[Xen-devel] Re: [PATCH v2] xen: Fix selfballooning and ensure it doesn't go too far

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 27/09/11 16:03, Dan Magenheimer wrote:
> Note: This patch is also now in a git tree at:
> 
> git://oss.oracle.com/git/djm/tmem.git#selfballoon-fix-v2
> 
> The balloon driver's "current_pages" is very different from
> totalram_pages.  Self-ballooning needs to be driven by
> the latter.

I don't think this part of the change makes any difference. It looks like it
rearranges the maths without changing the end result (other than
slightly increasing the rate of change).

I think this (partial, untested) patch is equivalent:

diff --git a/drivers/xen/xen-selfballoon.c b/drivers/xen/xen-selfballoon.c
index 1b4afd8..b207e89 100644
--- a/drivers/xen/xen-selfballoon.c
+++ b/drivers/xen/xen-selfballoon.c
@@ -194,14 +194,15 @@ __setup("selfballooning", xen_selfballooning_setup);
  */
 static void selfballoon_process(struct work_struct *work)
 {
-       unsigned long cur_pages, goal_pages, tgt_pages;
+       unsigned long cur_pages, goal_pages, tgt_pages, rsvd_pages, floor_pages;
        bool reset_timer = false;
 
        if (xen_selfballooning_enabled) {
                cur_pages = balloon_stats.current_pages;
                tgt_pages = cur_pages; /* default is no change */
-               goal_pages = percpu_counter_read_positive(&vm_committed_as) +
-                       balloon_stats.current_pages - totalram_pages;
+               rsvd_pages = cur_pages - totalram_pages;
+               goal_pages = percpu_counter_read_positive(&vm_committed_as)
+                       + rsvd_pages;
 #ifdef CONFIG_FRONTSWAP
                /* allow space for frontswap pages to be repatriated */
                if (frontswap_selfshrinking && frontswap_enabled)
@@ -216,6 +217,11 @@ static void selfballoon_process(struct work_struct *work)
                                ((goal_pages - cur_pages) /
                                  selfballoon_uphysteresis);
                /* else if cur_pages == goal_pages, no change */
+               floor_pages = rsvd_pages + totalreserve_pages +
+                       ((roundup_pow_of_two(max_pfn) / 128) *
+                               selfballoon_safety_margin);
+               if (tgt_pages < floor_pages)
+                       tgt_pages = floor_pages;
                balloon_set_new_target(tgt_pages);
                reset_timer = true;
        }

> Also, Committed_AS doesn't account for pages
> used by the kernel so enforce a floor for when there are little
> or no user-space threads using memory.  The floor function
> includes a "safety margin" tuneable in case we discover later
> that the floor function is still too aggressive in some workloads,
> though likely it will not be needed.

The sysfs file isn't documented (but then neither are any of the other
(self-)balloon driver sysfs files).

I don't think "safety_margin" is the right name.  Perhaps,
"min_reservation_ratio" or something like that?

David

> Changes since version 1:
> - tuneable safety margin added
> 
> [v2: konrad.wilk@xxxxxxxxxx: make safety margin tuneable]
> 
> Signed-off-by: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
> 
> diff --git a/drivers/xen/xen-selfballoon.c b/drivers/xen/xen-selfballoon.c
> index 6ea852e..ceaada6 100644
> --- a/drivers/xen/xen-selfballoon.c
> +++ b/drivers/xen/xen-selfballoon.c
> @@ -93,6 +93,12 @@ static unsigned int selfballoon_uphysteresis __read_mostly 
> = 1;
>  /* In HZ, controls frequency of worker invocation. */
>  static unsigned int selfballoon_interval __read_mostly = 5;
>  
> +/*
> + * Scale factor for safety margin for minimum selfballooning target for 
> balloon.
> + * Default adds about 3% (4/128) of max_pfn.
> + */
> +static unsigned int selfballoon_safety_margin = 4;
> +
>  static void selfballoon_process(struct work_struct *work);
>  static DECLARE_DELAYED_WORK(selfballoon_worker, selfballoon_process);
>  
> @@ -195,14 +201,13 @@ __setup("selfballooning", xen_selfballooning_setup);
>   */
>  static void selfballoon_process(struct work_struct *work)
>  {
> -     unsigned long cur_pages, goal_pages, tgt_pages;
> +     unsigned long cur_pages, goal_pages, tgt_pages, floor_pages;
>       bool reset_timer = false;
>  
>       if (xen_selfballooning_enabled) {
> -             cur_pages = balloon_stats.current_pages;
> +             cur_pages = totalram_pages;
>               tgt_pages = cur_pages; /* default is no change */
> -             goal_pages = percpu_counter_read_positive(&vm_committed_as) +
> -                     balloon_stats.current_pages - totalram_pages;
> +             goal_pages = percpu_counter_read_positive(&vm_committed_as);
>  #ifdef CONFIG_FRONTSWAP
>               /* allow space for frontswap pages to be repatriated */
>               if (frontswap_selfshrinking && frontswap_enabled)
> @@ -217,7 +222,13 @@ static void selfballoon_process(struct work_struct *work)
>                               ((goal_pages - cur_pages) /
>                                 selfballoon_uphysteresis);
>               /* else if cur_pages == goal_pages, no change */
> -             balloon_set_new_target(tgt_pages);
> +             floor_pages = totalreserve_pages +
> +                     ((roundup_pow_of_two(max_pfn) / 128) *
> +                             selfballoon_safety_margin);
> +             if (tgt_pages < floor_pages)
> +                     tgt_pages = floor_pages;
> +             balloon_set_new_target(tgt_pages +
> +                     balloon_stats.current_pages - totalram_pages);
>               reset_timer = true;
>       }
>  #ifdef CONFIG_FRONTSWAP
> @@ -340,6 +351,30 @@ static ssize_t store_selfballoon_uphys(struct sys_device 
> *dev,
>  static SYSDEV_ATTR(selfballoon_uphysteresis, S_IRUGO | S_IWUSR,
>                  show_selfballoon_uphys, store_selfballoon_uphys);
>  
> +SELFBALLOON_SHOW(selfballoon_safety_margin, "%d\n", 
> selfballoon_safety_margin);
> +
> +static ssize_t store_selfballoon_safety_margin(struct sys_device *dev,
> +                                            struct sysdev_attribute *attr,
> +                                            const char *buf,
> +                                            size_t count)
> +{
> +     unsigned long val;
> +     int err;
> +
> +     if (!capable(CAP_SYS_ADMIN))
> +             return -EPERM;
> +     err = strict_strtoul(buf, 10, &val);
> +     if (err || val == 0 || val > 128)
> +             return -EINVAL;
> +     selfballoon_safety_margin = val;
> +     return count;
> +}
> +
> +static SYSDEV_ATTR(selfballoon_safety_margin, S_IRUGO | S_IWUSR,
> +                show_selfballoon_safety_margin,
> +                store_selfballoon_safety_margin);
> +
> +
>  #ifdef CONFIG_FRONTSWAP
>  SELFBALLOON_SHOW(frontswap_selfshrinking, "%d\n", frontswap_selfshrinking);
>  
> @@ -421,6 +456,7 @@ static struct attribute *selfballoon_attrs[] = {
>       &attr_selfballoon_interval.attr,
>       &attr_selfballoon_downhysteresis.attr,
>       &attr_selfballoon_uphysteresis.attr,
> +     &attr_selfballoon_safety_margin.attr,
>  #ifdef CONFIG_FRONTSWAP
>       &attr_frontswap_selfshrinking.attr,
>       &attr_frontswap_hysteresis.attr,


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