migration: Maintain postcopy faulted addresses

Maintain a list of faulted addresses on the destination host for which we're
waiting on.  This is implemented using a GTree rather than a real list to make
sure even there're plenty of vCPUs/threads that are faulting, the lookup will
still be fast with O(log(N)) (because we'll do that after placing each page).
It should bring a slight overhead, but ideally that shouldn't be a big problem
simply because in most cases the requested page list will be short.

Actually we did similar things for postcopy blocktime measurements.  This patch
didn't use that simply because:

  (1) blocktime measurement is towards vcpu threads only, but here we need to
      record all faulted addresses, including main thread and external
      thread (like, DPDK via vhost-user).

  (2) blocktime measurement will require UFFD_FEATURE_THREAD_ID, but here we
      don't want to add that extra dependency on the kernel version since not
      necessary.  E.g., we don't need to know which thread faulted on which
      page, we also don't care about multiple threads faulting on the same
      page.  But we only care about what addresses are faulted so waiting for a
      page copying from src.

  (3) blocktime measurement is not enabled by default.  However we need this by
      default especially for postcopy recover.

Another thing to mention is that this patch introduced a new mutex to serialize
the receivedmap and the page_requested tree, however that serialization does
not cover other procedures like UFFDIO_COPY.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20201021212721.440373-4-peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

diff --git a/migration/migration.c b/migration/migration.c
index ed122ff8d9c5fe93d9331f18cf4cd914b8ded202..5421042d4a8869c160f2653b48a4483f91a775c4 100644 (file)
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -143,6 +143,13 @@ static int migration_maybe_pause(MigrationState *s,
                                  int new_state);
 static void migrate_fd_cancel(MigrationState *s);
 
+static gint page_request_addr_cmp(gconstpointer ap, gconstpointer bp)
+{
+    uintptr_t a = (uintptr_t) ap, b = (uintptr_t) bp;
+
+    return (a > b) - (a < b);
+}
+
 void migration_object_init(void)
 {
     MachineState *ms = MACHINE(qdev_get_machine());
@@ -165,6 +172,8 @@ void migration_object_init(void)
     qemu_event_init(&current_incoming->main_thread_load_event, false);
     qemu_sem_init(&current_incoming->postcopy_pause_sem_dst, 0);
     qemu_sem_init(&current_incoming->postcopy_pause_sem_fault, 0);
+    qemu_mutex_init(&current_incoming->page_request_mutex);
+    current_incoming->page_requested = g_tree_new(page_request_addr_cmp);
 
     if (!migration_object_check(current_migration, &err)) {
         error_report_err(err);
@@ -240,6 +249,11 @@ void migration_incoming_state_destroy(void)
 
     qemu_event_reset(&mis->main_thread_load_event);
 
+    if (mis->page_requested) {
+        g_tree_destroy(mis->page_requested);
+        mis->page_requested = NULL;
+    }
+
     if (mis->socket_address_list) {
         qapi_free_SocketAddressList(mis->socket_address_list);
         mis->socket_address_list = NULL;
@@ -354,8 +368,33 @@ int migrate_send_rp_message_req_pages(MigrationIncomingState *mis,
 }
 
 int migrate_send_rp_req_pages(MigrationIncomingState *mis,
-                              RAMBlock *rb, ram_addr_t start)
+                              RAMBlock *rb, ram_addr_t start, uint64_t haddr)
 {
+    void *aligned = (void *)(uintptr_t)(haddr & (-qemu_ram_pagesize(rb)));
+    bool received;
+
+    WITH_QEMU_LOCK_GUARD(&mis->page_request_mutex) {
+        received = ramblock_recv_bitmap_test_byte_offset(rb, start);
+        if (!received && !g_tree_lookup(mis->page_requested, aligned)) {
+            /*
+             * The page has not been received, and it's not yet in the page
+             * request list.  Queue it.  Set the value of element to 1, so that
+             * things like g_tree_lookup() will return TRUE (1) when found.
+             */
+            g_tree_insert(mis->page_requested, aligned, (gpointer)1);
+            mis->page_requested_count++;
+            trace_postcopy_page_req_add(aligned, mis->page_requested_count);
+        }
+    }
+
+    /*
+     * If the page is there, skip sending the message.  We don't even need the
+     * lock because as long as the page arrived, it'll be there forever.
+     */
+    if (received) {
+        return 0;
+    }
+
     return migrate_send_rp_message_req_pages(mis, rb, start);
 }
 

diff --git a/migration/migration.h b/migration/migration.h
index 9c1cba98d67205d24aecc8be233745cc2edf189b..d096b77f7469a52f0ef7ed3debb506338ceffa8a 100644 (file)
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -104,6 +104,23 @@ struct MigrationIncomingState {
 
