关于 Nginx 负载均衡的配置问题

我最近在学习 Nginx，想配置一下 Nginx 的负载均衡，在同一台机器上启动了两个 tomcat，但使用 localhost 访问时报错：502 Bad Gateway。求 v 友帮忙看一下问题出在哪里？一下是我的配置文件:
user www-data; worker_processes 4; pid /run/nginx.pid;

events { worker_connections 768;
# multi_accept on;
}

http { upstream test_A {
server 127.0.0.1:8000;
server 127.0.0.1:8001;
}

 server { listen 80; server_name localhost; location / { # proxy_pass http://127.0.0.1:8080; proxy_pass http://test_A; # index home.html; } } sendfile on; tcp_nopush on; tcp_nodelay on; keepalive_timeout 65; types_hash_max_size 2048; # server_tokens off; # server_names_hash_bucket_size 64; # server_name_in_redirect off; include /etc/nginx/mime.types; default_type application/octet-stream; ## # Logging Settings ## access_log /var/log/nginx/access.log; error_log /var/log/nginx/error.log; gzip on; gzip_disable "msie6"; include /etc/nginx/conf.d/*.conf; include /etc/nginx/sites-enabled/*; 

}