On 2022-01-25 16:26:44 [+0100], To linux-kernel@xxxxxxxxxxxxxxx wrote: > [ This is a repost of https://lkml.kernel.org/r/20211118143452.136421-1-bigeasy@xxxxxxxxxxxxx ] > > 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. > TS 124.918126, the first command is done, stack is ached on CPU19. > TS 124.918279, script's second command, ache from stack. > 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. > > 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 no new is allocated during its runtime and a cached stack is used. ping Sebastian
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