blk_lld_busy — Check if underlying low-level drivers of a device are busy
|struct request_queue * q|
Check if underlying low-level drivers of a device are busy.
If the drivers want to export their busy state, they must set own
exporting function using
Basically, this function is used only by request stacking drivers to stop dispatching requests to underlying devices when underlying devices are busy. This behavior helps more I/O merging on the queue of the request stacking driver and prevents I/O throughput regression on burst I/O load.