[PATCH v6 0/7] block: seriously improve savevm/loadvm performance

[PATCH v6 0/7] block: seriously improve savevm/loadvm performance

[Denis V. Lunev]

This series do standard basic things:
- it creates intermediate buffer for all writes from QEMU migration code
  to QCOW2 image,
- this buffer is sent to disk asynchronously, allowing several writes to
  run in parallel.

In general, migration code is fantastically inefficent (by observation),
buffers are not aligned and sent with arbitrary pieces, a lot of time
less than 100 bytes at a chunk, which results in read-modify-write
operations with non-cached operations. It should also be noted that all
operations are performed into unallocated image blocks, which also suffer
due to partial writes to such new clusters.

This patch series is an implementation of idea discussed in the RFC
posted by Denis Plotnikov
https://lists.gnu.org/archive/html/qemu-devel/2020-04/msg01925.html
Results with this series over NVME are better than original code
                original     rfc    this
cached:          1.79s      2.38s   1.27s
non-cached:      3.29s      1.31s   0.81s

Changes from v5:
- loadvm optimizations added with Vladimir comments included

Changes from v4:
- added patch 4 with blk_save_vmstate() cleanup
- added R-By
- bdrv_flush_vmstate -> bdrv_finalize_vmstate
- fixed return code of bdrv_co_do_save_vmstate
- fixed typos in comments (Eric, thanks!)
- fixed patchew warnings

Changes from v3:
- rebased to master
- added patch 3 which removes aio_task_pool_wait_one()
- added R-By to patch 1
- patch 4 is rewritten via bdrv_run_co
- error path in blk_save_vmstate() is rewritten to call bdrv_flush_vmstate
  unconditionally
- added some comments
- fixes initialization in bdrv_co_vmstate_save_task_entry as suggested

Changes from v2:
- code moved from QCOW2 level to generic block level
- created bdrv_flush_vmstate helper to fix 022, 029 tests
- added recursive for bs->file in bdrv_co_flush_vmstate (fix 267)
- fixed blk_save_vmstate helper
- fixed coroutine wait as Vladimir suggested with waiting fixes from me

Changes from v1:
- patchew warning fixed
- fixed validation that only 1 waiter is allowed in patch 1

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <fam@euphon.net>
CC: Juan Quintela <quintela@redhat.com>
CC: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
CC: Denis Plotnikov <dplotnikov@virtuozzo.com>

[PATCH 1/7] migration/savevm: respect qemu_fclose() error code in save_snapshot()

[Denis V. Lunev]

qemu_fclose() could return error, f.e. if bdrv_co_flush() will return
the error.

This validation will become more important once we will start waiting of
asynchronous IO operations, started from bdrv_write_vmstate(), which are
coming soon.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <fam@euphon.net>
CC: Juan Quintela <quintela@redhat.com>
CC: Denis Plotnikov <dplotnikov@virtuozzo.com>
---
 migration/savevm.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/migration/savevm.c b/migration/savevm.c
index b979ea6e7f..da3dead4e9 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -2628,7 +2628,7 @@ int save_snapshot(const char *name, Error **errp)
 {
     BlockDriverState *bs, *bs1;
     QEMUSnapshotInfo sn1, *sn = &sn1, old_sn1, *old_sn = &old_sn1;
-    int ret = -1;
+    int ret = -1, ret2;
     QEMUFile *f;
     int saved_vm_running;
     uint64_t vm_state_size;
@@ -2712,10 +2712,14 @@ int save_snapshot(const char *name, Error **errp)
     }
     ret = qemu_savevm_state(f, errp);
     vm_state_size = qemu_ftell(f);
-    qemu_fclose(f);
+    ret2 = qemu_fclose(f);
     if (ret < 0) {
         goto the_end;
     }
+    if (ret2 < 0) {
+        ret = ret2;
+        goto the_end;
+    }
 
     /* The bdrv_all_create_snapshot() call that follows acquires the AioContext
      * for itself.  BDRV_POLL_WHILE() does not support nested locking because
-- 
2.17.1

[PATCH 2/7] block/aio_task: allow start/wait task from any coroutine

[Denis V. Lunev]

From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

Currently, aio task pool assumes that there is a main coroutine, which
creates tasks and wait for them. Let's remove the restriction by using
CoQueue. Code becomes clearer, interface more obvious.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <fam@euphon.net>
CC: Juan Quintela <quintela@redhat.com>
CC: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
CC: Denis Plotnikov <dplotnikov@virtuozzo.com>
---
 block/aio_task.c | 21 ++++++---------------
 1 file changed, 6 insertions(+), 15 deletions(-)

diff --git a/block/aio_task.c b/block/aio_task.c
index 88989fa248..cf62e5c58b 100644
--- a/block/aio_task.c
+++ b/block/aio_task.c
@@ -27,11 +27,10 @@
 #include "block/aio_task.h"
 
 struct AioTaskPool {
-    Coroutine *main_co;
     int status;
     int max_busy_tasks;
     int busy_tasks;
-    bool waiting;
+    CoQueue waiters;
 };
 
 static void coroutine_fn aio_task_co(void *opaque)
@@ -52,31 +51,23 @@ static void coroutine_fn aio_task_co(void *opaque)
 
     g_free(task);
 
