MESSAGE
| DATE | 2015-04-23 |
| FROM | Ruben Safir
|
| SUBJECT | Subject: [LIU Comp Sci] Fwd: Re: Kernel Scheduler and wiat queues
|
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From: Ruben Safir
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To: Ruben Safir ,
Maneesh Kongara ,
learn-at-nylxs.com
Subject: [LIU Comp Sci] Fwd: Re: Kernel Scheduler and wiat queues
References: <20150422091305.302-at-kylheku.com> <87sibsays5.fsf-at-doppelsaurus.mobileactivedefense.com> <20150422110344.469-at-kylheku.com> <20150422160337.511-at-kylheku.com>
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From: Kaz Kylheku
Newsgroups: comp.unix.programmer
Subject: Re: Kernel Scheduler and wiat queues
Date: Wed, 22 Apr 2015 16:31:25 +0000 (UTC)
Organization: Aioe.org NNTP Server
Message-ID: <20150422091305.302-at-kylheku.com>
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On 2015-04-22, ruben safir wrote:
> DEFINE_WAIT(wait) //what IS wait EXACTLY in this context
A preprocessor token, used as the argument to a macro call.
To understand what it means, you must look at the macro expansion.
What the macro does is generate code to define a variable whose name is "wait".
It is abstracted away because, possibly, the variable is not only defined, but
requires initialization. It requires initialization in ways that could change
as the kernel is maintained, so it needs to be abstract.
> add_wait_queue(q, &wait); // in the current kernel this invovled
> // flag checking and a linked list
>
> while(!condition){ /* an event we are weighting for
> prepare_to_wait(&q, &wait, TASK_INTERRUPTIBLE);
> if(signal_pending(current))
> /* SIGNAl HANDLE */
> schedule();
> }
>
> finish_wait(&q, &wait);
>
> He also write how this proceeds to function and one part confuses me
>
> 5. When the taks awakens, it again checks whether the condition is
> true. If it is, it exists the loop. Otherwise it again calls schedule.
This is just the consequence of the code you have before you. When
the task awakens it returns out of the schedule() call, and continues
with the loop.
> This is not the order that it seems to follow according to the code.
>
> To me it looks like it should
> 1 - creat2 the wait queue
You really mean "wait queue node". The wait queue already exists, but
the task needs a queuing data structure so that it can add itself to
that queue.
(Why does a task need an extra structure for this rather than just some
link pointers inside its task structure. The answer is that a task
can wait on multiple wait queues queues at the same time:
DEFINE_WAIT(node0);
DEFINE_WAIT(node1);
/* ... */
DEFINE_WAIT(nodeN);
add_wait_queue(q0, &node0);
add_wait_queue(q1, &node1);
/* ... */
add_wait_queue(q1, &nodeN);
while (!condition) {
prepare_to_wait(&q0, &node0, TASK_INTERRUPTIBLE);
prepare_to_wait(&q1, &node1, TASK_INTERRUPTIBLE);
/* ... */
prepare_to_wait(&qn, &nodeN, TASK_INTERRUPTIBLE);
if(signal_pending(current)) { /* SIGNAl HANDLE */ }
schedule();
}
finish_wait(&q0, &node0);
finish_wait(&q1, &node1);
/* ... */
finish_wait(&qn, &nodeN);
> 5 check for signals ... notice the process is still moving. Does it
> stop and wait now?
No, it stops and waits somewhere inside the schedule() function, and only
there. Elsewhere, the code is running.
> 6 schedule itself on the runtime rbtree... which make NO sense unless
> there was a stopage I didn't know about.
It makes perfect sense. The task changes its status to "interruptible sleep",
and then informs the scheduler, by calling the schedule() function.
The scheduler puts that into effect, and removes the task from the run queue.
Then it makes a scheduling decision to dispatch a task. The task which
called it is not eligible for execution now, so another task is chosen.
> Isn't this reckless to leave this to users to write the code. Your
> begging for a race condition.
Yes, of course you are absolutely right; most kernel developers should not be
writing the above open code most of the time. The best justification for
it is that you're developing a new synchronization primitive (which itself
requires justification as to why the existing ones aren't good enough).
There are higher level primitives which wrap the above logic: semaphores,
read-write semaphores, mutexes, events, poll, ...
For instance, look for wait_event, wait_event_interruptible, and related
macros.
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From: Rainer Weikusat
Newsgroups: comp.unix.programmer
Subject: Re: Kernel Scheduler and wiat queues
Date: Wed, 22 Apr 2015 17:55:06 +0100
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References: <20150422091305.302-at-kylheku.com>
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Kaz Kylheku writes:
> On 2015-04-22, ruben safir wrote:
[...]
