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@citrix.com>

diff --git a/drivers/xen/balloon/balloon.c b/drivers/xen/balloon/balloon.c
index f2fbf318458edcd4cb5790145ed4a07759a2f626..0bb27345ac806abf2fab6559925bb6860d2e2bba 100644 (file)
--- a/drivers/xen/balloon/balloon.c
+++ b/drivers/xen/balloon/balloon.c
@@ -188,7 +188,7 @@ static void balloon_alarm(unsigned long unused)
 
 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 long nr_pages)
        }
 
        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 long nr_pages)
                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 @@ static void balloon_process(void *unused)
 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 **start, off_t off,
                "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 --git a/drivers/xen/balloon/common.h b/drivers/xen/balloon/common.h
index 4496d215e2ea82cebe4eacd83a4aa0a1716b0ae0..cf5d86f17a3bd877d6763c6a75538e4fa05e6c04 100644 (file)
--- a/drivers/xen/balloon/common.h
+++ b/drivers/xen/balloon/common.h
@@ -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 --git a/drivers/xen/balloon/sysfs.c b/drivers/xen/balloon/sysfs.c
index 16411cbcca2ac11f375d94f26c2d404c8394a04f..5aceedff54525491a0af40632e908ac37c8795c1 100644 (file)
--- a/drivers/xen/balloon/sysfs.c
+++ b/drivers/xen/balloon/sysfs.c
@@ -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 @@ static struct attribute *balloon_info_attrs[] = {
        &attr_current_kb.attr,
        &attr_low_kb.attr,
        &attr_high_kb.attr,
-       &attr_hard_limit_kb.attr,
        &attr_driver_kb.attr,
        NULL
 };