-    if (pool->waiting) {
-        pool->waiting = false;
-        aio_co_wake(pool->main_co);
-    }
+    qemu_co_queue_restart_all(&pool->waiters);
 }
 
 void coroutine_fn aio_task_pool_wait_one(AioTaskPool *pool)
 {
     assert(pool->busy_tasks > 0);
-    assert(qemu_coroutine_self() == pool->main_co);
 
-    pool->waiting = true;
-    qemu_coroutine_yield();
+    qemu_co_queue_wait(&pool->waiters, NULL);
 
-    assert(!pool->waiting);
     assert(pool->busy_tasks < pool->max_busy_tasks);
 }
 
 void coroutine_fn aio_task_pool_wait_slot(AioTaskPool *pool)
 {
-    if (pool->busy_tasks < pool->max_busy_tasks) {
-        return;
+    while (pool->busy_tasks >= pool->max_busy_tasks) {
+        aio_task_pool_wait_one(pool);
     }
-
-    aio_task_pool_wait_one(pool);
 }
 
 void coroutine_fn aio_task_pool_wait_all(AioTaskPool *pool)
@@ -98,8 +89,8 @@ AioTaskPool *coroutine_fn aio_task_pool_new(int max_busy_tasks)
 {
     AioTaskPool *pool = g_new0(AioTaskPool, 1);
 
-    pool->main_co = qemu_coroutine_self();
     pool->max_busy_tasks = max_busy_tasks;
+    qemu_co_queue_init(&pool->waiters);
 
     return pool;
 }
-- 
2.17.1

[PATCH 3/7] block/aio_task: drop aio_task_pool_wait_one() helper

[Denis V. Lunev]

It is not used outside the module.

Actually there are 2 kind of waiters:
- for a slot and
- for all tasks to finish
This patch limits external API to listed types.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <fam@euphon.net>
CC: Juan Quintela <quintela@redhat.com>
CC: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
CC: Denis Plotnikov <dplotnikov@virtuozzo.com>
---
 block/aio_task.c         | 13 ++-----------
 include/block/aio_task.h |  1 -
 2 files changed, 2 insertions(+), 12 deletions(-)

diff --git a/block/aio_task.c b/block/aio_task.c
index cf62e5c58b..7ba15ff41f 100644
--- a/block/aio_task.c
+++ b/block/aio_task.c
@@ -54,26 +54,17 @@ static void coroutine_fn aio_task_co(void *opaque)
     qemu_co_queue_restart_all(&pool->waiters);
 }
 
-void coroutine_fn aio_task_pool_wait_one(AioTaskPool *pool)
-{
-    assert(pool->busy_tasks > 0);
-
-    qemu_co_queue_wait(&pool->waiters, NULL);
-
-    assert(pool->busy_tasks < pool->max_busy_tasks);
-}
-
 void coroutine_fn aio_task_pool_wait_slot(AioTaskPool *pool)
 {
     while (pool->busy_tasks >= pool->max_busy_tasks) {
-        aio_task_pool_wait_one(pool);
+        qemu_co_queue_wait(&pool->waiters, NULL);
     }
 }
 
 void coroutine_fn aio_task_pool_wait_all(AioTaskPool *pool)
 {
     while (pool->busy_tasks > 0) {
-        aio_task_pool_wait_one(pool);
+        qemu_co_queue_wait(&pool->waiters, NULL);
     }
 }
 
diff --git a/include/block/aio_task.h b/include/block/aio_task.h
index 50bc1e1817..50b1c036c5 100644
--- a/include/block/aio_task.h
+++ b/include/block/aio_task.h
@@ -48,7 +48,6 @@ bool aio_task_pool_empty(AioTaskPool *pool);
 void coroutine_fn aio_task_pool_start_task(AioTaskPool *pool, AioTask *task);
 
 void coroutine_fn aio_task_pool_wait_slot(AioTaskPool *pool);
-void coroutine_fn aio_task_pool_wait_one(AioTaskPool *pool);
 void coroutine_fn aio_task_pool_wait_all(AioTaskPool *pool);
 
 #endif /* BLOCK_AIO_TASK_H */
-- 
2.17.1

[PATCH 4/7] block/block-backend: remove always true check from blk_save_vmstate

[Denis V. Lunev]

bdrv_save_vmstate() returns either error with negative return value or
size. Thus this check is useless.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Suggested-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <fam@euphon.net>
CC: Juan Quintela <quintela@redhat.com>
CC: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
CC: Denis Plotnikov <dplotnikov@virtuozzo.com>
---
 block/block-backend.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/block-backend.c b/block/block-backend.c
index 6936b25c83..1c6e53bbde 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -2188,7 +2188,7 @@ int blk_save_vmstate(BlockBackend *blk, const uint8_t *buf,
         return ret;
     }
 
-    if (ret == size && !blk->enable_write_cache) {
+    if (!blk->enable_write_cache) {
         ret = bdrv_flush(blk_bs(blk));
     }
 
-- 
2.17.1

[PATCH 5/7] block, migration: add bdrv_finalize_vmstate helper

[Denis V. Lunev]

Right now bdrv_fclose() is just calling bdrv_flush().

The problem is that migration code is working inefficiently from block
layer terms and are frequently called for very small pieces of
unaligned data. Block layer is capable to work this way, but this is very
slow.

This patch is a preparation for the introduction of the intermediate
buffer at block driver state. It would be beneficial to separate
conventional bdrv_flush() from closing QEMU file from migration code.

