[Xen-changelog] [linux-2.6.18-xen] balloon: try harder to balloon up under memory pressure.

["Xen patchbot-linux-2.6.18-xen" <patchbot-linux-2.6.18-xen@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1244206880 -3600
# Node ID 329ea0ccb344c636e918cc3cd2677c24b03af5bd
# Parent  f59c5daed527f3a7ddf1270480ec63028c206f31
balloon: 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.

Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
---
 drivers/xen/balloon/balloon.c |   40 +++++++++-------------------------------
 drivers/xen/balloon/common.h  |    2 --
 drivers/xen/balloon/sysfs.c   |    4 ----
 3 files changed, 9 insertions(+), 37 deletions(-)

diff -r f59c5daed527 -r 329ea0ccb344 drivers/xen/balloon/balloon.c
--- a/drivers/xen/balloon/balloon.c     Thu Jun 04 10:46:54 2009 +0100
+++ b/drivers/xen/balloon/balloon.c     Fri Jun 05 14:01:20 2009 +0100
@@ -188,7 +188,7 @@ static void balloon_alarm(unsigned long 
 
 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 @@ static int increase_reservation(unsigned
        }
 
        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;
-
-                       /* 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);
+       reservation.nr_extents = nr_pages;
+       rc = HYPERVISOR_memory_op(XENMEM_populate_physmap, &reservation);
+       if (rc < 0)
                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 @@ static int increase_reservation(unsigned
                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 lon
 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 @@ static int balloon_read(char *page, char
                "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 @@ static int __init balloon_init(void)
        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 f59c5daed527 -r 329ea0ccb344 drivers/xen/balloon/common.h
--- a/drivers/xen/balloon/common.h      Thu Jun 04 10:46:54 2009 +0100
+++ b/drivers/xen/balloon/common.h      Fri Jun 05 14:01:20 2009 +0100
@@ -35,8 +35,6 @@ struct balloon_stats {
        /* 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 f59c5daed527 -r 329ea0ccb344 drivers/xen/balloon/sysfs.c
--- a/drivers/xen/balloon/sysfs.c       Thu Jun 04 10:46:54 2009 +0100
+++ b/drivers/xen/balloon/sysfs.c       Fri Jun 05 14:01:20 2009 +0100
@@ -53,9 +53,6 @@ BALLOON_SHOW(current_kb, "%lu\n", PAGES2
 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 @@ static struct attribute *balloon_info_at
        &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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