when or what is iowait CPU time on Linux

basically, when any process need to do I/O and underlying library calls io_schedule() will put CPU into iowait status and then if the CPU is idle (no other active process running), iowait time increases.

below is related code snip from 3.x kernel:


// in kernel/sched/cputime.c                                                                                                                                                                  
/*
 * Account for idle time.
 * @cputime: the cpu time spent in idle wait
 */
void account_idle_time(cputime_t cputime)
{
    u64 *cpustat = kcpustat_this_cpu->cpustat;
    struct rq *rq = this_rq();                                      // get running queue of this processor/CPU

    if (atomic_read(&rq->nr_iowait) > 0)                 // if there's process in iowait status
        cpustat[CPUTIME_IOWAIT] += (__force u64) cputime;
    else
        cpustat[CPUTIME_IDLE] += (__force u64) cputime;
}

// in kernel/sched/core.c
//
void __sched io_schedule(void)
{
    struct rq *rq = raw_rq();

    delayacct_blkio_start();
    atomic_inc(&rq->nr_iowait);                // before entering schedule()
    blk_flush_plug(current);
    current->in_iowait = 1;
    schedule();
    current->in_iowait = 0;
    atomic_dec(&rq->nr_iowait);              // returned from schedule()
    delayacct_blkio_end();
}