As discussed in this answer, SRSTIKHANDA of Padma Purana says:
Shudras are not entitled to do penance in all the three yugas, except the Kali Yuga.
Does it mean Shudras are allowed to penance in Kali Yuga?
Shudras are not allowed to chant mantras. So how they do penance?
Isn't Jabala in Treta yuga was Shudra?