler/sched-deadline.txt >> +++ b/Documentation/scheduler/sched-deadline.txt >> @@ -165,39 +165,38 @@ CONTENTS >> >> In order for the -deadline scheduling to be effective and useful, it is >> important to have some method to keep 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 has a bandwidth >> - associated, calculated as a certain amount of runtime over a period. >> - Moreover, to make it possible to manipulate such bandwidth, readable/writable >> - controls have been added to both procfs (for system wide settings) and cgroupfs >> - (for per-group settings). >> - Therefore, the same interface is being used for controlling the bandwidth >> - distrubution to -deadline tasks. >> - >> - 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_DEADLINE >> - uses (for now) a less sophisticated, but actually very sensible, mechanism to >> - ensure that a certain utilization cap is not overcome per each root_domain. >> - >> - Another main difference between deadline bandwidth management and RT-throttling >> + 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. >> + >> + 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. 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). > > s/some_desired_value/global_dl_utilization_cap/ perhaps? > Ok, fixed. >> 4.1 System wide settings >> ------------------------ >> >> The system wide settings are configured under the /proc virtual file system. >> >> - For now the -rt knobs are used for dl admission control and the -deadline >> - runtime is accounted against the -rt runtime. We realise that this isn't >> - entirely desirable; however, it is better to have a small interface for now, >> - and be able to change it easily later. The ideal situation (see 5.) is to run >> - -rt tasks from a -deadline server; in which case the -rt bandwidth is a direct >> - subset of dl_bw. >> + For now the -rt knobs are used for -deadline admission control and the >> + -deadline runtime is accounted against the -rt runtime. We realise that this >> + isn't entirely desirable; however, it is better to have a small interface for >> + now, and be able to change it easily later. The ideal situation (see 5.) is to >> + run -rt tasks from a -deadline server; in which case the -rt bandwidth is a >> + direct subset of dl_bw. > > Isn't forking forbidden for _dl tasks? Or did I miss a memo somewhere? > Yes, forking is forbidden. But, that's mainly because it is not entirely clear how bandwidth has to be split between parent and child(ren). When we'll have -rt tasks running from a -deadline server this problem will disappear, as children will share bandwidth with other tasks in the same server. >> This means that, for a root_domain comprising M CPUs, -deadline tasks >> can be created while the sum of their bandwidths stays below: >> -- >> 2.0.4 >> >> > > All in all, looks pretty good to me! > Great! Thanks, - Juri -- 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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