Provided by: systemtap-doc_2.3-1ubuntu1.4_all 

NAME
probe::ioscheduler_trace.plug - Fires when a request queue is plugged;
SYNOPSIS
ioscheduler_trace.plug
VALUES
rq_queue
request queue
name
Name of the probe point
DESCRIPTION
ie, requests in the queue cannot be serviced by block driver.
SystemTap Tapset Reference March 2016 PROBE::IOSCHEDULER_T(3stap)