[Xen-devel] [PATCH v5 4/8] mm: Scrub memory from idle loop

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

Instead of scrubbing pages during guest destruction (from
free_heap_pages()) do this opportunistically, from the idle loop.

We might come to scrub_free_pages()from idle loop while another CPU
uses mapcache override, resulting in a fault while trying to do 
__map_domain_page() in scrub_one_page(). To avoid this, make mapcache
vcpu override a per-cpu variable.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
---
CC: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
---
Changes in v5:
* Added explanation in commit message for making mapcache override VCPU
  a per-cpu variable
* Fixed loop counting in scrub_free_pages()
* Fixed the off-by-one error in setting first_dirty in scrub_free_pages().
* Various style fixes
* Added a comment in node_to_scrub() explaining why it should be OK to
  prevent another CPU from scrubbing a node that ths current CPU temporarily
  claimed. (I decided against using locks there)


 xen/arch/arm/domain.c      |   2 +-
 xen/arch/x86/domain.c      |   2 +-
 xen/arch/x86/domain_page.c |   6 +--
 xen/common/page_alloc.c    | 118 ++++++++++++++++++++++++++++++++++++++++-----
 xen/include/xen/mm.h       |   1 +
 5 files changed, 111 insertions(+), 18 deletions(-)

diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
index 2dc8b0a..d282cd8 100644
--- a/xen/arch/arm/domain.c
+++ b/xen/arch/arm/domain.c
@@ -51,7 +51,7 @@ void idle_loop(void)
         /* Are we here for running vcpu context tasklets, or for idling? */
         if ( unlikely(tasklet_work_to_do(cpu)) )
             do_tasklet();
-        else
+        else if ( !softirq_pending(cpu) && !scrub_free_pages() )
         {
             local_irq_disable();
             if ( cpu_is_haltable(cpu) )
diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index f7873da..71f1ef4 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -122,7 +122,7 @@ static void idle_loop(void)
         /* Are we here for running vcpu context tasklets, or for idling? */
         if ( unlikely(tasklet_work_to_do(cpu)) )
             do_tasklet();
-        else
+        else if ( !softirq_pending(cpu) && !scrub_free_pages() )
             pm_idle();
         do_softirq();
         /*
diff --git a/xen/arch/x86/domain_page.c b/xen/arch/x86/domain_page.c
index 71baede..0783c1e 100644
--- a/xen/arch/x86/domain_page.c
+++ b/xen/arch/x86/domain_page.c
@@ -18,12 +18,12 @@
 #include <asm/hardirq.h>
 #include <asm/setup.h>
 
-static struct vcpu *__read_mostly override;
+static DEFINE_PER_CPU(struct vcpu *, override);
 
 static inline struct vcpu *mapcache_current_vcpu(void)
 {
     /* In the common case we use the mapcache of the running VCPU. */
-    struct vcpu *v = override ?: current;
+    struct vcpu *v = this_cpu(override) ?: current;
 
     /*
      * When current isn't properly set up yet, this is equivalent to
@@ -59,7 +59,7 @@ static inline struct vcpu *mapcache_current_vcpu(void)
 
 void __init mapcache_override_current(struct vcpu *v)
 {
-    override = v;
+    this_cpu(override) = v;
 }
 
 #define mapcache_l2_entry(e) ((e) >> PAGETABLE_ORDER)
diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 9aac196..4e2775f 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -1019,15 +1019,85 @@ static int reserve_offlined_page(struct page_info *head)
     return count;
 }
 
-static void scrub_free_pages(unsigned int node)
+static nodemask_t node_scrubbing;
+
+/*
+ * If get_node is true this will return closest node that needs to be scrubbed,
+ * with appropriate bit in node_scrubbing set.
+ * If get_node is not set, this will return *a* node that needs to be scrubbed.
+ * node_scrubbing bitmask will no be updated.
+ * If no node needs scrubbing then NUMA_NO_NODE is returned.
+ */
+static unsigned int node_to_scrub(bool get_node)
 {
-    struct page_info *pg;
-    unsigned int zone;
+    nodeid_t node = cpu_to_node(smp_processor_id()), local_node;
+    nodeid_t closest = NUMA_NO_NODE;
+    u8 dist, shortest = 0xff;
 
-    ASSERT(spin_is_locked(&heap_lock));
+    if ( node == NUMA_NO_NODE )
+        node = 0;
 
