diff --git a/libc/testlib/BUILD.mk b/libc/testlib/BUILD.mk
index 5de84b1d2..a81790c86 100644
--- a/libc/testlib/BUILD.mk
+++ b/libc/testlib/BUILD.mk
@@ -215,6 +215,7 @@ LIBC_TESTMAIN_DIRECTDEPS =					\
 	LIBC_LOG						\
 	LIBC_MEM						\
 	LIBC_NEXGEN32E						\
+	LIBC_PROC						\
 	LIBC_RUNTIME						\
 	LIBC_STDIO						\
 	LIBC_SYSV						\
@@ -222,7 +223,7 @@ LIBC_TESTMAIN_DIRECTDEPS =					\
 	LIBC_TESTLIB						\
 	LIBC_TESTLIB_RUNNER					\
 	THIRD_PARTY_DLMALLOC					\
-	THIRD_PARTY_GETOPT
+	THIRD_PARTY_GETOPT					\
 
 LIBC_TESTMAIN_DEPS :=						\
 	$(call uniq,$(foreach x,$(LIBC_TESTMAIN_DIRECTDEPS),$($(x))))
diff --git a/libc/testlib/testmain.c b/libc/testlib/testmain.c
index 923f02608..538ca3ec5 100644
--- a/libc/testlib/testmain.c
+++ b/libc/testlib/testmain.c
@@ -18,6 +18,7 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/assert.h"
 #include "libc/calls/calls.h"
+#include "libc/calls/struct/cpuset.h"
 #include "libc/calls/struct/rlimit.h"
 #include "libc/calls/struct/sigaction.h"
 #include "libc/calls/struct/siginfo.h"
@@ -31,6 +32,7 @@
 #include "libc/intrin/strace.h"
 #include "libc/intrin/ubsan.h"
 #include "libc/intrin/weaken.h"
+#include "libc/limits.h"
 #include "libc/log/log.h"
 #include "libc/macros.h"
 #include "libc/mem/leaks.h"
@@ -52,6 +54,8 @@
 #include "libc/thread/tls.h"
 #include "third_party/getopt/getopt.internal.h"
 
+#pragma weak main
+
 #define USAGE \
   " [FLAGS]\n\
 \n\
@@ -88,7 +92,41 @@ static void GetOpts(int argc, char *argv[]) {
   }
 }
 
-#pragma weak main
+static int rando(void) {
+  return _rand64() & INT_MAX;
+}
+
+static void limit_process_to_single_cpu(void) {
+  extern int disable_limit_process_to_single_cpu;
+  if (_weaken(disable_limit_process_to_single_cpu))
+    return;
+  if (!(IsLinux() || IsFreebsd() || IsNetbsd() || IsWindows()))
+    return;
+  if (IsFreebsd() && getuid())
+    return;
+  cpu_set_t legal;
+  if (sched_getaffinity(0, sizeof(cpu_set_t), &legal) == -1) {
+    perror("sched_setaffinity failed");
+    exit(1);
+  }
+  int count = CPU_COUNT(&legal);
+  cpu_set_t newset;
+  CPU_ZERO(&newset);
+  bool done = false;
+  while (!done) {
+    for (int i = 0; i < CPU_SETSIZE; ++i) {
+      if (CPU_ISSET(i, &legal) && !(rando() % count)) {
+        CPU_SET(rando() % count, &newset);
+        done = true;
+        break;
+      }
+    }
+  }
+  if (sched_setaffinity(0, sizeof(cpu_set_t), &newset) == -1) {
+    perror("sched_setaffinity failed");
+    exit(1);
+  }
+}
 
 /**
  * Generic test program main function.
@@ -108,8 +146,11 @@ int main(int argc, char *argv[]) {
     return 1;
   }
 
+  // // this sometimes helps tease out mt bugs
+  // limit_process_to_single_cpu();
+
   // test huge pointers by enabling pml5t
-  if (_rand64() % 2) {
+  if (rando() % 2) {
     errno_t e = errno;
     mmap((char *)0x80000000000000, 1, PROT_NONE,  //
          MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
diff --git a/test/libc/proc/sched_getaffinity_test.c b/test/libc/proc/sched_getaffinity_test.c
index 33d3c16e5..dad9c1b26 100644
--- a/test/libc/proc/sched_getaffinity_test.c
+++ b/test/libc/proc/sched_getaffinity_test.c
@@ -30,6 +30,8 @@
 #include "libc/thread/thread.h"
 #include "libc/thread/thread2.h"
 
+int disable_limit_process_to_single_cpu;
+
 void SetUp(void) {
   if (!IsLinux() && !IsFreebsd() && !IsWindows()) {
     exit(0);