[PATCH] vhost: Perform memory section dirty scans once per iteration

[Joao Martins]

On setups with one or more virtio-net devices with vhost on,
dirty tracking iteration increases cost the bigger the number
amount of queues are set up e.g. on idle guests migration the
following is observed with virtio-net with vhost=on:

48 queues -> 78.11%  [.] vhost_dev_sync_region.isra.13
8 queues -> 40.50%   [.] vhost_dev_sync_region.isra.13
1 queue -> 6.89%     [.] vhost_dev_sync_region.isra.13
2 devices, 1 queue -> 18.60%  [.] vhost_dev_sync_region.isra.14

With high memory rates the symptom is lack of convergence as soon
as it has a vhost device with a sufficiently high number of queues,
the sufficient number of vhost devices.

On every migration iteration (every 100msecs) it will redundantly
query the *shared log* the number of queues configured with vhost
that exist in the guest. For the virtqueue data, this is necessary,
but not for the memory sections which are the same. So
essentially we end up scanning the dirty log too often.

To fix that, select a vhost device responsible for scanning the
log with regards to memory sections dirty tracking. It is selected
when we enable the logger (during migration) and cleared when we
disable the logger.

The real problem, however, is exactly that: a device per vhost worker/qp,
when there should be a device representing a netdev (for N vhost workers).
Given this problem exists for any Qemu these days, figured a simpler
solution is better to increase stable tree's coverage; thus don't
change the device model of sw vhost to fix this "over log scan" issue.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
---
I am not fully sure the heuristic captures the myriad of different vhost
devices -- I think so. IIUC, the log is always shared, it's just whether
it's qemu head memory or via /dev/shm when other processes want to
access it.
---
 hw/virtio/vhost.c | 44 ++++++++++++++++++++++++++++++++++++++------
 1 file changed, 38 insertions(+), 6 deletions(-)

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index e2f6ffb446b7..70646c2b533c 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -44,6 +44,7 @@
 
 static struct vhost_log *vhost_log;
 static struct vhost_log *vhost_log_shm;
+static struct vhost_dev *vhost_log_dev;
 
 static unsigned int used_memslots;
 static QLIST_HEAD(, vhost_dev) vhost_devices =
@@ -124,6 +125,21 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev)
     }
 }
 
+static bool vhost_log_dev_enabled(struct vhost_dev *dev)
+{
+    return dev == vhost_log_dev;
+}
+
+static void vhost_log_set_dev(struct vhost_dev *dev)
+{
+    vhost_log_dev = dev;
+}
+
+static bool vhost_log_dev_is_set(void)
+{
+    return vhost_log_dev != NULL;
+}
+
 static int vhost_sync_dirty_bitmap(struct vhost_dev *dev,
                                    MemoryRegionSection *section,
                                    hwaddr first,
@@ -141,13 +157,16 @@ static int vhost_sync_dirty_bitmap(struct vhost_dev *dev,
     start_addr = MAX(first, start_addr);
     end_addr = MIN(last, end_addr);
 
-    for (i = 0; i < dev->mem->nregions; ++i) {
-        struct vhost_memory_region *reg = dev->mem->regions + i;
-        vhost_dev_sync_region(dev, section, start_addr, end_addr,
-                              reg->guest_phys_addr,
-                              range_get_last(reg->guest_phys_addr,
-                                             reg->memory_size));
+    if (vhost_log_dev_enabled(dev)) {
+        for (i = 0; i < dev->mem->nregions; ++i) {
+            struct vhost_memory_region *reg = dev->mem->regions + i;
+            vhost_dev_sync_region(dev, section, start_addr, end_addr,
+                                  reg->guest_phys_addr,
+                                  range_get_last(reg->guest_phys_addr,
+                                                 reg->memory_size));
+        }
     }
+
     for (i = 0; i < dev->nvqs; ++i) {
         struct vhost_virtqueue *vq = dev->vqs + i;
 
@@ -943,6 +962,19 @@ static int vhost_dev_set_log(struct vhost_dev *dev, bool 
enable_log)
             goto err_vq;
         }
     }
+
+    /*
+     * During migration devices can't be removed, so we at log start
+     * we select our vhost_device that will scan the memory sections
+     * and skip for the others. This is possible because the log is shared
+     * amongst all vhost devices.
+     */
+    if (enable_log && !vhost_log_dev_is_set()) {
+        vhost_log_set_dev(dev);
+    } else if (!enable_log) {
+        vhost_log_set_dev(NULL);
+    }
+
     return 0;
 err_vq:
     for (; i >= 0; --i) {
-- 
2.39.3