The patch also forces bdrv_finalize_vmstate() operation inside
synchronous blk_save_vmstate() operation. This helper is used from
qemu-io only.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <fam@euphon.net>
CC: Juan Quintela <quintela@redhat.com>
CC: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
CC: Denis Plotnikov <dplotnikov@virtuozzo.com>
---
 block/block-backend.c |  6 +++++-
 block/io.c            | 15 +++++++++++++++
 include/block/block.h |  5 +++++
 migration/savevm.c    |  4 ++++
 4 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/block/block-backend.c b/block/block-backend.c
index 1c6e53bbde..5bb11c8e01 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -2177,16 +2177,20 @@ int blk_truncate(BlockBackend *blk, int64_t offset, bool exact,
 int blk_save_vmstate(BlockBackend *blk, const uint8_t *buf,
                      int64_t pos, int size)
 {
-    int ret;
+    int ret, ret2;
 
     if (!blk_is_available(blk)) {
         return -ENOMEDIUM;
     }
 
     ret = bdrv_save_vmstate(blk_bs(blk), buf, pos, size);
+    ret2 = bdrv_finalize_vmstate(blk_bs(blk));
     if (ret < 0) {
         return ret;
     }
+    if (ret2 < 0) {
+        return ret2;
+    }
 
     if (!blk->enable_write_cache) {
         ret = bdrv_flush(blk_bs(blk));
diff --git a/block/io.c b/block/io.c
index df8f2a98d4..1f69268361 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2724,6 +2724,21 @@ int bdrv_readv_vmstate(BlockDriverState *bs, QEMUIOVector *qiov, int64_t pos)
     return bdrv_rw_vmstate(bs, qiov, pos, true);
 }
 
+static int coroutine_fn bdrv_co_finalize_vmstate(BlockDriverState *bs)
+{
+    return 0;
+}
+
+static int coroutine_fn bdrv_finalize_vmstate_co_entry(void *opaque)
+{
+    return bdrv_co_finalize_vmstate(opaque);
+}
+
+int bdrv_finalize_vmstate(BlockDriverState *bs)
+{
+    return bdrv_run_co(bs, bdrv_finalize_vmstate_co_entry, bs);
+}
+
 /**************************************************************/
 /* async I/Os */
 
diff --git a/include/block/block.h b/include/block/block.h
index e8fc814996..0119b68505 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -572,6 +572,11 @@ int bdrv_save_vmstate(BlockDriverState *bs, const uint8_t *buf,
 
 int bdrv_load_vmstate(BlockDriverState *bs, uint8_t *buf,
                       int64_t pos, int size);
+/*
+ * bdrv_finalize_vmstate() is mandatory to commit vmstate changes if
+ * bdrv_save_vmstate() was ever called.
+ */
+int bdrv_finalize_vmstate(BlockDriverState *bs);
 
 void bdrv_img_create(const char *filename, const char *fmt,
                      const char *base_filename, const char *base_fmt,
diff --git a/migration/savevm.c b/migration/savevm.c
index da3dead4e9..798a4cb402 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -150,6 +150,10 @@ static ssize_t block_get_buffer(void *opaque, uint8_t *buf, int64_t pos,
 
 static int bdrv_fclose(void *opaque, Error **errp)
 {
+    int err = bdrv_finalize_vmstate(opaque);
+    if (err < 0) {
+        return err;
+    }
     return bdrv_flush(opaque);
 }
 
-- 
2.17.1

[PATCH 6/7] block/io: improve savevm performance

[Denis V. Lunev]

This patch does 2 standard basic things:
- it creates intermediate buffer for all writes from QEMU migration code
  to block driver,
- this buffer is sent to disk asynchronously, allowing several writes to
  run in parallel.

Thus bdrv_vmstate_write() is becoming asynchronous. All pending operations
completion are performed in newly invented bdrv_finalize_vmstate().

In general, migration code is fantastically inefficent (by observation),
buffers are not aligned and sent with arbitrary pieces, a lot of time
less than 100 bytes at a chunk, which results in read-modify-write
operations if target file descriptor is opened with O_DIRECT. It should
also be noted that all operations are performed into unallocated image
blocks, which also suffer due to partial writes to such new clusters
even on cached file descriptors.

Snapshot creation time (2 GB Fedora-31 VM running over NVME storage):
                original     fixed
cached:          1.79s       1.27s
non-cached:      3.29s       0.81s

The difference over HDD would be more significant :)

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <fam@euphon.net>
CC: Juan Quintela <quintela@redhat.com>
CC: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
CC: Denis Plotnikov <dplotnikov@virtuozzo.com>
---
 block/io.c                | 126 +++++++++++++++++++++++++++++++++++++-
 include/block/block_int.h |   8 +++
 2 files changed, 132 insertions(+), 2 deletions(-)

diff --git a/block/io.c b/block/io.c
index 1f69268361..71a696deb7 100644
--- a/block/io.c
+++ b/block/io.c
@@ -26,6 +26,7 @@
 #include "trace.h"
 #include "sysemu/block-backend.h"
 #include "block/aio-wait.h"
+#include "block/aio_task.h"
 #include "block/blockjob.h"
 #include "block/blockjob_int.h"
 #include "block/block_int.h"
@@ -33,6 +34,7 @@
 #include "qapi/error.h"
 #include "qemu/error-report.h"
 #include "qemu/main-loop.h"
+#include "qemu/units.h"
 #include "sysemu/replay.h"
 
