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 at 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