[Qemu-devel] [PULL 42/57] Page request: Consume pages off the post-copy queue

[Juan Quintela <quintela@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

When transmitting RAM pages, consume pages that have been queued by
MIG_RPCOMM_REQPAGE commands and send them ahead of normal page scanning.

Note:
  a) After a queued page the linear walk carries on from after the
unqueued page; there is a reasonable chance that the destination
was about to ask for other closeby pages anyway.

  b) We have to be careful of any assumptions that the page walking
code makes, in particular it does some short cuts on its first linear
walk that break as soon as we do a queued page.

  c) We have to be careful to not break up host-page size chunks, since
this makes it harder to place the pages on the destination.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
---
 migration/ram.c | 249 +++++++++++++++++++++++++++++++++++++++++++++++++-------
 trace-events    |   2 +
 2 files changed, 220 insertions(+), 31 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index 8302d09..d09d5ab 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -548,9 +548,9 @@ static int save_xbzrle_page(QEMUFile *f, uint8_t **current_data,
  * Returns: byte offset within memory region of the start of a dirty page
  */
 static inline
-ram_addr_t migration_bitmap_find_and_reset_dirty(RAMBlock *rb,
-                                                 ram_addr_t start,
-                                                 ram_addr_t *ram_addr_abs)
+ram_addr_t migration_bitmap_find_dirty(RAMBlock *rb,
+                                       ram_addr_t start,
+                                       ram_addr_t *ram_addr_abs)
 {
     unsigned long base = rb->offset >> TARGET_PAGE_BITS;
     unsigned long nr = base + (start >> TARGET_PAGE_BITS);
@@ -567,15 +567,24 @@ ram_addr_t migration_bitmap_find_and_reset_dirty(RAMBlock *rb,
         next = find_next_bit(bitmap, size, nr);
     }

-    if (next < size) {
-        clear_bit(next, bitmap);
-        migration_dirty_pages--;
-    }
     *ram_addr_abs = next << TARGET_PAGE_BITS;
     return (next - base) << TARGET_PAGE_BITS;
 }

-/* Called with rcu_read_lock() to protect migration_bitmap */
+static inline bool migration_bitmap_clear_dirty(ram_addr_t addr)
+{
+    bool ret;
+    int nr = addr >> TARGET_PAGE_BITS;
+    unsigned long *bitmap = atomic_rcu_read(&migration_bitmap_rcu)->bmap;
+
+    ret = test_and_clear_bit(nr, bitmap);
+
+    if (ret) {
+        migration_dirty_pages--;
+    }
+    return ret;
+}
+
 static void migration_bitmap_sync_range(ram_addr_t start, ram_addr_t length)
 {
     unsigned long *bitmap;
@@ -974,9 +983,8 @@ static int ram_save_compressed_page(QEMUFile *f, RAMBlock *block,
 static bool find_dirty_block(QEMUFile *f, PageSearchStatus *pss,
                              bool *again, ram_addr_t *ram_addr_abs)
 {
-    pss->offset = migration_bitmap_find_and_reset_dirty(pss->block,
-                                                       pss->offset,
-                                                       ram_addr_abs);
+    pss->offset = migration_bitmap_find_dirty(pss->block, pss->offset,
+                                              ram_addr_abs);
     if (pss->complete_round && pss->block == last_seen_block &&
         pss->offset >= last_offset) {
         /*
@@ -1015,6 +1023,107 @@ static bool find_dirty_block(QEMUFile *f, PageSearchStatus *pss,
     }
 }