 /* Maximum bounce buffer for copy-on-read and write zeroes, in bytes */
@@ -2640,6 +2642,103 @@ typedef struct BdrvVmstateCo {
     bool                is_read;
 } BdrvVmstateCo;
 
+typedef struct BdrvVMStateTask {
+    AioTask task;
+
+    BlockDriverState *bs;
+    int64_t offset;
+    void *buf;
+    size_t bytes;
+} BdrvVMStateTask;
+
+typedef struct BdrvSaveVMState {
+    AioTaskPool *pool;
+    BdrvVMStateTask *t;
+} BdrvSaveVMState;
+
+
+static coroutine_fn int bdrv_co_vmstate_save_task_entry(AioTask *task)
+{
+    int err = 0;
+    BdrvVMStateTask *t = container_of(task, BdrvVMStateTask, task);
+
+    if (t->bytes != 0) {
+        QEMUIOVector qiov = QEMU_IOVEC_INIT_BUF(qiov, t->buf, t->bytes);
+
+        bdrv_inc_in_flight(t->bs);
+        err = t->bs->drv->bdrv_save_vmstate(t->bs, &qiov, t->offset);
+        bdrv_dec_in_flight(t->bs);
+    }
+
+    qemu_vfree(t->buf);
+    return err;
+}
+
+static BdrvVMStateTask *bdrv_vmstate_task_create(BlockDriverState *bs,
+                                                 int64_t pos, size_t size)
+{
+    BdrvVMStateTask *t = g_new(BdrvVMStateTask, 1);
+
+    *t = (BdrvVMStateTask) {
+        .task.func = bdrv_co_vmstate_save_task_entry,
+        .buf = qemu_blockalign(bs, size),
+        .offset = pos,
+        .bs = bs,
+    };
+
+    return t;
+}
+
+static int bdrv_co_do_save_vmstate(BlockDriverState *bs, QEMUIOVector *qiov,
+                                   int64_t pos)
+{
+    BdrvSaveVMState *state = bs->savevm_state;
+    BdrvVMStateTask *t;
+    size_t buf_size = MAX(bdrv_get_cluster_size(bs), 1 * MiB);
+    size_t to_copy, off;
+
+    if (state == NULL) {
+        state = g_new(BdrvSaveVMState, 1);
+        *state = (BdrvSaveVMState) {
+            .pool = aio_task_pool_new(BDRV_VMSTATE_WORKERS_MAX),
+            .t = bdrv_vmstate_task_create(bs, pos, buf_size),
+        };
+
+        bs->savevm_state = state;
+    }
+
+    if (aio_task_pool_status(state->pool) < 0) {
+        /*
+         * The operation as a whole is unsuccessful. Prohibit all futher
+         * operations. If we clean here, new useless ops will come again.
+         * Thus we rely on caller for cleanup here.
+         */
+        return aio_task_pool_status(state->pool);
+    }
+
+    t = state->t;
+    if (t->offset + t->bytes != pos) {
+        /* Normally this branch is not reachable from migration */
+        return bs->drv->bdrv_save_vmstate(bs, qiov, pos);
+    }
+
+    off = 0;
+    while (1) {
+        to_copy = MIN(qiov->size - off, buf_size - t->bytes);
+        qemu_iovec_to_buf(qiov, off, t->buf + t->bytes, to_copy);
+        t->bytes += to_copy;
+        if (t->bytes < buf_size) {
+            return 0;
+        }
+
+        aio_task_pool_start_task(state->pool, &t->task);
+
+        pos += to_copy;
+        off += to_copy;
+        state->t = t = bdrv_vmstate_task_create(bs, pos, buf_size);
+    }
+}
+
 static int coroutine_fn
 bdrv_co_rw_vmstate(BlockDriverState *bs, QEMUIOVector *qiov, int64_t pos,
                    bool is_read)
@@ -2655,7 +2754,7 @@ bdrv_co_rw_vmstate(BlockDriverState *bs, QEMUIOVector *qiov, int64_t pos,
         if (is_read) {
             ret = drv->bdrv_load_vmstate(bs, qiov, pos);
         } else {
-            ret = drv->bdrv_save_vmstate(bs, qiov, pos);
+            ret = bdrv_co_do_save_vmstate(bs, qiov, pos);
         }
     } else if (bs->file) {
         ret = bdrv_co_rw_vmstate(bs->file->bs, qiov, pos, is_read);
@@ -2726,7 +2825,30 @@ int bdrv_readv_vmstate(BlockDriverState *bs, QEMUIOVector *qiov, int64_t pos)
 
 static int coroutine_fn bdrv_co_finalize_vmstate(BlockDriverState *bs)
 {
-    return 0;
+    int err;
+    BdrvSaveVMState *state = bs->savevm_state;
+
+    if (bs->drv->bdrv_save_vmstate == NULL && bs->file != NULL) {
+        return bdrv_co_finalize_vmstate(bs->file->bs);
+    }
+    if (state == NULL) {
+        return 0;
+    }
+
+    if (aio_task_pool_status(state->pool) >= 0) {
+        /* We are on success path, commit last chunk if possible */
+        aio_task_pool_start_task(state->pool, &state->t->task);
+    }
+
+    aio_task_pool_wait_all(state->pool);
+    err = aio_task_pool_status(state->pool);
+
+    aio_task_pool_free(state->pool);
+    g_free(state);
+
+    bs->savevm_state = NULL;
+
+    return err;
 }
 
 static int coroutine_fn bdrv_finalize_vmstate_co_entry(void *opaque)
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 791de6a59c..f90f0e8b6a 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -61,6 +61,8 @@
 
