From: Martin Pieuchot <mpi@grenadille.net>
Subject: Read uvmexp.free only once in pdaemon
To: tech@openbsd.org
Date: Wed, 6 Nov 2024 10:01:32 +0100

Now that all "shrinkers" freeing non-managed memory return the amount of
freed pages we can use them.

Diff below update `shortage' based on return value and remove one more
access to `uvmexp.free'.

While here compute `inactive_shortage' early in the loop where
`uvmexp.inactarg' is calculated.  They are both part of the same logic
and since the "shrinkers" do not deal with managed memory it is fine.
Also do not trash the per-CPU cache if we already freed enough memory.
This is generally the case when the pagedaemon is awaken to free "low"
memory and the buffer cache satisfies the request.

Ok?

Index: uvm/uvm_pdaemon.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_pdaemon.c,v
diff -u -p -r1.121 uvm_pdaemon.c
--- uvm/uvm_pdaemon.c	5 Nov 2024 17:28:32 -0000	1.121
+++ uvm/uvm_pdaemon.c	6 Nov 2024 08:33:03 -0000
@@ -237,6 +237,7 @@ uvm_pageout(void *arg)
  			} else
  				constraint = no_constraint;
  		}
+		/* How many pages do we need to free during this round? */
  		shortage = uvmexp.freetarg - uvmexp.free + BUFPAGES_DEFICIT;
  		uvm_unlock_fpageq();
  @@ -248,6 +249,8 @@ uvm_pageout(void *arg)
  		if (uvmexp.inactarg <= uvmexp.freetarg) {
  			uvmexp.inactarg = uvmexp.freetarg + 1;
  		}
+		inactive_shortage =
+			uvmexp.inactarg - uvmexp.inactive - BUFPAGES_INACT;
  		uvm_unlock_pageq();
   		/* Reclaim pages from the buffer cache if possible. */
@@ -259,19 +262,17 @@ uvm_pageout(void *arg)
  		if (size == 0)
  			size = 16; /* XXX */
  -		(void) bufbackoff(&constraint, size * 2);
+		shortage -= bufbackoff(&constraint, size * 2);
  #if NDRM > 0
-		drmbackoff(size * 2);
+		shortage -= drmbackoff(size * 2);
  #endif
-		uvm_pmr_cache_drain();
+		if (shortage > 0)
+			shortage -= uvm_pmr_cache_drain();
   		/*
  		 * scan if needed
  		 */
  		uvm_lock_pageq();
-		shortage = uvmexp.freetarg - uvmexp.free + BUFPAGES_DEFICIT;
-		inactive_shortage =
-			uvmexp.inactarg - uvmexp.inactive - BUFPAGES_INACT;
  		if (pma != NULL || (shortage > 0) || (inactive_shortage > 0)) {
  			uvmpd_scan(pma, shortage, inactive_shortage,
  			    &constraint);