Re: [PATCH] drivers: xen: deaggressive selfballoon driver

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Wed, Jan 22, 2014 at 02:57:44PM +0800, Bob Liu wrote:
> Current xen-selfballoon driver is too aggressive which may cause OOM be
> triggered more often. Eg. this bug reported by James:
> https://lkml.org/lkml/2013/11/21/158
> 
> There are two mainly reasons:
> 1) The original goal_page didn't consider some pages used by kernel space, like
> slab pages and pages used by device drivers.
> 
> 2) The balloon driver may not give back memory to guest OS fast enough when the
> workload suddenly aquries a lot of physical memory.
> 
> In both cases, the guest OS will suffer from memory pressure and OOM may
> be triggered.
> 
> The fix is make xen-selfballoon driver not that aggressive by adding extra 10%
> of total ram pages to goal_page.
> It's more valuable to keep the guest system reliable and response faster than
> balloon out these 10% pages to XEN.
> 
> Signed-off-by: Bob Liu <bob.liu@oracle.com>

Looks OK to me.
> ---
>  drivers/xen/xen-selfballoon.c |   22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/drivers/xen/xen-selfballoon.c b/drivers/xen/xen-selfballoon.c
> index 21e18c1..745ad79 100644
> --- a/drivers/xen/xen-selfballoon.c
> +++ b/drivers/xen/xen-selfballoon.c
> @@ -175,6 +175,7 @@ static void frontswap_selfshrink(void)
>  #endif /* CONFIG_FRONTSWAP */
>  
>  #define MB2PAGES(mb)	((mb) << (20 - PAGE_SHIFT))
> +#define PAGES2MB(pages) ((pages) >> (20 - PAGE_SHIFT))
>  
>  /*
>   * Use current balloon size, the goal (vm_committed_as), and hysteresis
> @@ -525,6 +526,7 @@ EXPORT_SYMBOL(register_xen_selfballooning);
>  int xen_selfballoon_init(bool use_selfballooning, bool use_frontswap_selfshrink)
>  {
>  	bool enable = false;
> +	unsigned long reserve_pages;
>  
>  	if (!xen_domain())
>  		return -ENODEV;
> @@ -549,6 +551,26 @@ int xen_selfballoon_init(bool use_selfballooning, bool use_frontswap_selfshrink)
>  	if (!enable)
>  		return -ENODEV;
>  
> +	/*
> +	 * Give selfballoon_reserved_mb a default value(10% of total ram pages)
> +	 * to make selfballoon not so aggressive.
> +	 *
> +	 * There are mainly two reasons:
> +	 * 1) The original goal_page didn't consider some pages used by kernel
> +	 *    space, like slab pages and memory used by device drivers.
> +	 *
> +	 * 2) The balloon driver may not give back memory to guest OS fast
> +	 *    enough when the workload suddenly aquries a lot of physical memory.
> +	 *
> +	 * In both cases, the guest OS will suffer from memory pressure and
> +	 * OOM killer may be triggered.
> +	 * By reserving extra 10% of total ram pages, we can keep the system
> +	 * much more reliably and response faster in some cases.
> +	 */
> +	if (!selfballoon_reserved_mb) {
> +		reserve_pages = totalram_pages / 10;
> +		selfballoon_reserved_mb = PAGES2MB(reserve_pages);
> +	}
>  	schedule_delayed_work(&selfballoon_worker, selfballoon_interval * HZ);
>  
>  	return 0;
> -- 
> 1.7.10.4
>