Man page 9 : part_round_stats

part_round_stats - Round off the performance stats on a struct disk_stats.

void part_round_stats(int  cpu , struct hd_struct *  part );

cpu
    cpu number for stats access

part
    target partition

The average IO queue length and utilisation statistics are maintained by observing the current state of the queue length and the amount of time it has been in this state for.

Normally, that accounting is done on IO completion, but that can result in more than a secondAqs worth of IO being accounted for within any one second, leading to >100% utilisation. To deal with that, we call this function to do a round-off before returning the results when reading /proc/diskstats. This accounts immediately for all queue usage up to the current jiffies and restarts the counters again.