Need for thread_info structure in Linux 2.6 Kernel?

Question

Prior to the Linux 2.6 kernel, struct task_struct was present at the end of the kernel stack of each process. There was no thread_info struct concept. But in Linux 2.6 kernel, instead of task_struct being placed at the end of the kernel stack for the process, the thread_info struct is placed at the end. This thread_info struct contains a pointer to the task_struct structure.

What was the need for thread_info structure to be introduced ?. We could have accessed the task_struct structure using the stack pointer directly if task_struct was placed at the end of the kernel stack of the process.

In 2.6 Kernel, task_struct is dynamically allocated using slab_allocator. Prior to 2.6 Kernel, was it statically allocated?

Answer 1

FrankH, he is looking (out of pure interest as I am, I suspect) for a reason for this change. This if what I've found with my l33t google skills. A bit more info behind the link:

"task_struct is huge. it's around 1,7KB on a 32 bit machine. on the other hand, you can easily see that thread_info is much slimmer.

kernel stack is either 4 or 8KB, and either way a 1.7KB is pretty much, so storing a slimmer struct, that points to task_struct, immediately saves a lot of stack space and is a scalable solution."

(c) http://www.spinics.net/lists/newbies/msg22263.html

Answer 2

The reason why we need the thread_info is due to the fact that we are allocating the memory for task_struct using the Slab Allocator. Now you may ask what is the relation between these?

To understand that you need to understand how Slab Allocator works.

Without the Slab Allocator , the kernel developers could allocate memory for task_struct in the kernel stack for the particular process so that it can be accessed easily. Now with the advent of Slab Allocator , the memory is allocated to the task_struct as determined by the Slab Allocator. So with the Slab Allocator you have task_struct stored somewhere else and not in the kernel stack of the particular process. Now the Kernel developers introduced thread_info and placed a pointer in it to the place where the task_struct resides. And that is why we have to live with thread_info.

You can read about Slab Allocator in Robert Love's book Linux Kernel Development.