svn commit: r338802 - head/sys/vm

[Mateusz Guzik]

Author: mjg
Date: Wed Sep 19 16:02:33 2018
New Revision: 338802
URL: https://svnweb.freebsd.org/changeset/base/338802

Log:
  vm: check for empty kstack cache before locking
  
  The current cache logic checks the total number of stacks in the kernel,
  which even on small boxes significantly exceeds the 128 limit (e.g. an
  8-way box with zfs has almost 800 stacks allocated).
  
  Stacks are cached earlier for each main thread.
  
  As a result the code is rarely executed, but when it is then (on boxes like
  the above) it always fails. Since there are no provisions made for NUMA and
  release time is approaching, just do a quick check to avoid acquiring the
  lock.
  
  Approved by:	re (kib)

Modified:
  head/sys/vm/vm_glue.c

Modified: head/sys/vm/vm_glue.c
==============================================================================
--- head/sys/vm/vm_glue.c	Wed Sep 19 15:39:16 2018	(r338801)
+++ head/sys/vm/vm_glue.c	Wed Sep 19 16:02:33 2018	(r338802)
@@ -327,7 +327,7 @@ vm_thread_new(struct thread *td, int pages)
 	else if (pages > KSTACK_MAX_PAGES)
 		pages = KSTACK_MAX_PAGES;
 
-	if (pages == kstack_pages) {
+	if (pages == kstack_pages && kstack_cache != NULL) {
 		mtx_lock(&kstack_cache_mtx);
 		if (kstack_cache != NULL) {
 			ks_ce = kstack_cache;