I am going screen logs of devstack with following command.
screen -x stack
/ Vi-like search forward.
? Vi-like search backward.
C-a s Emacs style incremental search forward.
C-r Emacs style reverse i-search.
Once you are in scrollback mode with Ctrl + A, ESC, you should be able to search backwards through the buffer as described in the manual.
So to search for the word "string", press Ctrl + A, ESC, enter
?string and press Enter. It will take you backwards through the buffer to the word string. Pressing N will go to the next match (backwards) in the buffer.
To search forwards, the cursor should be anywhere but the end of the buffer, then use
/string to search down.