Kernel thread scheduling

[ruben@mrbrklyn.com (Ruben Safir)]

On 04/10/2015 09:09 AM, nick wrote:
> 
> 
> On 2015-04-09 11:37 PM, Ruben Safir wrote:
>> On 04/09/2015 10:52 PM, nick wrote:
>>> Before asking questions again like this please look into either using lxr or ctags
>>> to navigate the kernel tree for answers as can be faster then waiting for me or 
>>> someone else to respond.
>>
>>
>> well, I reading the text is ctags aren't much value there.
>>
> Ctags is useful for searching the code, which is why I am recommending it.
> Nick

I have it built into gvim, but you can't use it from a textbook.  I'm
finding it is not as useful as it could be for the kernel code.  There
are stacks of tags to get around.  Another 2 days to learn to get around
tags in vi is not in the agenda right now.  It is the tool I have so
I'll have to live with it right now.

I also have a question that is not obvious from the code I'm looking at.
 I'm not sure how these structs are attached together.  Or more
specifically, I'm not sure how pulling the correct sched_entity gets one
the coresponding task_entity

You have
struct task_struct with a
	struct sched_entity

struct sched_enitities are nodes in the RB tree
	which are a "container" for "struct rb_node run_node".

So a look at sched_entity ... is in ../linux/sched.h

1161 struct sched_entity {
1162    struct load_weight   load;    /* for load-balancing */
1163    struct rb_node    run_node;
1164    struct list_head  group_node;
1165    unsigned int      on_rq;
1166
1167    u64         exec_start;
1168    u64         sum_exec_runtime;
1169    u64         vruntime;
1170    u64         prev_sum_exec_runtime;
1171
1172    u64         nr_migrations;
1173
1174 #ifdef CONFIG_SCHEDSTATS
1175    struct sched_statistics statistics;
1176 #endif
1177
1178 #ifdef CONFIG_FAIR_GROUP_SCHED
1179    int         depth;
1180    struct sched_entity  *parent;
1181    /* rq on which this entity is (to be) queued: */
1182    struct cfs_rq     *cfs_rq;
1183    /* rq "owned" by this entity/group: */
1184    struct cfs_rq     *my_q;
1185 #endif
1186
1187 #ifdef CONFIG_SMP
1188    /* Per-entity load-tracking */
1189    struct sched_avg  avg;
1190 #endif
1191 };

I see no means of referencing a specific task from this struct that
forms the node.  So when you pull the node with the smallest vruntime
from the left most postion of the RB tree, by calling pick_next_task(),


static struct sched_entity *__pick_next_entity(struct sched_entity *se)
{
	struct rb_node *next = rb_next(&se->run_node);

	if (!next)
		return NULL;

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


how do we know what task we are attached to?

Ruben




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