location ~ .php$ {
   try_files $uri =404;
   fastcgi_pass unix:/var/run/php-fpm7.sock;
}

[www123]
user = app
group = app
listen = /var/run/php-fpm7.sock
listen.owner = app
listen.group = app
listen.mode = 0666

pm = dynamic
pm.max_children = 5
pm.start_servers = 2
pm.min_spare_servers = 1
pm.max_spare_servers = 3

slowlog = /app/logs/my_slow_log/$pool.log.slow
request_slowlog_timeout = 1s

/app/logs/my_slow_log # ls -al
total 4
drwxr-xr-x    2 app      app             28 May 16 19:32 .
drwxrwxrwx    3 app      app           4096 May 16 19:00 ..
-rw-------    1 root     root             0 May 16 19:32 www123.log.slow

app/logs/my_slow_log # ps -ef
PID   USER     TIME   COMMAND
    1 root       0:00 runsvdir /etc/service
    7 root       0:00 runsv nginx
    8 root       0:00 runsv javabridge
    9 root       0:00 runsv php7
   10 root       0:00 java -jar /usr/lib/jvm/java-1.8-openjdk/jre/lib/ext/JavaBridge.jar SERVLET_LOCAL:8080
   11 root       0:00 nginx: master process /usr/sbin/nginx -c /etc/nginx/nginx.conf -g daemon off;
   12 root       0:00 {php-fpm7} php-fpm: master process (/etc/php7/php-fpm.conf)
   13 root       0:00 nginx: worker process
   14 root       0:00 nginx: worker process
   15 root       0:00 nginx: worker process
   16 root       0:00 nginx: worker process
   22 app        0:00 {php-fpm7} php-fpm: pool www123
   23 app        0:00 {php-fpm7} php-fpm: pool www123

user = root
group = root
listen.owner = root
listen.group = root

ERROR: [pool www123] please specify user and group other than root
ERROR: FPM initialization failed