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

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Fri, 2009-06-05 at 08:41 -0400, Carsten Schiers wrote:
> Ian, sorry if this question is stupid, but might this patch be the remedy
> for an observation that I made:
> 
>   pv DomU with need for swiotlb memory (kernel option swiotlb=32,force) is
>   not starting with an error "not being able to allocate enough memory", 
>   especially during startup phase where some 7 DomUs are starting, but will
>   succeed to do so if you wait some minutes
> 
> To my current knowledge, swiotlb memory is just memory, although there are
> some further "reuirements", but I assume it's handled the same way.

It is similar but not the same, swiotlb uses XENMEM_exchange (via
xen_create_contiguous_region()) to swap pages for more suitable ones
while ballooning uses XENMEM_populate_physmap to simply allocate more
pages so this patch is unlikely to help.

I can't really explain why swiotlb-suitable pages would be unavailable
while a bunch of domains are starting but some become available later on
(without killing one or more of those domains).

Do you see "Cannot allocate SWIOTLB buffer!" or "No suitable physical
memory available for SWIOTLB buffer!"?

Ian.

> 
> BR,
> Carsten.
> 
> ----- Originalnachricht -----
> Von: Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>
> Gesendet: Fre, 5.6.2009 12:57
> An: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
> Cc: Keir Fraser <Keir.Fraser@xxxxxxxxxxxxx>
> Betreff: [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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