 #define BLOCK_PROBE_BUF_SIZE        512
 
+#define BDRV_VMSTATE_WORKERS_MAX    8
+
 enum BdrvTrackedRequestType {
     BDRV_TRACKED_READ,
     BDRV_TRACKED_WRITE,
@@ -784,6 +786,9 @@ struct BdrvChild {
     QLIST_ENTRY(BdrvChild) next_parent;
 };
 
+
+typedef struct BdrvSaveVMState BdrvSaveVMState;
+
 /*
  * Note: the function bdrv_append() copies and swaps contents of
  * BlockDriverStates, so if you add new fields to this struct, please
@@ -947,6 +952,9 @@ struct BlockDriverState {
 
     /* BdrvChild links to this node may never be frozen */
     bool never_freeze;
+
+    /* Intermediate buffer for VM state saving from snapshot creation code */
+    BdrvSaveVMState *savevm_state;
 };
 
 struct BlockBackendRootState {
-- 
2.17.1

[PATCH 7/7] block/io: improve loadvm performance

[Denis V. Lunev]

This patch creates intermediate buffer for reading from block driver
state and performs read-ahead to this buffer. Snapshot code performs
reads sequentially and thus we know what offsets will be required
and when they will become not needed.

Results are fantastic. Switch to snapshot times of 2GB Fedora 31 VM
over NVME storage are the following:
                original     fixed
cached:          1.84s       1.16s
non-cached:     12.74s       1.27s

The difference over HDD would be more significant :)

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <fam@euphon.net>
CC: Juan Quintela <quintela@redhat.com>
CC: Denis Plotnikov <dplotnikov@virtuozzo.com>
---
 block/io.c                | 254 +++++++++++++++++++++++++++++++++++++-
 include/block/block_int.h |   3 +
 2 files changed, 254 insertions(+), 3 deletions(-)

diff --git a/block/io.c b/block/io.c
index 71a696deb7..decb850f1a 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2739,6 +2739,209 @@ static int bdrv_co_do_save_vmstate(BlockDriverState *bs, QEMUIOVector *qiov,
     }
 }
 