     /* List of listening socket addresses  */
     SocketAddressList *socket_address_list;
+
+    /* A tree of pages that we requested to the source VM */
+    GTree *page_requested;
+    /* For debugging purpose only, but would be nice to keep */
+    int page_requested_count;
+    /*
+     * The mutex helps to maintain the requested pages that we sent to the
+     * source, IOW, to guarantee coherent between the page_requests tree and
+     * the per-ramblock receivedmap.  Note! This does not guarantee consistency
+     * of the real page copy procedures (using UFFDIO_[ZERO]COPY).  E.g., even
+     * if one bit in receivedmap is cleared, UFFDIO_COPY could have happened
+     * for that page already.  This is intended so that the mutex won't
+     * serialize and blocked by slow operations like UFFDIO_* ioctls.  However
+     * this should be enough to make sure the page_requested tree always
+     * contains valid information.
+     */
+    QemuMutex page_request_mutex;
 };
 
 MigrationIncomingState *migration_incoming_get_current(void);
@@ -331,7 +348,7 @@ void migrate_send_rp_shut(MigrationIncomingState *mis,
 void migrate_send_rp_pong(MigrationIncomingState *mis,
                           uint32_t value);
 int migrate_send_rp_req_pages(MigrationIncomingState *mis, RAMBlock *rb,
-                              ram_addr_t start);
+                              ram_addr_t start, uint64_t haddr);
 int migrate_send_rp_message_req_pages(MigrationIncomingState *mis,
                                       RAMBlock *rb, ram_addr_t start);
 void migrate_send_rp_recv_bitmap(MigrationIncomingState *mis,

diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index aed4cdaa38c9279d89e5ff40fdf4052554cd5065..d3bb3a744b11f28e8b65fbfaca92130942591968 100644 (file)
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -684,7 +684,7 @@ int postcopy_request_shared_page(struct PostCopyFD *pcfd, RAMBlock *rb,
                                         qemu_ram_get_idstr(rb), rb_offset);
         return postcopy_wake_shared(pcfd, client_addr, rb);
     }
-    migrate_send_rp_req_pages(mis, rb, aligned_rbo);
+    migrate_send_rp_req_pages(mis, rb, aligned_rbo, client_addr);
     return 0;
 }
 
@@ -979,7 +979,8 @@ retry:
              * Send the request to the source - we want to request one
              * of our host page sizes (which is >= TPS)
              */
-            ret = migrate_send_rp_req_pages(mis, rb, rb_offset);
+            ret = migrate_send_rp_req_pages(mis, rb, rb_offset,
+                                            msg.arg.pagefault.address);
             if (ret) {
                 /* May be network failure, try to wait for recovery */
                 if (ret == -EIO && postcopy_pause_fault_thread(mis)) {
@@ -1149,10 +1150,20 @@ static int qemu_ufd_copy_ioctl(MigrationIncomingState *mis, void *host_addr,
         ret = ioctl(userfault_fd, UFFDIO_ZEROPAGE, &zero_struct);
     }
     if (!ret) {
+        qemu_mutex_lock(&mis->page_request_mutex);
         ramblock_recv_bitmap_set_range(rb, host_addr,
                                        pagesize / qemu_target_page_size());
+        /*
+         * If this page resolves a page fault for a previous recorded faulted
+         * address, take a special note to maintain the requested page list.
+         */
+        if (g_tree_lookup(mis->page_requested, host_addr)) {
+            g_tree_remove(mis->page_requested, host_addr);
+            mis->page_requested_count--;
+            trace_postcopy_page_req_del(host_addr, mis->page_requested_count);
+        }
+        qemu_mutex_unlock(&mis->page_request_mutex);
         mark_postcopy_blocktime_end((uintptr_t)host_addr);
-
     }
     return ret;
 }

diff --git a/migration/trace-events b/migration/trace-events
index c7755116b84a39279f17b2b36b333ad742ad83cb..5f0001f8082b1d79c29315a2d1317d9b99763dee 100644 (file)
--- a/migration/trace-events
+++ b/migration/trace-events
@@ -162,6 +162,7 @@ postcopy_pause_return_path(void) ""
 postcopy_pause_return_path_continued(void) ""
 postcopy_pause_continued(void) ""
 postcopy_start_set_run(void) ""
+postcopy_page_req_add(void *addr, int count) "new page req %p total %d"
 source_return_path_thread_bad_end(void) ""
 source_return_path_thread_end(void) ""
 source_return_path_thread_entry(void) ""
@@ -272,6 +273,7 @@ postcopy_ram_incoming_cleanup_blocktime(uint64_t total) "total blocktime %" PRIu
 postcopy_request_shared_page(const char *sharer, const char *rb, uint64_t rb_offset) "for %s in %s offset 0x%"PRIx64
 postcopy_request_shared_page_present(const char *sharer, const char *rb, uint64_t rb_offset) "%s already %s offset 0x%"PRIx64
 postcopy_wake_shared(uint64_t client_addr, const char *rb) "at 0x%"PRIx64" in %s"
+postcopy_page_req_del(void *addr, int count) "resolved page req %p total %d"
 
 get_mem_fault_cpu_index(int cpu, uint32_t pid) "cpu: %d, pid: %u"