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

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

On 2024-06-21 10:20:58 [-0400], Kent Overstreet wrote:
> On Fri, Jun 21, 2024 at 12:27:50PM +0200, Sebastian Andrzej Siewior 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.
> 
> We sure we want it at the end? we do want it on a hot cacheline

Well, the front is bad.
Looking at pgalloc_tag_add() and its callers, there is no task_struct
touching. alloc_tag_save()/restore might be the critical one. This is
random code… Puh. So if the end is too cold, what about around the mm
pointer?
Other suggestions?

Sebastian