+
+typedef struct BdrvLoadVMChunk {
+    void *buf;
+    uint64_t offset;
+    ssize_t bytes;
+
+    QLIST_ENTRY(BdrvLoadVMChunk) list;
+} BdrvLoadVMChunk;
+
+typedef struct BdrvLoadVMState {
+    AioTaskPool *pool;
+
+    int64_t offset;
+    int64_t last_loaded;
+
+    int chunk_count;
+    QLIST_HEAD(, BdrvLoadVMChunk) chunks;
+    QLIST_HEAD(, BdrvLoadVMChunk) loading;
+    CoMutex lock;
+    CoQueue waiters;
+} BdrvLoadVMState;
+
+typedef struct BdrvLoadVMStateTask {
+    AioTask task;
+
+    BlockDriverState *bs;
+    BdrvLoadVMChunk *chunk;
+} BdrvLoadVMStateTask;
+
+static BdrvLoadVMChunk *bdrv_co_find_loadvmstate_chunk(int64_t pos,
+                                                       BdrvLoadVMChunk *c)
+{
+    for (; c != NULL; c = QLIST_NEXT(c, list)) {
+        if (c->offset <= pos && c->offset + c->bytes > pos) {
+            return c;
+        }
+    }
+
+    return NULL;
+}
+
+static void bdrv_free_loadvm_chunk(BdrvLoadVMChunk *c)
+{
+    qemu_vfree(c->buf);
+    g_free(c);
+}
+
+static coroutine_fn int bdrv_co_vmstate_load_task_entry(AioTask *task)
+{
+    int err = 0;
+    BdrvLoadVMStateTask *t = container_of(task, BdrvLoadVMStateTask, task);
+    BdrvLoadVMChunk *c = t->chunk;
+    BdrvLoadVMState *state = t->bs->loadvm_state;
+    QEMUIOVector qiov = QEMU_IOVEC_INIT_BUF(qiov, c->buf, c->bytes);
+
+    bdrv_inc_in_flight(t->bs);
+    err = t->bs->drv->bdrv_load_vmstate(t->bs, &qiov, c->offset);
+    bdrv_dec_in_flight(t->bs);
+
+    qemu_co_mutex_lock(&state->lock);
+    QLIST_REMOVE(c, list);
+    if (err == 0) {
+        QLIST_INSERT_HEAD(&state->chunks, c, list);
+    } else {
+        bdrv_free_loadvm_chunk(c);
+    }
+    qemu_co_mutex_unlock(&state->lock);
+    qemu_co_queue_restart_all(&state->waiters);
+
+    return err;
+}
+
+
+static void bdrv_co_loadvmstate_next(BlockDriverState *bs, BdrvLoadVMChunk *c)
+{
+    BdrvLoadVMStateTask *t = g_new(BdrvLoadVMStateTask, 1);
+    BdrvLoadVMState *state = bs->loadvm_state;
+
+    qemu_co_mutex_assert_locked(&state->lock);
+
+    c->offset = state->last_loaded;
+
+    *t = (BdrvLoadVMStateTask) {
+        .task.func = bdrv_co_vmstate_load_task_entry,
+        .bs = bs,
+        .chunk = c,
+    };
+
+    QLIST_INSERT_HEAD(&state->loading, t->chunk, list);
+    state->chunk_count++;
+    state->last_loaded += c->bytes;
+
+    qemu_co_mutex_unlock(&state->lock);
+    aio_task_pool_start_task(state->pool, &t->task);
+    qemu_co_mutex_lock(&state->lock);
+}
+
+
+static void bdrv_co_loadvmstate_start(BlockDriverState *bs)
+{
+    int i;
+    size_t buf_size = MAX(bdrv_get_cluster_size(bs), 1 * MiB);
+    BdrvLoadVMState *state = bs->loadvm_state;
+
+    qemu_co_mutex_lock(&state->lock);
+    for (i = 0; i < BDRV_VMSTATE_WORKERS_MAX; i++) {
+        BdrvLoadVMChunk *c = g_new0(BdrvLoadVMChunk, 1);
+
+        c->buf = qemu_blockalign(bs, buf_size);
+        c->bytes = buf_size;
+
+        bdrv_co_loadvmstate_next(bs, c);
+    }
+    qemu_co_mutex_unlock(&state->lock);
+}
+
+static int bdrv_co_do_load_vmstate(BlockDriverState *bs, QEMUIOVector *qiov,
+                                   int64_t pos)
+{
+    BdrvLoadVMState *state = bs->loadvm_state;
+    BdrvLoadVMChunk *c;
+    size_t off;
+    int64_t start_pos = pos;
+
+    if (state == NULL) {
+        if (pos != 0) {
+            goto slow_path;
+        }
+
+        state = g_new(BdrvLoadVMState, 1);
+        *state = (BdrvLoadVMState) {
+            .pool = aio_task_pool_new(BDRV_VMSTATE_WORKERS_MAX),
+            .chunks = QLIST_HEAD_INITIALIZER(state->chunks),
+            .loading = QLIST_HEAD_INITIALIZER(state->loading),
+        };
+        qemu_co_mutex_init(&state->lock);
+        qemu_co_queue_init(&state->waiters);
+
+        bs->loadvm_state = state;
+
+        bdrv_co_loadvmstate_start(bs);
+    }
+
+    if (state->offset != pos) {
+        goto slow_path;
+    }
+
+    off = 0;
+
+    qemu_co_mutex_lock(&state->lock);
+    while (off < qiov->size && aio_task_pool_status(state->pool) == 0) {
+        c = bdrv_co_find_loadvmstate_chunk(pos, QLIST_FIRST(&state->chunks));
+        if (c != NULL) {
+            ssize_t chunk_off = pos - c->offset;
+            ssize_t to_copy = MIN(qiov->size - off, c->bytes - chunk_off);
+
+            qemu_iovec_from_buf(qiov, off, c->buf + chunk_off, to_copy);
+
+            off += to_copy;
+            pos += to_copy;
+
+            if (pos == c->offset + c->bytes) {
+                state->chunk_count--;
+                /* End of buffer, discard it from the list */
+                QLIST_REMOVE(c, list);
+
+                /*
+                 * Start loading next chunk. The slot in the pool should
+                 * always be free for this purpose at the moment.
+                 *
+                 * There is a problem with the end of the stream. This code
+                 * starts to read the data beyond the end of the saved state.
+                 * The code in low level should be ready to such behavior but
+                 * we will have unnecessary BDRV_VMSTATE_WORKERS_MAX chunks
+                 * fully zeroed. This is not good, but acceptable.
+                 */
+                bdrv_co_loadvmstate_next(bs, c);
+            }
+
+            state->offset += to_copy;
+            continue;
+        }
+
+        c = bdrv_co_find_loadvmstate_chunk(pos, QLIST_FIRST(&state->loading));
+        if (c != NULL) {
+            qemu_co_queue_wait(&state->waiters, &state->lock);
+            continue;
+        }
+
+        /*
+         * This should not happen normally. This point could be reached only
+         * if we have had some parallel requests. Fallback to slow load.
+         */
+        qemu_co_mutex_unlock(&state->lock);
+
+slow_path:
+        return bs->drv->bdrv_load_vmstate(bs, qiov, start_pos);
+    }
+    qemu_co_mutex_unlock(&state->lock);
+
+    return aio_task_pool_status(state->pool);
+}
+
 static int coroutine_fn
 bdrv_co_rw_vmstate(BlockDriverState *bs, QEMUIOVector *qiov, int64_t pos,
                    bool is_read)
@@ -2752,7 +2955,7 @@ bdrv_co_rw_vmstate(BlockDriverState *bs, QEMUIOVector *qiov, int64_t pos,
         ret = -ENOMEDIUM;
     } else if (drv->bdrv_load_vmstate) {
         if (is_read) {
-            ret = drv->bdrv_load_vmstate(bs, qiov, pos);
+            ret = bdrv_co_do_load_vmstate(bs, qiov, pos);
         } else {
             ret = bdrv_co_do_save_vmstate(bs, qiov, pos);
         }
@@ -2823,13 +3026,13 @@ int bdrv_readv_vmstate(BlockDriverState *bs, QEMUIOVector *qiov, int64_t pos)
     return bdrv_rw_vmstate(bs, qiov, pos, true);
 }
 
