[PATCH] base-files: flush kernel memory cache during sysupgrade

[Hannu Nyman]

Flush kernel memory caches during sysupgrade in order
to mitigate the impact from memory consumption spikes
in low-RAM devices.

This may help to prevent sysupgrade causing a reboot
before the actual flashing starts.

Signed-off-by: Hannu Nyman <hannu.nyman at iki.fi>
---

I have noticed this to help in 64 MB WNDR3700v2, where sysupgrade
typically has failed when the router has been running some time,
but succeeded when sysupgrade is done right after a reboot.

The cache flushing is non-destructive and as the router is going
to sysupgrade, there aren't "long-term" performance issues.

Reference to mailing list discussion in 
http://lists.openwrt.org/pipermail/openwrt-devel/2020-November/032266.html

Kernel documentation:
https://www.kernel.org/doc/Documentation/sysctl/vm.txt


 package/base-files/files/lib/upgrade/common.sh | 1 +
 package/base-files/files/lib/upgrade/stage2    | 1 +
 2 files changed, 2 insertions(+)

diff --git a/package/base-files/files/lib/upgrade/common.sh b/package/base-files/files/lib/upgrade/common.sh
index a5c27dc2fb..b44a5998f4 100644
--- a/package/base-files/files/lib/upgrade/common.sh
+++ b/package/base-files/files/lib/upgrade/common.sh
@@ -297,6 +297,7 @@ indicate_upgrade() {
 # $(2): (optional) pipe command to extract firmware, e.g. dd bs=n skip=m
 default_do_upgrade() {
 	sync
+	echo 3 > /proc/sys/vm/drop_caches
 	if [ -n "$UPGRADE_BACKUP" ]; then
 		get_image "$1" "$2" | mtd $MTD_ARGS $MTD_CONFIG_ARGS -j "$UPGRADE_BACKUP" write - "${PART_NAME:-image}"
 	else
diff --git a/package/base-files/files/lib/upgrade/stage2 b/package/base-files/files/lib/upgrade/stage2
index c7629c383f..23d356a447 100755
--- a/package/base-files/files/lib/upgrade/stage2
+++ b/package/base-files/files/lib/upgrade/stage2
@@ -123,6 +123,7 @@ kill_remaining KILL 1
 
 sleep 1
 
+echo 3 > /proc/sys/vm/drop_caches
 
 if [ -n "$IMAGE" ] && type 'platform_pre_upgrade' >/dev/null 2>/dev/null; then
 	platform_pre_upgrade "$IMAGE"
-- 
2.27.0