Re: [PATCH v3 8/9] block/block-copy: reduce intersecting request lock

[Max Reitz]

On 06.03.20 08:38, 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.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> Reviewed-by: Andrey Shinkevich <address@hidden>
> ---
>  block/block-copy.c | 128 ++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 104 insertions(+), 24 deletions(-)
> 
> diff --git a/block/block-copy.c b/block/block-copy.c
> index 2b29131653..d66b8eb691 100644
> --- a/block/block-copy.c
> +++ b/block/block-copy.c

[...]

> +/* Called only on full-dirty region */
>  static void block_copy_inflight_req_begin(BlockCopyState *s,
>                                            BlockCopyInFlightReq *req,
>                                            int64_t offset, int64_t bytes)
>  {
> +    assert(!find_conflicting_inflight_req(s, offset, bytes));
> +
> +    bdrv_reset_dirty_bitmap(s->copy_bitmap, offset, bytes);
> +    s->in_flight_bytes += 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)
> +/*
> + * block_copy_inflight_req_shrink
> + *
> + * Drop the tail of the request to be handled later. Set dirty bits back and
> + * wake up all requests waiting for us (may be some of them are not 
> intersecting
> + * with shrunk request)
> + */
> +static void coroutine_fn block_copy_inflight_req_shrink(BlockCopyState *s,
> +        BlockCopyInFlightReq *req, int64_t new_bytes)
>  {
> +    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);

I think we need to reduce in_flight_bytes here.

> +
> +    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)
> +{
> +    s->in_flight_bytes -= req->bytes;
> +    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);
>  }

[...]

> @@ -432,7 +479,40 @@ int coroutine_fn block_copy(BlockCopyState *s,
>          bytes -= cur_bytes;
>      }
>  
> -    block_copy_inflight_req_end(&req);
> +    return found_dirty;
> +}
> +
> +/*
> + * block_copy
> + *
> + * Copy requested region, accordingly to dirty bitmap.
> + * Collaborate with parallel block_copy requests: if they success it help 
> us. If

s/success/succeed/, s/it help/it will help/

> + * they fail, we retry not-copied regions. So, if we return error, it means 
> that

s/retry/will retry/

(In theory also s/it means/it will mean/, but I suppose that also works
as-is.)

> + * io operation failed in context of _this_ block_copy call, not some 
> parallel

Perhaps rather “some I/O operation failed in the context of […]”?

> + * operation.
> + */
> +int coroutine_fn block_copy(BlockCopyState *s, int64_t offset, int64_t bytes,
> +                            bool *error_is_read)
> +{
> +    int ret;
> +
> +    do {
> +        ret = block_copy_dirty_clusters(s, offset, bytes, error_is_read);
> +
> +        if (ret == 0) {
> +            ret = block_copy_wait_one(s, offset, bytes);
> +        }
> +
> +        /*
> +         * We retry in two cases:
> +         * 1. Some progress done
> +         *    Something was copied, which means that there were yield points
> +         *    and some new dirty bits may have appeared (due to failed 
> parallel
> +         *    block-copy requests).
> +         * 2. We have waited for some intersecting block-copy request
> +         *    It may have failed and produced new dirty bits.
> +         */
> +    } while (ret > 0);
>  
>      return ret;
>  }

This new code looks good.

Max

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