From: Mark Kettenis <mark.kettenis@xs4all.nl>
Subject: Re: uvm_swapout_threads() & freed pages
To: Martin Pieuchot <mpi@grenadille.net>
Cc: tech@openbsd.org
Date: Tue, 22 Oct 2024 21:14:45 +0200

> Date: Sun, 20 Oct 2024 11:25:41 +0200
> From: Martin Pieuchot <mpi@grenadille.net>
> 
> Return the number of freed pages in uvm_swapout_threads().  This is
> part of my ongoing work to reduce accesses to global variables inside 
> the page deamon loop.
> 
> ok?

Hmm, but this still accesses uvmexp.free inside uvm_swapout_threads().
So I'm not sure this is an improvement.  And there is no guarantee
that the returned number of freed pages is accurate.

Wouldn't it be better to make pmap_collect() return the number of
freed pages?  This is only actually implemented on i386 and it looks
fairly simple to count the number of ptp pages freed in
pmap_do_remove_86() and pmap_do_remove_pae() and have them return that
number.

> Index: uvm/uvm_glue.c
> ===================================================================
> RCS file: /cvs/src/sys/uvm/uvm_glue.c,v
> diff -u -p -r1.85 uvm_glue.c
> --- uvm/uvm_glue.c	8 Oct 2024 02:29:10 -0000	1.85
> +++ uvm/uvm_glue.c	20 Oct 2024 09:20:09 -0000
> @@ -339,13 +339,13 @@ int	swapdebug = 0;
>   *   are swapped... otherwise the longest-sleeping or stopped process
>   *   is swapped, otherwise the longest resident process...
>   */
> -void
> +int
>  uvm_swapout_threads(void)
>  {
>  	struct process *pr;
>  	struct proc *p, *slpp;
>  	struct process *outpr;
> -	int outpri;
> +	int free, outpri;
>  	int didswap = 0;
>  	extern int maxslp; 
>  	/* XXXCDC: should move off to uvmexp. or uvm., also in uvm_meter */
> @@ -355,6 +355,8 @@ uvm_swapout_threads(void)
>  		return;
>  #endif
>  
> +	free = uvmexp.free;
> +
>  	/*
>  	 * outpr/outpri  : stop/sleep process whose most active thread has
>  	 *	the largest sleeptime < maxslp
> @@ -403,8 +405,7 @@ next_process:	;
>  	 * if we are real low on memory since we don't gain much by doing
>  	 * it.
>  	 */
> -	if (didswap == 0 && uvmexp.free <= atop(round_page(USPACE)) &&
> -	    outpr != NULL) {
> +	if (didswap == 0 && free <= atop(round_page(USPACE)) && outpr != NULL) {
>  #ifdef DEBUG
>  		if (swapdebug & SDB_SWAPOUT)
>  			printf("swapout_threads: no duds, try procpr %p\n",
> @@ -412,6 +413,8 @@ next_process:	;
>  #endif
>  		pmap_collect(outpr->ps_vmspace->vm_map.pmap);
>  	}
> +
> +	return (uvmexp.free - free);
>  }
>  
>  #endif	/* __HAVE_PMAP_COLLECT */
> Index: uvm/uvm_glue.h
> ===================================================================
> RCS file: /cvs/src/sys/uvm/uvm_glue.h,v
> diff -u -p -r1.9 uvm_glue.h
> --- uvm/uvm_glue.h	11 Jul 2014 16:35:40 -0000	1.9
> +++ uvm/uvm_glue.h	20 Oct 2024 09:20:09 -0000
> @@ -37,7 +37,7 @@
>   * uvm_glue.h
>   */
>  
> -void uvm_swapout_threads(void);
> +int uvm_swapout_threads(void);
>  
>  struct vm_page	*uvm_atopg(vaddr_t);
>  
> 
>