Can I extend my 90-day S2 visa in Chengdu?

Generally yes. The documents required for the visa extension usually include your passport or other international travel documents, the temporary accommodation registration form issued by a local police station or a hotel, completely filled VISA / STAY PERMIT / RESIDENCE PERMIT APPLICATION FORM with recent half-length, white background, bareheaded, full-faced 2-inch color photo and other relevant certification documents that the Exit-entry Administration Department. And you can go to local by Exit-entry Administration Department to apply for that. And you need to fill in the apllication form 7 day prior to the expiry of your old one.

For more info, you can consult on the spot. But I have to remind you that they may refuse your application sometimes, in which case you have to go back within the expiry of your visa.