-static int coroutine_fn bdrv_co_finalize_vmstate(BlockDriverState *bs)
+static int coroutine_fn bdrv_co_finalize_save_vmstate(BlockDriverState *bs)
 {
     int err;
     BdrvSaveVMState *state = bs->savevm_state;
 
     if (bs->drv->bdrv_save_vmstate == NULL && bs->file != NULL) {
-        return bdrv_co_finalize_vmstate(bs->file->bs);
+        return bdrv_co_finalize_save_vmstate(bs->file->bs);
     }
     if (state == NULL) {
         return 0;
@@ -2851,6 +3054,51 @@ static int coroutine_fn bdrv_co_finalize_vmstate(BlockDriverState *bs)
     return err;
 }
 
+static int coroutine_fn bdrv_co_finalize_load_vmstate(BlockDriverState *bs)
+{
+    int err;
+    BdrvLoadVMState *state = bs->loadvm_state;
+    BdrvLoadVMChunk *c, *tmp;
+
+    if (bs->drv->bdrv_load_vmstate == NULL && bs->file != NULL) {
+        return bdrv_co_finalize_load_vmstate(bs->file->bs);
+    }
+    if (state == NULL) {
+        return 0;
+    }
+
+    aio_task_pool_wait_all(state->pool);
+    err = aio_task_pool_status(state->pool);
+    aio_task_pool_free(state->pool);
+
+    QLIST_FOREACH(c, &state->loading, list) {
+        assert(1);  /* this list must be empty as all tasks are committed */
+    }
+    QLIST_FOREACH_SAFE(c, &state->chunks, list, tmp) {
+        QLIST_REMOVE(c, list);
+        bdrv_free_loadvm_chunk(c);
+    }
+
+    g_free(state);
+
+    bs->loadvm_state = NULL;
+
+    return err;
+}
+
+static int coroutine_fn bdrv_co_finalize_vmstate(BlockDriverState *bs)
+{
+    int err1 = bdrv_co_finalize_save_vmstate(bs);
+    int err2 = bdrv_co_finalize_load_vmstate(bs);
+    if (err1 < 0) {
+        return err1;
+    }
+    if (err2 < 0) {
+        return err2;
+    }
+    return 0;
+}
+
 static int coroutine_fn bdrv_finalize_vmstate_co_entry(void *opaque)
 {
     return bdrv_co_finalize_vmstate(opaque);
diff --git a/include/block/block_int.h b/include/block/block_int.h
index f90f0e8b6a..0942578a74 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -788,6 +788,7 @@ struct BdrvChild {
 
 
 typedef struct BdrvSaveVMState BdrvSaveVMState;
+typedef struct BdrvLoadVMState BdrvLoadVMState;
 
 /*
  * Note: the function bdrv_append() copies and swaps contents of
@@ -955,6 +956,8 @@ struct BlockDriverState {
 
     /* Intermediate buffer for VM state saving from snapshot creation code */
     BdrvSaveVMState *savevm_state;
+    /* Intermediate buffer for VM state loading */
+    BdrvLoadVMState *loadvm_state;
 };
 
 struct BlockBackendRootState {
-- 
2.17.1

Re: [PATCH v6 0/7] block: seriously improve savevm/loadvm performance

[no-reply]

Patchew URL: https://patchew.org/QEMU/20200703161130.23772-1-den@openvz.org/



Hi,

This series failed the docker-quick@centos7 build test. Please find the testing commands and
their output below. If you have Docker installed, you can probably reproduce it
locally.

=== TEST SCRIPT BEGIN ===
#!/bin/bash
make docker-image-centos7 V=1 NETWORK=1
time make docker-test-quick@centos7 SHOW_ENV=1 J=14 NETWORK=1
=== TEST SCRIPT END ===

 --        snap0                  SIZE yyyy-mm-dd hh:mm:ss   00:00:00.000
 (qemu) loadvm snap0
+Not a migration stream
+Error: Error -22 while loading VM state
 (qemu) quit
 
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
---
Not run: 259
Failures: 267
Failed 1 of 119 iotests
make: *** [check-tests/check-block.sh] Error 1
make: *** Waiting for unfinished jobs....
  TEST    check-qtest-aarch64: tests/qtest/qos-test
Traceback (most recent call last):
---
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['sudo', '-n', 'docker', 'run', '--label', 'com.qemu.instance.uuid=833b593236c549cfaff096556467168b', '-u', '1003', '--security-opt', 'seccomp=unconfined', '--rm', '-e', 'TARGET_LIST=', '-e', 'EXTRA_CONFIGURE_OPTS=', '-e', 'V=', '-e', 'J=14', '-e', 'DEBUG=', '-e', 'SHOW_ENV=1', '-e', 'CCACHE_DIR=/var/tmp/ccache', '-v', '/home/patchew2/.cache/qemu-docker-ccache:/var/tmp/ccache:z', '-v', '/var/tmp/patchew-tester-tmp-y5isxowg/src/docker-src.2020-07-03-12.37.49.32506:/var/tmp/qemu:z,ro', 'qemu:centos7', '/var/tmp/qemu/run', 'test-quick']' returned non-zero exit status 2.
filter=--filter=label=com.qemu.instance.uuid=833b593236c549cfaff096556467168b
make[1]: *** [docker-run] Error 1
make[1]: Leaving directory `/var/tmp/patchew-tester-tmp-y5isxowg/src'
make: *** [docker-run-test-quick@centos7] Error 2

real    17m38.244s
user    0m8.803s


The full log is available at
http://patchew.org/logs/20200703161130.23772-1-den@openvz.org/testing.docker-quick@centos7/?type=message.
---
Email generated automatically by Patchew [https://patchew.org/].
Please send your feedback to patchew-devel@redhat.com