-    if ( !node_need_scrub[node] )
-        return;
+    if ( node_need_scrub[node] &&
+         (!get_node || !node_test_and_set(node, node_scrubbing)) )
+        return node;
+
+    /*
+     * See if there are memory-only nodes that need scrubbing and choose
+     * the closest one.
+     */
+    local_node = node;
+    for ( ; ; )
+    {
+        do {
+            node = cycle_node(node, node_online_map);
+        } while ( !cpumask_empty(&node_to_cpumask(node)) &&
+                  (node != local_node) );
+
+        if ( node == local_node )
+            break;
+
+        /*
+         * Grab the node right away. If we find a closer node later we will
+         * release this one. While there is a chance that another CPU will
+         * not be able to scrub that node when it is searching for scrub work
+         * at the same time it will be able to do so next time it wakes up.
+         * The alternative would be to perform this search under a lock but
+         * then we'd need to take this lock every time we come in here.
+         */
+        if ( node_need_scrub[node] )
+        {
+            if ( !get_node )
+                return node;
+
+            dist = __node_distance(local_node, node);
+            if ( (dist < shortest || closest == NUMA_NO_NODE) &&
+                 !node_test_and_set(node, node_scrubbing) )
+            {
+                if ( closest != NUMA_NO_NODE )
+                    node_clear(closest, node_scrubbing);
+                shortest = dist;
+                closest = node;
+            }
+        }
+    }
+
+    return closest;
+}
+
+bool scrub_free_pages(void)
+{
+    struct page_info *pg;
+    unsigned int zone;
+    unsigned int cpu = smp_processor_id();
+    bool preempt = false;
+    nodeid_t node;
+    unsigned int cnt = 0;
+  
+    node = node_to_scrub(true);
+    if ( node == NUMA_NO_NODE )
+        return false;
+ 
+    spin_lock(&heap_lock);
 
     for ( zone = 0; zone < NR_ZONES; zone++ )
     {
@@ -1050,17 +1120,42 @@ static void scrub_free_pages(unsigned int node)
                         scrub_one_page(&pg[i]);
                         pg[i].count_info &= ~PGC_need_scrub;
                         node_need_scrub[node]--;
+                        cnt += 100; /* scrubbed pages add heavier weight. */
+                    }
+                    else
+                        cnt++;
+
+                    /*
+                     * Scrub a few (8) pages before becoming eligible for
+                     * preemption. But also count non-scrubbing loop iterations
+                     * so that we don't get stuck here with an almost clean
+                     * heap.
+                     */
+                    if ( cnt > 800 && softirq_pending(cpu) )
+                    {
+                        preempt = true;
+                        break;
                     }
                 }
 
-                page_list_del(pg, &heap(node, zone, order));
-                page_list_add_scrub(pg, node, zone, order, INVALID_DIRTY_IDX);
+                if ( i >= (1U << order) - 1 )
+                {
+                    page_list_del(pg, &heap(node, zone, order));
+                    page_list_add_scrub(pg, node, zone, order, 
INVALID_DIRTY_IDX);
+                }
+                else
+                    pg->u.free.first_dirty = i + 1;
 
-                if ( node_need_scrub[node] == 0 )
-                    return;
+                if ( preempt || (node_need_scrub[node] == 0) )
+                    goto out;
             }
         } while ( order-- != 0 );
     }
+
+ out:
+    spin_unlock(&heap_lock);
+    node_clear(node, node_scrubbing);
+    return softirq_pending(cpu) || (node_to_scrub(false) != NUMA_NO_NODE);
 }
 
 /* Free 2^@order set of pages. */
@@ -1175,9 +1270,6 @@ static void free_heap_pages(
     if ( tainted )
         reserve_offlined_page(pg);
 
-    if ( need_scrub )
-        scrub_free_pages(node);
-
     spin_unlock(&heap_lock);
 }
 
diff --git a/xen/include/xen/mm.h b/xen/include/xen/mm.h
index 5f3d84a..a9829c2 100644
--- a/xen/include/xen/mm.h
+++ b/xen/include/xen/mm.h
@@ -150,6 +150,7 @@ void init_xenheap_pages(paddr_t ps, paddr_t pe);
 void xenheap_max_mfn(unsigned long mfn);
 void *alloc_xenheap_pages(unsigned int order, unsigned int memflags);
 void free_xenheap_pages(void *v, unsigned int order);
+bool scrub_free_pages(void);
 #define alloc_xenheap_page() (alloc_xenheap_pages(0,0))
 #define free_xenheap_page(v) (free_xenheap_pages(v,0))
 /* Map machine page range in Xen virtual address space. */
-- 
1.8.3.1


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