RE: [Xen-devel] linux: try harder to balloon up under memory pressure.

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

Excellent!  I think this explains a weirdness I saw a year
ago when working on self-ballooning, and also a problem
I saw under certain conditions for tmem, that I had
communicated offline to Jeremy last month (because I was
looking at the pvops driver at the time) but never got
around to understanding in detail.

Suggestion:  Perhaps the balloon_timer interval should
be configurable via sysfs?

> -----Original Message-----
> From: Ian Campbell [mailto:Ian.Campbell@xxxxxxxxxxxxx]
> Sent: Friday, June 05, 2009 4:57 AM
> To: xen-devel
> Cc: Keir Fraser
> Subject: [Xen-devel] linux: try harder to balloon up under memory
> pressure.
> 
> 
> Currently if the balloon driver is unable to increase the guest's
> reservation it assumes the failure was due to reaching its full
> allocation, gives up on the ballooning operation and records the limit
> it reached as the "hard limit". The driver will not try again until
> the target is set again (even to the same value).
> 
> However it is possible that ballooning has in fact failed due to
> memory pressure in the host and therefore it is desirable to keep
> attempting to reach the target in case memory becomes available. The
> most likely scenario is that some guests are ballooning down while
> others are ballooning up and therefore there is temporary memory
> pressure while things stabilise. You would not expect a well behaved
> toolstack to ask a domain to balloon to more than its allocation nor
> would you expect it to deliberately over-commit memory by setting
> balloon targets which exceed the total host memory.
> 
> This patch drops the concept of a hard limit and causes the balloon
> driver to retry increasing the reservation on a timer in the same
> manner as when decreasing the reservation.
> 
> Also if we partially succeed in increasing the reservation
> (i.e. receive less pages than we asked for) then we may as well keep
> those pages rather than returning them to Xen.
> 
> This patch is for the 2.6.18-xen tree.
> 
> Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> 
> diff -r fd9c565657b8 drivers/xen/balloon/balloon.c
> --- a/drivers/xen/balloon/balloon.c   Thu Jun 04 16:26:49 2009 +0100
> +++ b/drivers/xen/balloon/balloon.c   Fri Jun 05 11:56:22 2009 +0100
> @@ -188,7 +188,7 @@
>  
>  static unsigned long current_target(void)
>  {
> -     unsigned long target = min(bs.target_pages, bs.hard_limit);
> +     unsigned long target = bs.target_pages;
>       if (target > (bs.current_pages + bs.balloon_low + 
> bs.balloon_high))
>               target = bs.current_pages + bs.balloon_low + 
> bs.balloon_high;
>       return target;
> @@ -255,26 +255,12 @@
>       }
>  
>       set_xen_guest_handle(reservation.extent_start, frame_list);
> -     reservation.nr_extents   = nr_pages;
> -     rc = HYPERVISOR_memory_op(
> -             XENMEM_populate_physmap, &reservation);
> -     if (rc < nr_pages) {
> -             if (rc > 0) {
> -                     int ret;
> +     reservation.nr_extents = nr_pages;
> +     rc = HYPERVISOR_memory_op(XENMEM_populate_physmap, 
> &reservation);
> +     if (rc < 0)
> +             goto out;
>  
> -                     /* We hit the Xen hard limit: reprobe. */
> -                     reservation.nr_extents = rc;
> -                     ret = 
> HYPERVISOR_memory_op(XENMEM_decrease_reservation,
> -                                     &reservation);
> -                     BUG_ON(ret != rc);
> -             }
> -             if (rc >= 0)
