Where is run_node defined in the sched_fair.c of Linux 2.6.33?

Where is run_node defined in the sched_fair.c of Linux 2.6.33?

[Sreyan Chakravarty]

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I am reading Robert Love's Book Linux Kernel Development(
https://rads.stackoverflow.com/amzn/click/com/B003V4ATI0).

It uses the 2.6.33 kernel for demonstration.

I have been going through certain parts of the source and can't find out
where is the initial definition of many things. A lot of things are just
used, like "magic" without me finding the definition.

One example:

static struct sched_entity *__pick_next_entity(struct cfs_rq *cfs_rq)
{
struct rb_node *left = cfs_rq->rb_leftmost;

if (!left)
return NULL;

return rb_entry(left, struct sched_entity, run_node);
}

static struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq)
{
struct rb_node *last = rb_last(&cfs_rq->tasks_timeline);

if (!last)
return NULL;

return rb_entry(last, struct sched_entity, run_node);
}

This in kernel/sched_fair.c lines 384 and 394 in 2.6.33 kernel.

Where is the run_node coming from ?

I have grep ed(
https://livegrep.com/search/linuxq=run_node&fold_case=auto&regex=false&context=true
<https://livegrep.com/search/linux?q=run_node&fold_case=auto&regex=false&context=true>)
the entire source base over here, and I have not found any definition of
run_node that would allow it to be used like this.

There is a declaration(
https://github.com/torvalds/linux/blob/219d54332a09e8d8741c1e1982f5eae56099de85/include/linux/sched.h#L448)
in the sched_entity structure, but nothing outside of it that would allow
it to be used like this.

I cannot understand how things are organized, its really confusing.

What is going on ?
-- 
Regards,
Sreyan Chakravarty

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Re: Where is run_node defined in the sched_fair.c of Linux 2.6.33?

[Valentin Vidić]

On Wed, May 06, 2020 at 05:32:26PM +0530, Sreyan Chakravarty wrote:
> I am reading Robert Love's Book Linux Kernel Development(
> https://rads.stackoverflow.com/amzn/click/com/B003V4ATI0).
> 
> It uses the 2.6.33 kernel for demonstration.
> 
> I have been going through certain parts of the source and can't find out
> where is the initial definition of many things. A lot of things are just
> used, like "magic" without me finding the definition.
> 
> One example:
> 
> static struct sched_entity *__pick_next_entity(struct cfs_rq *cfs_rq)
> {
> struct rb_node *left = cfs_rq->rb_leftmost;
> 
> if (!left)
> return NULL;
> 
> return rb_entry(left, struct sched_entity, run_node);
> }

include/linux/rbtree.h:#define  rb_entry(ptr, type, member) container_of(ptr, type, member)

So rb_entry is a macro and it basically rewrites the return line
into another macro:

  return container_of(left, struct sched_entity, run_node);

And that one gives the address of the struct sched_entity that
left is a member of by subtracting the offset from the struct
start to run_node member:

struct sched_entity {       -
 ...                        |
 ...                        | offset
 ...                        |
 ...                        -
 struct rb_node run_node;   <=== left points here
 ...
}

https://stackoverflow.com/questions/15832301/understanding-container-of-macro-in-the-linux-kernel

-- 
Valentin

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Re: Where is run_node defined in the sched_fair.c of Linux 2.6.33?

[Lukas Bulwahn]

On Wed, May 6, 2020 at 2:13 PM Sreyan Chakravarty <sreyan32@gmail.com> wrote:
>
> I am reading Robert Love's Book Linux Kernel Development(https://rads.stackoverflow.com/amzn/click/com/B003V4ATI0).
>
> It uses the 2.6.33 kernel for demonstration.
>
> I have been going through certain parts of the source and can't find out where is the initial definition of many things. A lot of things are just used, like "magic" without me finding the definition.
>
> One example:
>
> static struct sched_entity *__pick_next_entity(struct cfs_rq *cfs_rq)
> {
> struct rb_node *left = cfs_rq->rb_leftmost;
>
> if (!left)
> return NULL;
>
> return rb_entry(left, struct sched_entity, run_node);
> }
>
> static struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq)
> {
> struct rb_node *last = rb_last(&cfs_rq->tasks_timeline);
>
> if (!last)
> return NULL;
>
> return rb_entry(last, struct sched_entity, run_node);
> }
>
>
> This in kernel/sched_fair.c lines 384 and 394 in 2.6.33 kernel.
>
> Where is the run_node coming from ?
>
> I have grep ed(https://livegrep.com/search/linuxq=run_node&fold_case=auto&regex=false&context=true) the entire source base over here, and I have not found any definition of run_node that would allow it to be used like this.
>
> There is a declaration(https://github.com/torvalds/linux/blob/219d54332a09e8d8741c1e1982f5eae56099de85/include/linux/sched.h#L448) in the sched_entity structure, but nothing outside of it that would allow it to be used like this.
>
> I cannot understand how things are organized, its really confusing.
>
> What is going on ?

I believe you are assuming "rb_entry" is a C function and run_node is
a variable, but it is not. Instead rb_entry is a macro that takes the
argument as "plain source code strings" and creates an C-code
expression out of that. If you look at the macro definitions of
rb_entry and follow that to the truth, you will how everything
magically resolves to C code that a C compiler could actually parse
and make sense of.

I hope that helps.

Lukas

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