On Tue, 2015-11-10 at 12:01 -0800, David Rientjes wrote:

On Sat, 31 Oct 2015, Ben Hutchings wrote:

quoted

If we need to increase the number of huge pages, drop caches first
to reduce fragmentation and then check that we actually allocated
as many as we wanted.  Retry once if that doesn't work.

Signed-off-by: Ben Hutchings <redacted>
---
The test always fails for me in a 1 GB VM without this.

Ben.

 tools/testing/selftests/vm/run_vmtests | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/vm/run_vmtests b/tools/testing/selftests/vm/run_vmtests
index 9179ce8..97ed1b2 100755
--- a/tools/testing/selftests/vm/run_vmtests
+++ b/tools/testing/selftests/vm/run_vmtests

@@ -20,13 +20,26 @@ done < /proc/meminfo

 if [ -n "$freepgs" ] && [ -n "$pgsize" ]; then
      nr_hugepgs=`cat /proc/sys/vm/nr_hugepages`
      needpgs=`expr $needmem / $pgsize`
-     if [ $freepgs -lt $needpgs ]; then
+     tries=2
+     while [ $tries -gt 0 ] && [ $freepgs -lt $needpgs ]; do
              lackpgs=$(( $needpgs - $freepgs ))
+             echo 3 > /proc/sys/vm/drop_caches
              echo $(( $lackpgs + $nr_hugepgs )) > /proc/sys/vm/nr_hugepages
              if [ $? -ne 0 ]; then
                      echo "Please run this test as root"
                      exit 1
              fi
+             while read name size unit; do
+                     if [ "$name" = "HugePages_Free:" ]; then
+                             freepgs=$size
+                     fi
+             done < /proc/meminfo
+             tries=$((tries - 1))
+     done
+     if [ $freepgs -lt $needpgs ]; then
+             printf "Not enough huge pages available (%d < %d)\n" \
+                    $freepgs $needpgs
+             exit 1
      fi
 else
      echo "no hugetlbfs support in kernel?"

I know this patch is in -mm and hasn't been merged by Linus yet, but I'm 
wondering why the multiple /proc/sys/vm/drop_caches is helping?  Would it 
simply suffice to put a sleep in there instead or is drop_caches actually 
doing something useful a second time around?

Initially I just retried setting nr_hugepages up to 10 times, which
wasn't sufficient.  Then I added the drop_caches, and after that
setting nr_hugepages tended to worked first time so I reduced the retry
count.  It might not be necessary to retry at all.

Ben.

-- 
Ben Hutchings
All the simple programs have been written, and all the good names taken.