Re: [PATCH v2 6/7] block/block-copy: reduce intersecting request lock

[Max Reitz]

On 27.11.19 19:08, Vladimir Sementsov-Ogievskiy wrote:
> Currently, block_copy operation lock the whole requested region. But
> there is no reason to lock clusters, which are already copied, it will
> disturb other parallel block_copy requests for no reason.
> 
> Let's instead do the following:
> 
> Lock only sub-region, which we are going to operate on. Then, after
> copying all dirty sub-regions, we should wait for intersecting
> requests block-copy, if they failed, we should retry these new dirty
> clusters.

Just a thought spoken aloud:

I would expect the number of intersecting CBW requests to be low in
general, so I don’t know how useful this change is in practice.  OTOH,
it makes block_copy call the existing implementation in a loop, which
seems just worse.

But then again, in the common case, block_copy_dirty_clusters() won’t
copy anything because it’s all been copied already, so there is no
change; and even if something is copied, the second call will just
re-check the dirty bitmap to see that the area’s clean (which will be
quick compared to the copy operation).  So there’s probably nothing to
worry about.

> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
>  block/block-copy.c | 116 +++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 95 insertions(+), 21 deletions(-)
> 
> diff --git a/block/block-copy.c b/block/block-copy.c
> index 20068cd699..aca44b13fb 100644
> --- a/block/block-copy.c
> +++ b/block/block-copy.c
> @@ -39,29 +39,62 @@ static BlockCopyInFlightReq 
> *block_copy_find_inflight_req(BlockCopyState *s,
>      return NULL;
>  }
>  
> -static void coroutine_fn block_copy_wait_inflight_reqs(BlockCopyState *s,
> -                                                       int64_t offset,
> -                                                       int64_t bytes)
> +/*
> + * If there are no intersecting requests return false. Otherwise, wait for 
> the
> + * first found intersecting request to finish and return true.
> + */
> +static bool coroutine_fn block_copy_wait_one(BlockCopyState *s, int64_t 
> start,
> +                                             int64_t end)

s/end/bytes/?

(And maybe s/start/offset/, too)

>  {
> -    BlockCopyInFlightReq *req;
> +    BlockCopyInFlightReq *req = block_copy_find_inflight_req(s, start, end);
>  
> -    while ((req = block_copy_find_inflight_req(s, offset, bytes))) {
> -        qemu_co_queue_wait(&req->wait_queue, NULL);
> +    if (!req) {
> +        return false;
>      }
> +
> +    qemu_co_queue_wait(&req->wait_queue, NULL);
> +
> +    return true;
>  }
>  
> +/* Called only on full-dirty region */
>  static void block_copy_inflight_req_begin(BlockCopyState *s,
>                                            BlockCopyInFlightReq *req,
>                                            int64_t offset, int64_t bytes)
>  {
> +    assert(!block_copy_find_inflight_req(s, offset, bytes));
> +
> +    bdrv_reset_dirty_bitmap(s->copy_bitmap, offset, bytes);
> +
>      req->offset = offset;
>      req->bytes = bytes;
>      qemu_co_queue_init(&req->wait_queue);
>      QLIST_INSERT_HEAD(&s->inflight_reqs, req, list);
>  }
>  
> -static void coroutine_fn block_copy_inflight_req_end(BlockCopyInFlightReq 
> *req)
> +static void coroutine_fn block_copy_inflight_req_shrink(BlockCopyState *s,
> +        BlockCopyInFlightReq *req, int64_t new_bytes)

It took me a while to understand that this is operation drops the tail
of the request.  I think there should be a comment on this.

(I thought it would successively drop the head after each copy, and so I
was wondering why the code didn’t match that.)

