Hello!

    2. When a packet was SACKed but not retransmitted, the linux also
calculate the seq_rtt from it when there is no unSACKed packet in the queue.
     I cannot understand the second situation: The packet was SACKed before,
that means it arrived the receiver and triggered a SACK sometime before. The
interval between when packet is sent and when the SACK is received should be
the RTT for experienced this packet. Even now the packet is ACKed, I don't
think this ACK is triggered by this packet. Why is it used to calculate the
RTT?

It is not used. When a segment fills a hole, tcp uses skb->when of the segment
which _filled_ the hole. See?

What's about using SACKs to give additional feed to rtt estimator,
even when ACK is duplicate, it is intersting idea, I even read about
this somewhere. But we do not use this.

Alexey