Ack: [Xenial][SRU][PATCH] UBUNTU: SAUCE(no-up): If zone is so small that watermarks are the same, stop zone balance.

[Leann Ogasawara]

On 10/11/2016 06:13 PM, Gavin Guo wrote:
> BugLink: http://bugs.launchpad.net/bugs/1518457
> 
> On an AWS t2.micro instance (Xeon E5-2670, 991MiB of memory).
> Occasionally (about once a day), kswapd0 falls into a busy loop and
> spins on 100% CPU usage indefinitely. Reject to do the zone balance
> when the memory is too small.
> 
> Signed-off-by: Dan Streetman <dan.streetman at canonical.com>
> Signed-off-by: Gavin Guo <gavin.guo at canonical.com>

Acked-by: Leann Ogasawara <leann.ogasawara at canonical.com>

> ---
>  mm/vmscan.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 0c114e2b01d3..21755dca16c2 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2959,6 +2959,13 @@ static void age_active_anon(struct zone *zone, struct scan_control *sc)
>  static bool zone_balanced(struct zone *zone, int order,
>  			  unsigned long balance_gap, int classzone_idx)
>  {
> +	/*
> +	 * if zone is so small that watermarks are the same, don't bother trying
> +	 * to balance; kswapd would just spin continuously trying to balance it.
> +	 */
> +	if (low_wmark_pages(zone) == high_wmark_pages(zone))
> +		return true;
> +
>  	if (!zone_watermark_ok_safe(zone, order, high_wmark_pages(zone) +
>  				    balance_gap, classzone_idx))
>  		return false;
>