How to get full query text from performance_schema.threads? According to documentation:
Access to the threads table does not require a mutex 
and has minimal impact on server  performance. The other
sources have negative performance consequences because they require a mutex. 

So yes, I want to use threads table to examine running queries. The trouble is, the PROCESSLIST_INFO column does not contain the full query. It just contains a truncated version of the query. There is a flag viz performance_schema_max_sql_text_length which controls how many bytes are allocated for the SQL query. But it seems to affect the events_statements_history_long table. There is no change in the thread table output. There is also the events_statement_current table. But it is not clear if accessing that table affects performance or not.
Digging more into documentation:
The Performance Schema is intended to provide access to 
useful information about server execution while having minimal
impact on server performance

It looks safe to use performance_schema.events_statement_current for monitoring currently running queries and events_statement_current.SQL_TEXT gives full query text as long as it fits performance_schema_max_sql_text_length bytes.