Re: [PATCH] sched/task_struct: Move alloc_tag to the end of the struct.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2024-06-21 12:27:52 [+0200], To linux-mm@kvack.org wrote:
> The alloc_tag member has been added to task_struct at the very
> beginning. This is a pointer and on 64bit architectures it forces 4 byte
> padding after `ptrace' and then forcing another another 4 byte padding
> after `on_cpu'. A few members later, `se' requires a cacheline aligned
> due to struct sched_avg resulting in 52 hole before `se'.
> 
> This is the case on 64bit-SMP architectures.
> The 52 byte hole can be avoided by moving alloc_tag away where it
> currently resides.
> 
> Move alloc_tag to the end of task_struct. There is likely a hole before
> `thread' due to its alignment requirement and the previous members are
> likely to be already pointer-aligned.
> 
> Fixes: 22d407b164ff7 ("lib: add allocation tagging support for memory allocation profiling")
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Could we please get this merged and worry about possible performance
regression later? Or once there is a test case or an idea where this
pointer might fit better but clearly the current situation is worse.

Sebastian