> The patch below is a result of a discussion off-line with Christos.
> The granularity of the mtime doesn't need to be down to the nanosecond
> - using the kernel "time" variable (which is updated on each
> hardclock call) saves a potentially relatively expensive call to
> microtime/nanotime for each write, and is more than accurate enough to
> track pty idle times.

it isn't good to mix "time" and nanotime
because it can make timestamps go backward sometimes.

YAMAMOTO Takashi