Re: [PATCH 7/6] block/io: improve loadvm performance

[Vladimir Sementsov-Ogievskiy]

22.06.2020 11:33, Denis V. Lunev wrote:
> 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: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> CC: Denis Plotnikov <dplotnikov@virtuozzo.com>
> ---
>
>   block/io.c                | 225 +++++++++++++++++++++++++++++++++++++-
>   include/block/block_int.h |   3 +
>   2 files changed, 225 insertions(+), 3 deletions(-)
>
> diff --git a/block/io.c b/block/io.c
> index 71a696deb7..bb06f750d8 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -2739,6 +2739,180 @@ 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_start_loadvmstate(BlockDriverState *bs,
> +                                      BdrvLoadVMState *state)
> +{
> +    int i;
> +    size_t buf_size = MAX(bdrv_get_cluster_size(bs), 1 * MiB);
> +
> +    qemu_co_mutex_assert_locked(&state->lock);
> +    for (i = state->chunk_count; i < BDRV_VMSTATE_WORKERS_MAX; i++) {
> +        BdrvLoadVMStateTask *t = g_new(BdrvLoadVMStateTask, 1);
> +
> +        *t = (BdrvLoadVMStateTask) {
> +            .task.func = bdrv_co_vmstate_load_task_entry,
> +            .bs = bs,
> +            .chunk = g_new(BdrvLoadVMChunk, 1),
> +        };
> +
> +        *t->chunk = (BdrvLoadVMChunk) {
> +            .buf = qemu_blockalign(bs, buf_size),
> +            .offset = state->last_loaded,
> +            .bytes = buf_size,
> +        };
> +        /* FIXME: tail of stream */
> +
> +        QLIST_INSERT_HEAD(&state->loading, t->chunk, list);
> +        state->chunk_count++;
> +        state->last_loaded += buf_size;
> +
> +        qemu_co_mutex_unlock(&state->lock);
> +        aio_task_pool_start_task(state->pool, &t->task);

Hmm, so, we unlock the mutex here. Am I missing something, or if there are
two parallel load-state requests in parallel (yes, this shouldn't happen
with current migration code, but if don't care at all the mutex is not needed),

we may have two such loops in parallel, which will create more than
BDRV_VMSTATE_WORKERS_MAX chunks, as each has own local "i" variable?

> +        qemu_co_mutex_lock(&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;
> +
> +    if (state == NULL) {
> +        if (pos != 0) {
> +            /* Normally this branch is not reachable from migration */
> +            return bs->drv->bdrv_load_vmstate(bs, qiov, pos);
> +        }
> +
> +        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;
> +    }
> +
> +    if (state->offset != pos) {
> +        /* Normally this branch is not reachable from migration */
> +        return bs->drv->bdrv_load_vmstate(bs, qiov, pos);
> +    }
> +
> +    off = 0;
> +    qemu_co_mutex_lock(&state->lock);
> +    bdrv_co_start_loadvmstate(bs, state);
> +
> +    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);
> +                bdrv_free_loadvm_chunk(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;
> +        }
> +
> +        bdrv_co_start_loadvmstate(bs, state);

Hmm, so, you assume that if we are in a loop, we'll definitely find our chunk 
at some moment.
It should work with normal migration stream, sequentially reading chunk by 
chunk. But if not,
it may hang:

Consider two reads at same position, running in parallel: they both pass 
"state->offset != pos"
check and wait on mutex. Then the first one will go into critical section, find 
and remove
corresponding chunk. Then the second will go into critical section and loop 
forever, because
corresponding chunk will never be generated again.

So, seems, we should check (state->offset == pos) on each iteration, or 
something like this.

====

Another idea came into my mind is that you create new requests when we can't 
find our chunk.
It may be a bit more optimal to create new requests as soon as we get a free 
slot after
QLIST_REMOVE(..)

And, if we do so, we don't need "loading" list: if there is no corresponding 
chunk in chunks,
we will just wait in "waiters" queue.



> +    }
> +    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 +2926,7 @@ 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 +2997,13 @@ 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 +3025,51 @@ 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 */

assert(1) never fail, abort() you mean..

or better assert(QLIST_EMPTY(&state->loading))

> +    }
> +    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 {

Also, need call finalize from blk_load_vmstate(), like it's done for 
blk_save_vmstate().

--
Best regards,
Vladimir