> -                     bs.hard_limit = (bs.current_pages + rc -
> -                                      bs.driver_pages);
> -             goto out;
> -     }
> -
> -     for (i = 0; i < nr_pages; i++) {
> +     for (i = 0; i < rc; i++) {
>               page = balloon_retrieve();
>               BUG_ON(page == NULL);
>  
> @@ -302,13 +288,13 @@
>               balloon_free_page(page);
>       }
>  
> -     bs.current_pages += nr_pages;
> +     bs.current_pages += rc;
>       totalram_pages = bs.current_pages;
>  
>   out:
>       balloon_unlock(flags);
>  
> -     return 0;
> +     return rc < 0 ? rc : rc != nr_pages;
>  }
>  
>  static int decrease_reservation(unsigned long nr_pages)
> @@ -420,7 +406,6 @@
>  void balloon_set_new_target(unsigned long target)
>  {
>       /* No need for lock. Not read-modify-write updates. */
> -     bs.hard_limit   = ~0UL;
>       bs.target_pages = max(target, minimum_target());
>       schedule_work(&balloon_worker);
>  }
> @@ -498,17 +483,11 @@
>               "Requested target:   %8lu kB\n"
>               "Low-mem balloon:    %8lu kB\n"
>               "High-mem balloon:   %8lu kB\n"
> -             "Driver pages:       %8lu kB\n"
> -             "Xen hard limit:     ",
> +             "Driver pages:       %8lu kB\n",
>               PAGES2KB(bs.current_pages), PAGES2KB(bs.target_pages), 
>               PAGES2KB(bs.balloon_low), PAGES2KB(bs.balloon_high),
>               PAGES2KB(bs.driver_pages));
>  
> -     if (bs.hard_limit != ~0UL)
> -             len += sprintf(page + len, "%8lu kB\n",
> -                            PAGES2KB(bs.hard_limit));
> -     else
> -             len += sprintf(page + len, "     ??? kB\n");
>  
>       *eof = 1;
>       return len;
> @@ -539,7 +518,6 @@
>       bs.balloon_low   = 0;
>       bs.balloon_high  = 0;
>       bs.driver_pages  = 0UL;
> -     bs.hard_limit    = ~0UL;
>  
>       init_timer(&balloon_timer);
>       balloon_timer.data = 0;
> diff -r fd9c565657b8 drivers/xen/balloon/common.h
> --- a/drivers/xen/balloon/common.h    Thu Jun 04 16:26:49 2009 +0100
> +++ b/drivers/xen/balloon/common.h    Fri Jun 05 11:56:22 2009 +0100
> @@ -35,8 +35,6 @@
>       /* We aim for 'current allocation' == 'target allocation'. */
>       unsigned long current_pages;
>       unsigned long target_pages;
> -     /* We may hit the hard limit in Xen. If we do then we 
> remember it. */
> -     unsigned long hard_limit;
>       /*
>        * Drivers may alter the memory reservation 
> independently, but they
>        * must inform the balloon driver so we avoid hitting 
> the hard limit.
> diff -r fd9c565657b8 drivers/xen/balloon/sysfs.c
> --- a/drivers/xen/balloon/sysfs.c     Thu Jun 04 16:26:49 2009 +0100
> +++ b/drivers/xen/balloon/sysfs.c     Fri Jun 05 11:56:22 2009 +0100
> @@ -53,9 +53,6 @@
>  BALLOON_SHOW(current_kb, "%lu\n", PAGES2KB(bs.current_pages));
>  BALLOON_SHOW(low_kb, "%lu\n", PAGES2KB(bs.balloon_low));
>  BALLOON_SHOW(high_kb, "%lu\n", PAGES2KB(bs.balloon_high));
> -BALLOON_SHOW(hard_limit_kb,
> -          (bs.hard_limit!=~0UL) ? "%lu\n" : "???\n",
> -          (bs.hard_limit!=~0UL) ? PAGES2KB(bs.hard_limit) : 0);
>  BALLOON_SHOW(driver_kb, "%lu\n", PAGES2KB(bs.driver_pages));
>  
>  static ssize_t show_target_kb(struct sys_device *dev, char *buf)
> @@ -96,7 +93,6 @@
>       &attr_current_kb.attr,
>       &attr_low_kb.attr,
>       &attr_high_kb.attr,
> -     &attr_hard_limit_kb.attr,
>       &attr_driver_kb.attr,
>       NULL
>  };
> 
> 
> 
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>

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