>  {
> +    if (new_bytes == req->bytes) {
> +        return;
> +    }
> +
> +    assert(new_bytes > 0 && new_bytes < req->bytes);
> +
> +    bdrv_set_dirty_bitmap(s->copy_bitmap,
> +                          req->offset + new_bytes, req->bytes - new_bytes);> 
> +
> +    req->bytes = new_bytes;
> +    qemu_co_queue_restart_all(&req->wait_queue);
> +}
> +
> +static void coroutine_fn block_copy_inflight_req_end(BlockCopyState *s,
> +                                                     BlockCopyInFlightReq 
> *req,
> +                                                     int ret)
> +{
> +    if (ret < 0) {
> +        bdrv_set_dirty_bitmap(s->copy_bitmap, req->offset, req->bytes);
> +    }
>      QLIST_REMOVE(req, list);
>      qemu_co_queue_restart_all(&req->wait_queue);
>  }
> @@ -344,12 +377,19 @@ int64_t block_copy_reset_unallocated(BlockCopyState *s,
>      return ret;
>  }
>  
> -int coroutine_fn block_copy(BlockCopyState *s,
> -                            int64_t offset, uint64_t bytes,
> -                            bool *error_is_read)
> +/*
> + * block_copy_dirty_clusters
> + *
> + * Copy dirty clusters in @start/@bytes range.
> + * Returns 1 if dirty clusters found and successfully copied, 0 if no dirty
> + * clusters found and -errno on failure.
> + */
> +static int coroutine_fn block_copy_dirty_clusters(BlockCopyState *s,
> +                                                  int64_t offset, int64_t 
> bytes,
> +                                                  bool *error_is_read)
>  {
>      int ret = 0;
> -    BlockCopyInFlightReq req;
> +    bool found_dirty = false;
>  
>      /*
>       * block_copy() user is responsible for keeping source and target in same
> @@ -361,10 +401,8 @@ int coroutine_fn block_copy(BlockCopyState *s,
>      assert(QEMU_IS_ALIGNED(offset, s->cluster_size));
>      assert(QEMU_IS_ALIGNED(bytes, s->cluster_size));
>  
> -    block_copy_wait_inflight_reqs(s, offset, bytes);
> -    block_copy_inflight_req_begin(s, &req, offset, bytes);
> -
>      while (bytes) {
> +        BlockCopyInFlightReq req;
>          int64_t next_zero, cur_bytes, status_bytes;
>  
>          if (!bdrv_dirty_bitmap_get(s->copy_bitmap, offset)) {
> @@ -374,6 +412,8 @@ int coroutine_fn block_copy(BlockCopyState *s,
>              continue; /* already copied */
>          }
>  
> +        found_dirty = true;
> +
>          cur_bytes = MIN(bytes, s->copy_size);
>  
>          next_zero = bdrv_dirty_bitmap_next_zero(s->copy_bitmap, offset,
> @@ -383,10 +423,12 @@ int coroutine_fn block_copy(BlockCopyState *s,
>              assert(next_zero < offset + cur_bytes); /* no need to do MIN() */
>              cur_bytes = next_zero - offset;
>          }
> +        block_copy_inflight_req_begin(s, &req, offset, cur_bytes);
>  
>          ret = block_copy_block_status(s, offset, cur_bytes, &status_bytes);
> +        block_copy_inflight_req_shrink(s, &req, status_bytes);

block_copy_inflight_req_shrink() asserts that status_bytes <= cur_bytes.
 That isn’t necessarily correct, as block_copy_block_status() rounds up
on the last cluster.  So this should use the same MIN() as for the
cur_bytes update after the next block.

Would it make sense to move the block_copy_inflight_req_shrink() there
and pass the updated cur_bytes to it?

>          if (s->skip_unallocated && !(ret & BDRV_BLOCK_ALLOCATED)) {
> -            bdrv_reset_dirty_bitmap(s->copy_bitmap, offset, status_bytes);
> +            block_copy_inflight_req_end(s, &req, 0);
>              s->progress_reset_callback(s->progress_opaque);
>              trace_block_copy_skip_range(s, offset, status_bytes);
>              offset += status_bytes;
> @@ -398,15 +440,13 @@ int coroutine_fn block_copy(BlockCopyState *s,
>  
>          trace_block_copy_process(s, offset);
>  
> -        bdrv_reset_dirty_bitmap(s->copy_bitmap, offset, cur_bytes);
> -
>          co_get_from_shres(s->mem, cur_bytes);
>          ret = block_copy_do_copy(s, offset, cur_bytes, ret & BDRV_BLOCK_ZERO,
>                                   error_is_read);
>          co_put_to_shres(s->mem, cur_bytes);
> +        block_copy_inflight_req_end(s, &req, ret);
>          if (ret < 0) {
> -            bdrv_set_dirty_bitmap(s->copy_bitmap, offset, cur_bytes);
> -            break;
> +            return ret;
>          }
>  
>          s->progress_bytes_callback(cur_bytes, s->progress_opaque);
> @@ -414,7 +454,41 @@ int coroutine_fn block_copy(BlockCopyState *s,
>          bytes -= cur_bytes;
>      }
>  
> -    block_copy_inflight_req_end(&req);
> +    return found_dirty;
> +}
>  
> -    return ret;
> +int coroutine_fn block_copy(BlockCopyState *s, int64_t start, uint64_t bytes,
> +                            bool *error_is_read)
> +{
> +    while (true) {
> +        int ret = block_copy_dirty_clusters(s, start, bytes, error_is_read);
> +
> +        if (ret < 0) {
> +            /*
> +             * IO operation failed, which means the whole block_copy request
> +             * failed.
> +             */
> +            return ret;
> +        }
> +        if (ret) {
> +            /*
> +             * Something was copied, which means that there were yield points
> +             * and some new dirty bits may appered (due to failed parallel

s/appered/have appeared/

> +             * block-copy requests).
> +             */
> +            continue;
> +        }
> +
> +        /*
> +         * Here ret == 0, which means that there is no dirty clusters in
> +         * requested region.
> +         */
> +
> +        if (!block_copy_wait_one(s, start, bytes)) {
> +            /* No dirty bits and nothing to wait: the whole request is done 
> */

Wouldn’t it make more sense to keep block_copy_wait_one() a loop (i.e.,
keep it as block_copy_wait_inflight_reqs()) that returns whether it
waited or not?  Because I suppose if we had to wait for anything, we
might as well wait for everything in the range.

> +            break;
> +        }
> +    }

Continuing my loud thought from the beginning, I would have written this
as a tail-recursive function to stress that this isn’t really a
(potentially expensive) loop but more of a re-check to be sure.

(i.e.

int ret = block_copy_dirty...();
if (ret < 0) {
    return ret;
}

if (ret || block_copy_wait_one()) {
    /* Something might have changed, re-check */
    return block_copy();
}

/* Done */
return 0;
)

But who cares.

Max

> +
> +    return 0;
>  }
>

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