>> 6 schedule itself on the runtime rbtree... which make NO sense unless
>> there was a stopage I didn't know about.
>
> It makes perfect sense. The task changes its status to "interruptible sleep",
> and then informs the scheduler, by calling the schedule() function.
> The scheduler puts that into effect, and removes the task from the run
> queue.
schedule() is the top-level entry point of the scheduler, so, a more
accurate description would be "it calls into the scheduler in order to
make that select some task to run". Maybe a higher priority task than
the one which called schedule is runnable. In this case, the higher
priority task will run instead of the one which called
schedule. Otherwise, this task will continue to run unless it changed
its state to something other than 'runnable' first.
>> Isn't this reckless to leave this to users to write the code. Your
>> begging for a race condition.
>
> Yes, of course you are absolutely right; most kernel developers should not be
> writing the above open code most of the time. The best justification for
> it is that you're developing a new synchronization primitive (which itself
> requires justification as to why the existing ones aren't good
> enough).
I completely disagree with this: Just because more or less useful and
more or less buggy of bug-free 'special-case wait for this, that or
something else' code keeps piling up in in include/linux/wait.h doesn't
mean anyone ought to be required to read through all of this whenever
some kernel code has to wait for something to select the currently least
worse bit of infrastructure code from there, considering that the simple
solution of just coding the wait loop will work for every situation and
is all over the kernel for historical reasons, anyway.
Some people can't ever do anything without producing a "general purpose
library" (or a couple of these) [or can't ever do anything except
that]. Considering this obvious lack of practically applicable skills,
no code deserves to be used just because someone wrote it.
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From: Kaz Kylheku
Newsgroups: comp.unix.programmer
Subject: Re: Kernel Scheduler and wiat queues
Date: Wed, 22 Apr 2015 18:20:48 +0000 (UTC)
Organization: Aioe.org NNTP Server
Message-ID: <20150422110344.469-at-kylheku.com>
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On 2015-04-22, Rainer Weikusat wrote:
> Kaz Kylheku writes:
>> On 2015-04-22, ruben safir wrote:
>>> Isn't this reckless to leave this to users to write the code. Your
>>> begging for a race condition.
>>
>> Yes, of course you are absolutely right; most kernel developers should not be
>> writing the above open code most of the time. The best justification for
>> it is that you're developing a new synchronization primitive (which itself
>> requires justification as to why the existing ones aren't good
>> enough).
>
> I completely disagree with this: Just because more or less useful and
> more or less buggy of bug-free 'special-case wait for this, that or
> something else' code keeps piling up in in include/linux/wait.h doesn't
> mean anyone ought to be required to read through all of this whenever
> some kernel code has to wait for something to select the currently least
> worse bit of infrastructure code from there, considering that the simple
> solution of just coding the wait loop will work for every situation and
> is all over the kernel for historical reasons, anyway.
I'm the opposite. Give me a large number of primitives to choose from!
By the way, I used mutexes and condition variables in the kernel back in 1998,
before any such a thing existed in the kernel, and made a little package of
that. It supported a driver I was working on:
http://www.kylheku.com/~kaz/lmc.html
A few years ago, on another job, I also wrote a "super poll function".
With this bugger, you could, simultaneously:
- wait on a set of sockets (struct socket *)
- wait on a set of files (struct file *)
- wait on a condition variable
- with a timeout
with this mega-function, I multiplexed a ton of functionality onto a single
kernel thread.
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From: Kaz Kylheku
Newsgroups: comp.unix.programmer
Subject: Re: Kernel Scheduler and wiat queues
Date: Wed, 22 Apr 2015 23:04:32 +0000 (UTC)
Organization: Aioe.org NNTP Server
Message-ID: <20150422160337.511-at-kylheku.com>
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<20150422110344.469-at-kylheku.com>
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On 2015-04-22, Kaz Kylheku wrote:
> By the way, I used mutexes and condition variables in the kernel back in 1998,
> before any such a thing existed in the kernel, and made a little package of
> that. It supported a driver I was working on:
>
> http://www.kylheku.com/~kaz/lmc.html
>
> A few years ago, on another job, I also wrote a "super poll function".
> With this bugger, you could, simultaneously:
>
> - wait on a set of sockets (struct socket *)
> - wait on a set of files (struct file *)
> - wait on a condition variable
> - with a timeout
I just noticed that I had added that function to the above "LMC" project, haha.
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