Re: [PATCH 4/6] timers/nohz: Add a comment about broken iowait counter update race

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Feb 10, 2023 at 03:39:43PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 10, 2023 at 03:09:15PM +0100, Frederic Weisbecker wrote:
> > The per-cpu iowait task counter is incremented locally upon sleeping.
> > But since the task can be woken to (and by) another CPU, the counter may
> > then be decremented remotely. This is the source of a race involving
> > readers VS writer of idle/iowait sleeptime.
> > 
> > The following scenario shows an example where a /proc/stat reader
> > observes a pending sleep time as IO whereas that pending sleep time
> > later eventually gets accounted as non-IO.
> > 
> >     CPU 0                       CPU  1                    CPU 2
> >     -----                       -----                     ------
> >     //io_schedule() TASK A
> >     current->in_iowait = 1
> >     rq(0)->nr_iowait++
> >     //switch to idle
> >                         // READ /proc/stat
> >                         // See nr_iowait_cpu(0) == 1
> >                         return ts->iowait_sleeptime +
> >                                ktime_sub(ktime_get(), ts->idle_entrytime)
> > 
> >                                                           //try_to_wake_up(TASK A)
> >                                                           rq(0)->nr_iowait--
> >     //idle exit
> >     // See nr_iowait_cpu(0) == 0
> >     ts->idle_sleeptime += ktime_sub(ktime_get(), ts->idle_entrytime)
> > 
> > As a result subsequent reads on /proc/stat may expose backward progress.
> > 
> > This is unfortunately hardly fixable. Just add a comment about that
> > condition.
> 
> It is far worse than that, the whole concept of per-cpu iowait is
> absurd. Also see the comment near nr_iowait().

Alas I know :-(