+/*
+ * Helper for 'get_queued_page' - gets a page off the queue
+ *      ms:      MigrationState in
+ * *offset:      Used to return the offset within the RAMBlock
+ * ram_addr_abs: global offset in the dirty/sent bitmaps
+ *
+ * Returns:      block (or NULL if none available)
+ */
+static RAMBlock *unqueue_page(MigrationState *ms, ram_addr_t *offset,
+                              ram_addr_t *ram_addr_abs)
+{
+    RAMBlock *block = NULL;
+
+    qemu_mutex_lock(&ms->src_page_req_mutex);
+    if (!QSIMPLEQ_EMPTY(&ms->src_page_requests)) {
+        struct MigrationSrcPageRequest *entry =
+                                QSIMPLEQ_FIRST(&ms->src_page_requests);
+        block = entry->rb;
+        *offset = entry->offset;
+        *ram_addr_abs = (entry->offset + entry->rb->offset) &
+                        TARGET_PAGE_MASK;
+
+        if (entry->len > TARGET_PAGE_SIZE) {
+            entry->len -= TARGET_PAGE_SIZE;
+            entry->offset += TARGET_PAGE_SIZE;
+        } else {
+            memory_region_unref(block->mr);
+            QSIMPLEQ_REMOVE_HEAD(&ms->src_page_requests, next_req);
+            g_free(entry);
+        }
+    }
+    qemu_mutex_unlock(&ms->src_page_req_mutex);
+
+    return block;
+}
+
+/*
+ * Unqueue a page from the queue fed by postcopy page requests; skips pages
+ * that are already sent (!dirty)
+ *
+ *      ms:      MigrationState in
+ *     pss:      PageSearchStatus structure updated with found block/offset
+ * ram_addr_abs: global offset in the dirty/sent bitmaps
+ *
+ * Returns:      true if a queued page is found
+ */
+static bool get_queued_page(MigrationState *ms, PageSearchStatus *pss,
+                            ram_addr_t *ram_addr_abs)
+{
+    RAMBlock  *block;
+    ram_addr_t offset;
+    bool dirty;
+
+    do {
+        block = unqueue_page(ms, &offset, ram_addr_abs);
+        /*
+         * We're sending this page, and since it's postcopy nothing else
+         * will dirty it, and we must make sure it doesn't get sent again
+         * even if this queue request was received after the background
+         * search already sent it.
+         */
+        if (block) {
+            unsigned long *bitmap;
+            bitmap = atomic_rcu_read(&migration_bitmap_rcu)->bmap;
+            dirty = test_bit(*ram_addr_abs >> TARGET_PAGE_BITS, bitmap);
+            if (!dirty) {
+                trace_get_queued_page_not_dirty(
+                    block->idstr, (uint64_t)offset,
+                    (uint64_t)*ram_addr_abs,
+                    test_bit(*ram_addr_abs >> TARGET_PAGE_BITS,
+                         atomic_rcu_read(&migration_bitmap_rcu)->unsentmap));
+            } else {
+                trace_get_queued_page(block->idstr,
+                                      (uint64_t)offset,
+                                      (uint64_t)*ram_addr_abs);
+            }
+        }
+
+    } while (block && !dirty);
+
+    if (block) {
+        /*
+         * As soon as we start servicing pages out of order, then we have
+         * to kill the bulk stage, since the bulk stage assumes
+         * in (migration_bitmap_find_and_reset_dirty) that every page is
+         * dirty, that's no longer true.
+         */
+        ram_bulk_stage = false;
+
+        /*
+         * We want the background search to continue from the queued page
+         * since the guest is likely to want other pages near to the page
+         * it just requested.
+         */
+        pss->block = block;
+        pss->offset = offset;
+    }
+
+    return !!block;
+}
+
 /**
  * flush_page_queue: Flush any remaining pages in the ram request queue
  *    it should be empty at the end anyway, but in error cases there may be
@@ -1099,6 +1208,90 @@ err:
     return -1;
 }

+/**
+ * ram_save_target_page: Save one target page
+ *
+ *
+ * @f: QEMUFile where to send the data
+ * @block: pointer to block that contains the page we want to send
+ * @offset: offset inside the block for the page;
+ * @last_stage: if we are at the completion stage
+ * @bytes_transferred: increase it with the number of transferred bytes
+ * @dirty_ram_abs: Address of the start of the dirty page in ram_addr_t space
+ *
+ * Returns: Number of pages written.
+ */
+static int ram_save_target_page(MigrationState *ms, QEMUFile *f,
+                                RAMBlock *block, ram_addr_t offset,
+                                bool last_stage,
+                                uint64_t *bytes_transferred,
+                                ram_addr_t dirty_ram_abs)
+{
+    int res = 0;
+
+    /* Check the pages is dirty and if it is send it */
+    if (migration_bitmap_clear_dirty(dirty_ram_abs)) {
+        unsigned long *unsentmap;
+        if (compression_switch && migrate_use_compression()) {
+            res = ram_save_compressed_page(f, block, offset,
+                                           last_stage,
+                                           bytes_transferred);
+        } else {
+            res = ram_save_page(f, block, offset, last_stage,
+                                bytes_transferred);
+        }
+
+        if (res < 0) {
+            return res;
+        }
+        unsentmap = atomic_rcu_read(&migration_bitmap_rcu)->unsentmap;
+        if (unsentmap) {
+            clear_bit(dirty_ram_abs >> TARGET_PAGE_BITS, unsentmap);
+        }
+    }
+
+    return res;
+}
+
+/**
+ * ram_save_host_page: Starting at *offset send pages upto the end
+ *                     of the current host page.  It's valid for the initial
+ *                     offset to point into the middle of a host page
+ *                     in which case the remainder of the hostpage is sent.
+ *                     Only dirty target pages are sent.
+ *
+ * Returns: Number of pages written.
+ *
+ * @f: QEMUFile where to send the data
+ * @block: pointer to block that contains the page we want to send
+ * @offset: offset inside the block for the page; updated to last target page
+ *          sent
+ * @last_stage: if we are at the completion stage
+ * @bytes_transferred: increase it with the number of transferred bytes
+ * @dirty_ram_abs: Address of the start of the dirty page in ram_addr_t space
+ */
+static int ram_save_host_page(MigrationState *ms, QEMUFile *f, RAMBlock *block,
+                              ram_addr_t *offset, bool last_stage,
+                              uint64_t *bytes_transferred,
+                              ram_addr_t dirty_ram_abs)
+{
+    int tmppages, pages = 0;
+    do {
+        tmppages = ram_save_target_page(ms, f, block, *offset, last_stage,
+                                        bytes_transferred, dirty_ram_abs);
+        if (tmppages < 0) {
+            return tmppages;
+        }
+
+        pages += tmppages;
+        *offset += TARGET_PAGE_SIZE;
+        dirty_ram_abs += TARGET_PAGE_SIZE;
+    } while (*offset & (qemu_host_page_size - 1));
+
+    /* The offset we leave with is the last one we looked at */
+    *offset -= TARGET_PAGE_SIZE;
+    return pages;
+}

