[PATCH 0/8] kernel/fork: Move thread stack free otu of the scheduler path.

[Sebastian Andrzej Siewior]

This is a follup-up on the patch
    sched: Delay task stack freeing on RT 
    https://lkml.kernel.org/r/20210928122411.593486363@xxxxxxxxxxxxx

It addresses the review feedback:
- Decouple stack accounting from its free invocation. The accounting
  happens in do_exit(), the final free call happens later.

- Add put_task_stack_sched() to finish_task_switch(). Here the VMAP
  stack is cached only. If it fails, or in the !VMAP case then the final
  free happens in delayed_put_task_struct(). This is also an oportunity
  to cache the stack.

>From testing I observe the following:

|            bash-1715    [006] .....   124.901510: copy_process: allocC ffffc90007e70000
|      sh-cmds.sh-1746    [007] .....   124.907389: copy_process: allocC ffffc90007dc4000
|          <idle>-0       [019] ...1.   124.918126: free_thread_stack: cache ffffc90007dc4000
|      sh-cmds.sh-1746    [007] .....   124.918279: copy_process: allocC ffffc90007de8000
|          <idle>-0       [004] ...1.   124.920121: free_thread_stack: delay ffffc90007de8001
|          <idle>-0       [007] ...1.   124.920299: free_thread_stack: cache ffffc90007e70000
|          <idle>-0       [007] ..s1.   124.945433: free_thread_stack: cache ffffc90007de8000

TS 124.901510, bash started sh-cmds.sh, obtained stack from cache.
TS 124.907389, script invokes its first command, obtained stacak from
cache. As you can see bash was running on CPU6 but its child was moved
CPU7. This happens a lot, maybe because my test system is too idle.
TS 124.918126, the first command is done, stack is ached on CPU19.
TS 124.918279, script's second command, stack from cache.
TS 124.920121, the command is done. The stack cache on CPU4 is full.
TS 124.920299, the script is done, caches stack on CPU7.
TS 124.945433, the RCU-callback of last command is now happening. On
CPU7, which is where the command was invoked (but not running). Instead
of freeing the stack, it was cached since CPU7 had an empty slot. \o/.

If I pin the script to CPU5 and run it with multiple commands then it
works as expected:

|            bash-1799    [005] .....   993.608131: copy_process: allocC ffffc90007fa0000
|      sh-cmds.sh-1827    [005] .....   993.608888: copy_process: allocC ffffc90007fa8000
|      sh-cmds.sh-1827    [005] .....   993.610734: copy_process: allocV ffffc90007ff4000
|      sh-cmds.sh-1829    [005] ...1.   993.610757: free_thread_stack: cache ffffc90007fa8000
|      sh-cmds.sh-1827    [005] .....   993.612401: copy_process: allocC ffffc90007fa8000
|           <...>-1830    [005] ...1.   993.612416: free_thread_stack: cache ffffc90007ff4000
|      sh-cmds.sh-1827    [005] .....   993.613707: copy_process: allocC ffffc90007ff4000
|      sh-cmds.sh-1831    [005] ...1.   993.613723: free_thread_stack: cache ffffc90007fa8000
|      sh-cmds.sh-1827    [005] .....   993.615024: copy_process: allocC ffffc90007fa8000
|           <...>-1832    [005] ...1.   993.615040: free_thread_stack: cache ffffc90007ff4000
|      sh-cmds.sh-1827    [005] .....   993.616380: copy_process: allocC ffffc90007ff4000
|           <...>-1833    [005] ...1.   993.616397: free_thread_stack: cache ffffc90007fa8000
|            bash-1799    [005] ...1.   993.617759: free_thread_stack: cache ffffc90007fa0000
|          <idle>-0       [005] ...1.   993.617871: free_thread_stack: delay ffffc90007ff4001
|          <idle>-0       [005] ..s1.   993.638311: free_thread_stack: free ffffc90007ff4000

and after a warm up no new stack is allocated during its runtime and a
cached stack is used instead.

Sebastian