Re: [PATCH v3 2/4] Documentation/scheduler/sched-deadline.txt: Rewrite section 4 intro

[Juri Lelli <juri.lelli@arm.com>]

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@arm.com>
>> Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
>> Cc: Randy Dunlap <rdunlap@infradead.org>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Henrik Austad <henrik@austad.us>
>> Cc: Dario Faggioli <raistlin@linux.it>
>> Cc: Juri Lelli <juri.lelli@gmail.com>
>> Cc: linux-doc@vger.kernel.org
>> Cc: linux-kernel@vger.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