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Re: From cooperative to preemptive scheduler: a real example

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On 2020-01-10 1:32, pozz wrote:
> Il 09/01/2020 13:03, Niklas Holsti ha scritto:
>> On 2020-01-09 13:19, pozz wrote:
>>> Il 08/01/2020 23:54, Niklas Holsti ha scritto:
>>>> On 2020-01-08 1:02, pozz wrote:
>>>>> Il 07/01/2020 08:38, Niklas Holsti ha scritto:
>>
>>     [snip]
>>
>>>>>> Let's assume these requirements and properties of the environment:
>>>>>>
>>>>>> A. The function "serial_rx" polls the one-character reception 
>>>>>> buffer of the serial line once, and returns the received 
>>>>>> character, if any, and EOF otherwise. It must be called at least 
>>>>>> as often as characters arrive (that is, depending on baud rate) to 
>>>>>> avoid overrun and loss of some characters.
>>>>
>>>> You asked about possible advantages of pre-emption; I made my 
>>>> assumptions, above, such that the (incomplete) example you gave 
>>>> shows this advantage, under these assumptions (which could be true 
>>>> for other, otherwise similar example applications).
>>>>
>>>>> No, serial driver works in interrupt mode and already use a FIFO 
>>>>> buffer, sufficiently big. serial_rx() pop a single element from the 
>>>>> FIFO, if any.
>>>>
>>>> Ah, then your *system* is intrinsically pre-emptive (the interrupts 
>>>> pre-empt the tasks), even if the *code you showed* does not show 
>>>> this pre-emption.
>>>
>>> Ah yes, interrupts are preemptive and I use them a lot, but they are 
>>> confined in their works, they are very lightweight and fast.
>>
>> That's a design decision. Some systems do most of their work in 
>> interrupt handlers, and use "background" processing only for some 
>> non-critical housekeeping tasks.
>>
> 
> Yes, I imagine there are a pletora of possibilites. Anyway I thought 
> there was two typical approaches: superloop that continuously calls 
> non-blocking functions (an example of cooperative scheduler) and a full 
> preemptive scheduler (most of the time a full RTOS).

The approach where all real-time "tasks" are interrupts is called 
"foreground-background", I believe. It was popular in the microcomputer 
era, before real-time kernels became popular -- it gave most of the same 
benefits (pre-emption) but scheduled by HW (interrupt priorities).

> Interrupts are always present, even in superloop.

Again a design choice. Some critical systems don't want *any* 
pre-emption, even by interrupts -- simplicity reigns. So they poll, and 
accept the performance hit. For example, some colleagues of mine 
recently completed the "recovery SW" for ESA's ExoMars rover, which 
takes over if the "real" SW crashes and is intended to be super-safe and 
just support problem analysis and recovery. Using any kind of real-time 
kernel was forbidden -- only sequential coding was allowed. Any new, 
nice SW "feature" that my colleagues suggested, for example to speed up 
communication with Earth, was denied by the customer because it would 
complicate the SW a little bit -- perhaps one more conditional 
statement. No-no.

> Maybe I am wrong, but I implement ISRs with great care and attention,

Of course you are right to use great care and attention, and especially 
in ISRs.

> but they are very small and limited.

Many tasks in pre-emptive systems are also small and limited -- for 
example, one of my designs has a task whose main loop just takes a 
data-packet from an input queue, computes its CRC and appends it to the 
packet, and puts the updated packet in an output queue. A handful of 
source lines.

I would say that having a pre-emptive system relieves the designer of 
the great care and attention needed to slice computations into 
(state-mnachine-driven) "tasks" that execute their computations little 
by little.

> Normally the biggest part of the firmware is related to the 
> application/tasks, not interrupts.

Except in the aforementioned foreground-background designs.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .

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From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-07 02:08 +0100
  Re: From cooperative to preemptive scheduler: a real example Don Y <blockedofcourse@foo.invalid> - 2020-01-06 19:37 -0700
    Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-07 10:11 +0100
      Re: From cooperative to preemptive scheduler: a real example Don Y <blockedofcourse@foo.invalid> - 2020-01-07 07:51 -0700
        Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-07 23:51 +0100
          Re: From cooperative to preemptive scheduler: a real example Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-09 01:39 +0200
            Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-10 00:42 +0100
              Re: From cooperative to preemptive scheduler: a real example Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-15 18:39 +0200
          Re: From cooperative to preemptive scheduler: a real example Don Y <blockedofcourse@foo.invalid> - 2020-01-08 23:12 -0700
            Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-10 00:49 +0100
          Re: From cooperative to preemptive scheduler: a real example Don Y <blockedofcourse@foo.invalid> - 2020-01-08 23:12 -0700
          Re: From cooperative to preemptive scheduler: a real example Don Y <blockedofcourse@foo.invalid> - 2020-01-09 01:33 -0700
  Re: From cooperative to preemptive scheduler: a real example Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-07 09:38 +0200
    Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-08 00:02 +0100
      Re: From cooperative to preemptive scheduler: a real example Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-09 00:54 +0200
        Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-09 12:19 +0100
          Re: From cooperative to preemptive scheduler: a real example Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-09 14:03 +0200
            Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-10 00:32 +0100
              Re: From cooperative to preemptive scheduler: a real example Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-15 18:53 +0200
                Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-16 14:48 +0100
                Re: From cooperative to preemptive scheduler: a real example upsidedown@downunder.com - 2020-01-16 21:04 +0200
                Re: From cooperative to preemptive scheduler: a real example Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-16 22:24 +0200
                Re: From cooperative to preemptive scheduler: a real example pozz <pozzugno@gmail.com> - 2020-01-17 02:05 +0100
                Re: From cooperative to preemptive scheduler: a real example Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-18 23:03 +0200
  Re: From cooperative to preemptive scheduler [Stack] pozz <pozzugno@gmail.com> - 2020-01-14 13:27 +0100
    Re: From cooperative to preemptive scheduler [Stack] Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2020-01-14 20:10 +0100
      Re: From cooperative to preemptive scheduler [Stack] pozz <pozzugno@gmail.com> - 2020-01-15 09:36 +0100
        Re: From cooperative to preemptive scheduler [Stack] Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> - 2020-01-15 11:30 +0100
          Re: From cooperative to preemptive scheduler [Stack] pozz <pozzugno@gmail.com> - 2020-01-15 11:51 +0100
        Re: From cooperative to preemptive scheduler [Stack] Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2020-01-15 21:51 +0100
    Re: From cooperative to preemptive scheduler [Stack] Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-15 21:26 +0200
      Re: From cooperative to preemptive scheduler [Stack] Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2020-01-15 22:12 +0100
        Re: From cooperative to preemptive scheduler [Stack] Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-01-15 23:30 +0200

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