sched/core: split iowait state into two states

sched/core: split iowait state into two states

iowait is a bogus metric, but it's helpful in the sense that it allows
short waits to not enter sleep states that have a higher exit latency
than we would've picked for iowait'ing tasks. However, it's harmless in
that lots of applications and monitoring assumes that iowait is busy
time, or otherwise use it as a health metric. Particularly for async
IO it's entirely nonsensical.

Split the iowait part into two parts - one that tracks whether we need
boosting for short waits, and one that says we need to account the task
as such. ->in_iowait_acct nests inside of ->in_iowait, both for
efficiency reasons, but also so that the relationship between the two
is clear. A waiter may set ->in_wait alone and not care about the
accounting.

Existing users of nr_iowait() for accounting purposes are switched to
use nr_iowait_acct(), which leaves the governor using nr_iowait() as it
only cares about iowaiters, not the accounting side.

io_schedule_prepare() and io_schedule_finish() are changed to return
a simple mask of two state bits, as we now have more than one state to
manage. Outside of that, no further changes are needed to suppor this
generically.

Signed-off-by: Jens Axboe <axboe@kernel.dk>