Global virq for low memory situations

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. If the user specifies
zero via the command line, the virq is disabled.

We define a new virq VIRQ_ENOMEM. Potential listeners include
squeezed, xenballoond, or anything else that can be fired through
xencommons.

We error-check the low mem virq against initial available heap (after
dom0 allocation), to avoid firing immediately.

Virq issuing is controlled by a hysteresis algorithm: when memory dips
below a threshold, the virq is issued and the next virq will fire when
memory shrinks another order of magnitude. The virq will not fire
again in the current "band" until memory grows over the next higher
order of magnitude.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Committed-by: Keir Fraser <keir@xen.org>

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 249bb355dddf5eb6cbd244b20cd8cb4eea75b404..7cb16940c8db8d9b929b41eec2e5eb53612ecccc 100644 (file)
--- 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 +301,107 @@ static unsigned long init_node_heap(int node, unsigned long mfn,
     return needed;
 }
 
+/* Default to 64 MiB */
+#define DEFAULT_LOW_MEM_VIRQ    (((paddr_t) 64)   << 20)
+#define MAX_LOW_MEM_VIRQ        (((paddr_t) 1024) << 20)
+
+static paddr_t __read_mostly opt_low_mem_virq = ((paddr_t) -1);
+size_param("low_mem_virq_limit", opt_low_mem_virq);
+
+/* Thresholds to control hysteresis. In pages */
+/* When memory grows above this threshold, reset hysteresis.
+ * -1 initially to not reset until at least one virq issued. */
+static unsigned long low_mem_virq_high      = -1UL;
+/* Threshold at which we issue virq */
+static unsigned long low_mem_virq_th        = 0;
+/* Original threshold after all checks completed */
+static unsigned long low_mem_virq_orig      = 0;
+/* Order for current threshold */
+static unsigned int  low_mem_virq_th_order  = 0;
+
+/* Perform bootstrapping checks and set bounds */
+static void __init setup_low_mem_virq(void)
+{
+    unsigned int order;
+    paddr_t threshold;
+    bool_t halve;
+
+    /* If the user specifies zero, then he/she doesn't want this virq
+     * to ever trigger. */
+    if ( opt_low_mem_virq == 0 )
+    {
+        low_mem_virq_th = -1UL;
+        return;
+    }
+
+    /* If the user did not specify a knob, remember that */
+    halve = (opt_low_mem_virq == ((paddr_t) -1));
+    threshold = halve ? DEFAULT_LOW_MEM_VIRQ : opt_low_mem_virq;
+
+    /* Dom0 has already been allocated by now. So check we won't be
+     * complaining immediately with whatever's left of the heap. */
+    threshold = min(threshold,
+                    ((paddr_t) total_avail_pages) << PAGE_SHIFT);
+
+    /* Then, cap to some predefined maximum */
+    threshold = min(threshold, MAX_LOW_MEM_VIRQ);
+
+    /* If the user specified no knob, and we are at the current available
+     * level, halve the threshold. */
+    if ( halve &&
+         (threshold == (((paddr_t) total_avail_pages) << PAGE_SHIFT)) )
+        threshold >>= 1;
+
+    /* Zero? Have to fire immediately */
+    threshold = max(threshold, (paddr_t) PAGE_SIZE);
+
+    /* Threshold bytes -> pages */
+    low_mem_virq_th = threshold >> PAGE_SHIFT;
+
+    /* Next, round the threshold down to the next order */
+    order = get_order_from_pages(low_mem_virq_th);
+    if ( (1UL << order) > low_mem_virq_th )
+        order--;
+
+    /* Set bounds, ready to go */
+    low_mem_virq_th = low_mem_virq_orig = 1UL << order;
+    low_mem_virq_th_order = order;
+
+    printk("Initial low memory virq threshold set at 0x%lx pages.\n",
+            low_mem_virq_th);
+}
+
+static void check_low_mem_virq(void)
+{
+    if ( unlikely(total_avail_pages <= low_mem_virq_th) )
+    {
+        send_global_virq(VIRQ_ENOMEM);
+
+        /* Update thresholds. Next warning will be when we drop below
+         * next order. However, we wait until we grow beyond one
+         * order above us to complain again at the current order */
+        low_mem_virq_high   = 1UL << (low_mem_virq_th_order + 1);
+        if ( low_mem_virq_th_order > 0 )
+            low_mem_virq_th_order--;
+        low_mem_virq_th     = 1UL << low_mem_virq_th_order;
+        return;
+    }
+
+    if ( unlikely(total_avail_pages >= low_mem_virq_high) )
+    {
+        /* Reset hysteresis. Bring threshold up one order.
+         * If we are back where originally set, set high
+         * threshold to -1 to avoid further growth of
+         * virq threshold. */
+        low_mem_virq_th_order++;
+        low_mem_virq_th = 1UL << low_mem_virq_th_order;
+        if ( low_mem_virq_th == low_mem_virq_orig )
+            low_mem_virq_high = -1UL;
+        else
+            low_mem_virq_high = 1UL << (low_mem_virq_th_order + 2);
+    }
+}
+
 /* Allocate 2^@order contiguous pages. */
 static struct page_info *alloc_heap_pages(
     unsigned int zone_lo, unsigned int zone_hi,
@@ -420,6 +522,8 @@ static struct page_info *alloc_heap_pages(
     total_avail_pages -= request;
     ASSERT(total_avail_pages >= 0);
 
+    check_low_mem_virq();
+
     if ( d != NULL )
         d->last_alloc_node = node;
 
@@ -1022,6 +1126,10 @@ void __init scrub_heap_pages(void)
     }
 
     printk("done.\n");
+
+    /* Now that the heap is initialized, run checks and set bounds
+     * for the low mem virq algorithm. */
+    setup_low_mem_virq();
 }
 
 

diff --git a/xen/include/public/xen.h b/xen/include/public/xen.h
index 041ad0be33d1e7a6c7be4aa220f625eb49136165..b2f6c507b9f7ad973db5dae42793d45945a68f9a 100644 (file)
--- a/xen/include/public/xen.h
+++ b/xen/include/public/xen.h
@@ -158,6 +158,7 @@ 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) Low on heap memory       */
 
 /* Architecture-specific VIRQ definitions. */
 #define VIRQ_ARCH_0    16