[PATCH] virtio-blk: iothread-vq-mapping coroutine pool sizing

[Stefan Hajnoczi]

It is possible to hit the sysctl vm.max_map_count limit when the
coroutine pool size becomes large. Each coroutine requires two mappings
(one for the stack and one for the guard page). QEMU can crash with
"failed to set up stack guard page" or "failed to allocate memory for
stack" when this happens.

Coroutine pool sizing is simple when there is only one AioContext: sum
up all I/O requests across all virtqueues.

When the iothread-vq-mapping option is used we should calculate tighter
bounds: take the maximum number of the number of I/O requests across all
virtqueues. This number is lower than simply summing all virtqueues when
only a subset of the virtqueues is handled by each AioContext.

This is not a solution to hitting vm.max_map_count, but it helps. A
guest with 64 vCPUs (hence 64 virtqueues) across 4 IOThreads with one
iothread-vq-mapping virtio-blk device and a root disk without goes from
pool_max_size 16,448 to 10,304.

Reported-by: Sanjay Rao <srao@redhat.com>
Reported-by: Boaz Ben Shabat <bbenshab@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 include/hw/virtio/virtio-blk.h |  2 ++
 hw/block/virtio-blk.c          | 34 ++++++++++++++++++++++++++++++++--
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
index 5c14110c4b..ac29700ad4 100644
--- a/include/hw/virtio/virtio-blk.h
+++ b/include/hw/virtio/virtio-blk.h
@@ -74,6 +74,8 @@ struct VirtIOBlock {
     uint64_t host_features;
     size_t config_size;
     BlockRAMRegistrar blk_ram_registrar;
+
+    unsigned coroutine_pool_size;
 };
 
 typedef struct VirtIOBlockReq {
diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 738cb2ac36..0a14b2b175 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -1957,6 +1957,35 @@ static void virtio_blk_stop_ioeventfd(VirtIODevice *vdev)
     s->ioeventfd_stopping = false;
 }
 
+/* Increase the coroutine pool size to include our I/O requests */
+static void virtio_blk_inc_coroutine_pool_size(VirtIOBlock *s)
+{
+    VirtIOBlkConf *conf = &s->conf;
+    unsigned max_requests = 0;
+
+    /* Tracks the total number of requests for AioContext */
+    g_autoptr(GHashTable) counters = g_hash_table_new(NULL, NULL);
+
+    /* Call this function after setting up vq_aio_context[] */
+    assert(s->vq_aio_context);
+
+    for (unsigned i = 0; i < conf->num_queues; i++) {
+        AioContext *ctx = s->vq_aio_context[i];
+        unsigned n = GPOINTER_TO_UINT(g_hash_table_lookup(counters, ctx));
+
+        n += conf->queue_size / 2; /* this is a heuristic */
+
+        g_hash_table_insert(counters, ctx, GUINT_TO_POINTER(n));
+
+        if (n > max_requests) {
+            max_requests = n;
+        }
+    }
+
+    qemu_coroutine_inc_pool_size(max_requests);
+    s->coroutine_pool_size = max_requests; /* stash it for ->unrealize() */
+}
+
 static void virtio_blk_device_realize(DeviceState *dev, Error **errp)
 {
     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
@@ -2048,7 +2077,6 @@ static void virtio_blk_device_realize(DeviceState *dev, 
Error **errp)
     for (i = 0; i < conf->num_queues; i++) {
         virtio_add_queue(vdev, conf->queue_size, virtio_blk_handle_output);
     }
-    qemu_coroutine_inc_pool_size(conf->num_queues * conf->queue_size / 2);
 
     /* Don't start ioeventfd if transport does not support notifiers. */
     if (!virtio_device_ioeventfd_enabled(vdev)) {
@@ -2065,6 +2093,8 @@ static void virtio_blk_device_realize(DeviceState *dev, 
Error **errp)
         return;
     }
 
+    virtio_blk_inc_coroutine_pool_size(s);
+
     /*
      * This must be after virtio_init() so virtio_blk_dma_restart_cb() gets
      * called after ->start_ioeventfd() has already set blk's AioContext.
@@ -2096,7 +2126,7 @@ static void virtio_blk_device_unrealize(DeviceState *dev)
     for (i = 0; i < conf->num_queues; i++) {
         virtio_del_queue(vdev, i);
     }
-    qemu_coroutine_dec_pool_size(conf->num_queues * conf->queue_size / 2);
+    qemu_coroutine_dec_pool_size(s->coroutine_pool_size);
     qemu_mutex_destroy(&s->rq_lock);
     blk_ram_registrar_destroy(&s->blk_ram_registrar);
     qemu_del_vm_change_state_handler(s->change);
-- 
2.44.0