Re: [PATCH selftests 5/6] selftests: vm: Try harder to allocate huge pages

[David Rientjes <rientjes@google.com>]

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On Sat, 31 Oct 2015, Ben Hutchings wrote:

> 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.A A Retry once if that doesn't work.
> 
> Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
> ---
> The test always fails for me in a 1 GB VM without this.
> 
> Ben.
> 
> A tools/testing/selftests/vm/run_vmtests | 15 ++++++++++++++-
> A 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
> A if [ -n "$freepgs" ] && [ -n "$pgsize" ]; then
> A 	nr_hugepgs=`cat /proc/sys/vm/nr_hugepages`
> A 	needpgs=`expr $needmem / $pgsize`
> -	if [ $freepgs -lt $needpgs ]; then
> +	tries=2
> +	while [ $tries -gt 0 ] && [ $freepgs -lt $needpgs ]; do
> A 		lackpgs=$(( $needpgs - $freepgs ))
> +		echo 3 > /proc/sys/vm/drop_caches
> A 		echo $(( $lackpgs + $nr_hugepgs )) > /proc/sys/vm/nr_hugepages
> A 		if [ $? -ne 0 ]; then
> A 			echo "Please run this test as root"
> A 			exit 1
> A 		fi
> +		while read name size unit; do
> +			if [ "$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" \
> +		A A A A A A A $freepgs $needpgs
> +		exit 1
> A 	fi
> A else
> A 	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?