nginx 502 when working with gunicorn + uvicorn worker + fastAPI

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Hi team
I am facing an issue where when a user requests a fast API endpoint through Nginx, the Nginx server log emits 502 errors,which highlight "upstream prematurely closed connection while reading response header from upstream " in the Nginx log.
but I do not find what happened at that time.,as I can not find the corresponding logs in backend application log.Can someone help me how to investigate it?