django和nginx仍然返回502

Question

在上网后我发现了这些：
nginx upload client_max_body_size issue
https://www.scalescale.com/tips/nginx/502-bad-gateway-error-using-nginx/#

user www-data;
worker_processes auto;
pid /run/nginx.pid;

events {
    worker_connections 768;
}

http {
    sendfile on;
    tcp_nopush on;
    tcp_nodelay on;
    keepalive_timeout 65;

    client_body_temp_path      /tmp/;
    client_body_in_file_only   on;
    client_body_buffer_size    128K;
    client_max_body_size 30M;

    fastcgi_buffers 8 16k;
    fastcgi_buffer_size 32k;
    fastcgi_connect_timeout 300;
    fastcgi_send_timeout 300;
    fastcgi_read_timeout 300;    
    types_hash_max_size 2048;

    include /etc/nginx/mime.types;
    default_type application/octet-stream;

    ssl_protocols TLSv1 TLSv1.1 TLSv1.2; # Dropping SSLv3, ref: POODLE
    ssl_prefer_server_ciphers on;

    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/*;
}

./conf.d/default.conf

upstream app {
    server localhost:8000;
}

server {
    listen 80;
    server_name jobs.siamsbrand.com;
    return 301 https://$server_name$request_uri;
}

server {
    listen 443 ssl;
    server_name jobs.siamsbrand.com;
    add_header Strict-Transport-Security "max-age=31536000";
    proxy_pass_request_headers on;
    root /www/data;
    ssl_certificate /root/fullchain.pem;
    ssl_certificate_key /root/privkey.pem;
    include snippets/ssl-params.conf;

    location / {
            proxy_pass_request_headers on;
            proxy_set_header           Host $host;
            proxy_pass                 http://app;
    }

    location /static {
            try_files $uri $uri/ =404;
    }

    location /media {
            try_files $uri $uri/ =404;
    }

    location ~ /.well-known {
                allow all;
    }

}

gunicorn：

gunicorn --bind 0.0.0.0:8000 --workers 3 --worker-class gevent wsgi:application --log-level=INFO

问题： 30秒后我得到了502。

我哪里错了？