Re: [PATCH 1/7] migration/savevm: respect qemu_fclose() error code in save_snapshot()

[Juan Quintela]

"Denis V. Lunev" <den@openvz.org> wrote:
> qemu_fclose() could return error, f.e. if bdrv_co_flush() will return
> the error.
>
> This validation will become more important once we will start waiting of
> asynchronous IO operations, started from bdrv_write_vmstate(), which are
> coming soon.
>
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> Reviewed-by: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Max Reitz <mreitz@redhat.com>
> CC: Stefan Hajnoczi <stefanha@redhat.com>
> CC: Fam Zheng <fam@euphon.net>
> CC: Juan Quintela <quintela@redhat.com>
> CC: Denis Plotnikov <dplotnikov@virtuozzo.com>

Reviewed-by: Juan Quintela <quintela@redhat.com>

queued


> ---
>  migration/savevm.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/migration/savevm.c b/migration/savevm.c
> index b979ea6e7f..da3dead4e9 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -2628,7 +2628,7 @@ int save_snapshot(const char *name, Error **errp)
>  {
>      BlockDriverState *bs, *bs1;
>      QEMUSnapshotInfo sn1, *sn = &sn1, old_sn1, *old_sn = &old_sn1;
> -    int ret = -1;
> +    int ret = -1, ret2;
>      QEMUFile *f;
>      int saved_vm_running;
>      uint64_t vm_state_size;
> @@ -2712,10 +2712,14 @@ int save_snapshot(const char *name, Error **errp)
>      }
>      ret = qemu_savevm_state(f, errp);
>      vm_state_size = qemu_ftell(f);
> -    qemu_fclose(f);
> +    ret2 = qemu_fclose(f);
>      if (ret < 0) {
>          goto the_end;
>      }
> +    if (ret2 < 0) {
> +        ret = ret2;
> +        goto the_end;
> +    }
>  
>      /* The bdrv_all_create_snapshot() call that follows acquires the AioContext
>       * for itself.  BDRV_POLL_WHILE() does not support nested locking because

[PATCH 3/7] block/aio_task: drop aio_task_pool_wait_one() helper

[Denis V. Lunev]

It is not used outside the module.

Actually there are 2 kind of waiters:
- for a slot and
- for all tasks to finish
This patch limits external API to listed types.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <fam@euphon.net>
CC: Juan Quintela <quintela@redhat.com>
CC: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
CC: Denis Plotnikov <dplotnikov@virtuozzo.com>
---
 block/aio_task.c         | 13 ++-----------
 include/block/aio_task.h |  1 -
 2 files changed, 2 insertions(+), 12 deletions(-)

diff --git a/block/aio_task.c b/block/aio_task.c
index cf62e5c58b..7ba15ff41f 100644
--- a/block/aio_task.c
+++ b/block/aio_task.c
@@ -54,26 +54,17 @@ static void coroutine_fn aio_task_co(void *opaque)
     qemu_co_queue_restart_all(&pool->waiters);
 }
 
-void coroutine_fn aio_task_pool_wait_one(AioTaskPool *pool)
-{
-    assert(pool->busy_tasks > 0);
-
-    qemu_co_queue_wait(&pool->waiters, NULL);
-
-    assert(pool->busy_tasks < pool->max_busy_tasks);
-}
-
 void coroutine_fn aio_task_pool_wait_slot(AioTaskPool *pool)
 {
     while (pool->busy_tasks >= pool->max_busy_tasks) {
-        aio_task_pool_wait_one(pool);
+        qemu_co_queue_wait(&pool->waiters, NULL);
     }
 }
 
 void coroutine_fn aio_task_pool_wait_all(AioTaskPool *pool)
 {
     while (pool->busy_tasks > 0) {
-        aio_task_pool_wait_one(pool);
+        qemu_co_queue_wait(&pool->waiters, NULL);
     }
 }
 
diff --git a/include/block/aio_task.h b/include/block/aio_task.h
index 50bc1e1817..50b1c036c5 100644
--- a/include/block/aio_task.h
+++ b/include/block/aio_task.h
@@ -48,7 +48,6 @@ bool aio_task_pool_empty(AioTaskPool *pool);
 void coroutine_fn aio_task_pool_start_task(AioTaskPool *pool, AioTask *task);
 
 void coroutine_fn aio_task_pool_wait_slot(AioTaskPool *pool);
-void coroutine_fn aio_task_pool_wait_one(AioTaskPool *pool);
 void coroutine_fn aio_task_pool_wait_all(AioTaskPool *pool);
 
 #endif /* BLOCK_AIO_TASK_H */
-- 
2.17.1