[Xen-devel] [PATCH 1 of 2 RFC] xen, pod: Zero-check recently populated pages (checklast)

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

When demand-populating pages due to guest accesses, check recently populated
pages to see if we can reclaim them for the cache.  This should keep the PoD
cache filled when the start-of-day scrubber is going through.

The number 128 was chosen by experiment.  Windows does its page
scrubbing in parallel; while a small nubmer like 4 works well for
single VMs, it breaks down as multiple vcpus are scrubbing different
pages in parallel.  Increasing to 128 works well for higher numbers of
vcpus.

Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

diff --git a/xen/arch/x86/mm/p2m-pod.c b/xen/arch/x86/mm/p2m-pod.c
--- a/xen/arch/x86/mm/p2m-pod.c
+++ b/xen/arch/x86/mm/p2m-pod.c
@@ -909,6 +909,26 @@ p2m_pod_emergency_sweep_super(struct p2m
     p2m->pod.reclaim_super = i ? i - SUPERPAGE_PAGES : 0;
 }
 
+/* When populating a new superpage, look at recently populated superpages
+ * hoping that they've been zeroed.  This will snap up zeroed pages as soon as 
+ * the guest OS is done with them. */
+static void
+p2m_pod_check_last_super(struct p2m_domain *p2m, unsigned long gfn_aligned)
+{
+    unsigned long check_gfn;
+
+    ASSERT(p2m->pod.last_populated_index < POD_HISTORY_MAX);
+
+    check_gfn = p2m->pod.last_populated[p2m->pod.last_populated_index];
+
+    p2m->pod.last_populated[p2m->pod.last_populated_index] = gfn_aligned;
+
+    p2m->pod.last_populated_index = ( p2m->pod.last_populated_index + 1 ) % 
POD_HISTORY_MAX;
+
+    p2m->pod.reclaim_super += p2m_pod_zero_check_superpage(p2m, check_gfn);
+}
+
+
 #define POD_SWEEP_STRIDE  16
 static void
 p2m_pod_emergency_sweep(struct p2m_domain *p2m)
@@ -1066,6 +1086,12 @@ p2m_pod_demand_populate(struct p2m_domai
         __trace_var(TRC_MEM_POD_POPULATE, 0, sizeof(t), &t);
     }
 
+    /* Check the last guest demand-populate */
+    if ( p2m->pod.entry_count > p2m->pod.count 
+         && order == 9
+         && q & P2M_ALLOC )
+        p2m_pod_check_last_super(p2m, gfn_aligned);
+
     pod_unlock(p2m);
     return 0;
 out_of_memory:
diff --git a/xen/include/asm-x86/p2m.h b/xen/include/asm-x86/p2m.h
--- a/xen/include/asm-x86/p2m.h
+++ b/xen/include/asm-x86/p2m.h
@@ -287,6 +287,9 @@ struct p2m_domain {
         unsigned         reclaim_super; /* Last gpfn of a scan */
         unsigned         reclaim_single; /* Last gpfn of a scan */
         unsigned         max_guest;    /* gpfn of max guest demand-populate */
+#define POD_HISTORY_MAX 128
+        unsigned         last_populated[POD_HISTORY_MAX]; /* gpfn of last 
guest page demand-populated */
+        int              last_populated_index;
         mm_lock_t        lock;         /* Locking of private pod structs,   *
                                         * not relying on the p2m lock.      */
     } pod;

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