[PATCH 4 of 4] Global virq for low memory situations

[Andres Lagar-Cavilla <andres@lagarcavilla.org>]

 xen/common/page_alloc.c  |  10 ++++++++++
 xen/include/public/xen.h |   1 +
 2 files changed, 11 insertions(+), 0 deletions(-)


When a low memory threshold on the Xen heap is reached, we fire a global dom0
virq. If someone's listening, they can free up some more memory. The low
threshold is configurable via the command line token 'low_mem_virq_limit", and
defaults to 64MiB. We define a new virq VIRQ_ENOMEM. Potential listeners
include squeezed, xenballoond, or anything else that can be fired through
xencommons.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>

diff -r 407b5ac709aa -r 65c15f3c9611 xen/common/page_alloc.c
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -35,6 +35,7 @@
 #include <xen/perfc.h>
 #include <xen/numa.h>
 #include <xen/nodemask.h>
+#include <xen/event.h>
 #include <xen/tmem.h>
 #include <xen/tmem_xen.h>
 #include <public/sysctl.h>
@@ -300,6 +301,12 @@ static unsigned long init_node_heap(int 
     return needed;
 }
 
+/* Default to 64 MiB */
+#define DEFAULT_LOW_MEM_VIRQ_MIB    64
+static unsigned long long __read_mostly opt_low_mem_virq = 
+                                        (DEFAULT_LOW_MEM_VIRQ_MIB << 20);
+size_param("low_mem_virq_limit", opt_low_mem_virq);
+
 /* Allocate 2^@order contiguous pages. */
 static struct page_info *alloc_heap_pages(
     unsigned int zone_lo, unsigned int zone_hi,
@@ -420,6 +427,9 @@ static struct page_info *alloc_heap_page
     total_avail_pages -= request;
     ASSERT(total_avail_pages >= 0);
 
+    if ( (total_avail_pages << PAGE_SHIFT) <= opt_low_mem_virq )
+        send_global_virq(VIRQ_ENOMEM);
+
     if ( d != NULL )
         d->last_alloc_node = node;
 
diff -r 407b5ac709aa -r 65c15f3c9611 xen/include/public/xen.h
--- a/xen/include/public/xen.h
+++ b/xen/include/public/xen.h
@@ -157,6 +157,7 @@ DEFINE_XEN_GUEST_HANDLE(xen_pfn_t);
 #define VIRQ_PCPU_STATE 9  /* G. (DOM0) PCPU state changed                   */
 #define VIRQ_MEM_EVENT  10 /* G. (DOM0) A memory event has occured           */
 #define VIRQ_XC_RESERVED 11 /* G. Reserved for XenClient                     */
+#define VIRQ_ENOMEM     12 /* G. (DOM0) Dangerously low on heap memory       */
 
 /* Architecture-specific VIRQ definitions. */
 #define VIRQ_ARCH_0    16