 /**
  * ram_find_and_save_block: Finds a dirty page and sends it to f
@@ -1111,12 +1304,16 @@ err:
  * @f: QEMUFile where to send the data
  * @last_stage: if we are at the completion stage
  * @bytes_transferred: increase it with the number of transferred bytes
+ *
+ * On systems where host-page-size > target-page-size it will send all the
+ * pages in a host page that are dirty.
  */

 static int ram_find_and_save_block(QEMUFile *f, bool last_stage,
                                    uint64_t *bytes_transferred)
 {
     PageSearchStatus pss;
+    MigrationState *ms = migrate_get_current();
     int pages = 0;
     bool again, found;
     ram_addr_t dirty_ram_abs; /* Address of the start of the dirty page in
@@ -1131,28 +1328,18 @@ static int ram_find_and_save_block(QEMUFile *f, bool last_stage,
     }

     do {
-        found = find_dirty_block(f, &pss, &again, &dirty_ram_abs);
+        again = true;
+        found = get_queued_page(ms, &pss, &dirty_ram_abs);
+
+        if (!found) {
+            /* priority queue empty, so just search for something dirty */
+            found = find_dirty_block(f, &pss, &again, &dirty_ram_abs);
+        }

         if (found) {
-            if (compression_switch && migrate_use_compression()) {
-                pages = ram_save_compressed_page(f, pss.block, pss.offset,
-                                                 last_stage,
-                                                 bytes_transferred);
-            } else {
-                pages = ram_save_page(f, pss.block, pss.offset, last_stage,
-                                      bytes_transferred);
-            }
-
-            /* if page is unmodified, continue to the next */
-            if (pages > 0) {
-                unsigned long *unsentmap;
-
-                unsentmap = atomic_rcu_read(&migration_bitmap_rcu)->unsentmap;
-                last_sent_block = pss.block;
-                if (unsentmap) {
-                    clear_bit(dirty_ram_abs >> TARGET_PAGE_BITS, unsentmap);
-                }
-            }
+            pages = ram_save_host_page(ms, f, pss.block, &pss.offset,
+                                       last_stage, bytes_transferred,
+                                       dirty_ram_abs);
         }
     } while (!pages && again);

diff --git a/trace-events b/trace-events
index 0914e0a..3df3656 100644
--- a/trace-events
+++ b/trace-events
@@ -1252,6 +1252,8 @@ vmstate_subsection_load_good(const char *parent) "%s"
 qemu_file_fclose(void) ""

 # migration/ram.c
+get_queued_page(const char *block_name, uint64_t tmp_offset, uint64_t ram_addr) "%s/%" PRIx64 " ram_addr=%" PRIx64
+get_queued_page_not_dirty(const char *block_name, uint64_t tmp_offset, uint64_t ram_addr, int sent) "%s/%" PRIx64 " ram_addr=%" PRIx64 " (sent=%d)"
 migration_bitmap_sync_start(void) ""
 migration_bitmap_sync_end(uint64_t dirty_pages) "dirty_pages %" PRIu64""
 migration_throttle(void) ""
-- 
2.5.0