Re: [PATCH v2] sched: Don't try to catch up excess steal time.

David Woodhouse
2024-09-25T14:27:04Z

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On Wed, 2024-09-25 at 13:25 +0000, Suleiman Souhlal wrote:
> On Wed, Sep 25, 2024 at 12:45:55PM +0100, David Woodhouse wrote:
> > On Wed, 2024-09-11 at 20:15 +0900, Suleiman Souhlal wrote:
> > > When steal time exceeds the measured delta when updating clock_task,
> > > we
> > > currently try to catch up the excess in future updates.
> > > However, this results in inaccurate run times for the future things
> > > using
> > > clock_task, as they end up getting additional steal time that did not
> > > actually happen.
> > >
> > > For example, suppose a task in a VM runs for 10ms and had 15ms of
> > > steal
> > > time reported while it ran. clock_task rightly doesn't advance. Then,
> > > a
> > > different taks runs on the same rq for 10ms without any time stolen
> > > in
> > > the host.
> > > Because of the current catch up mechanism, clock_sched inaccurately
> > > ends
> > > up advancing by only 5ms instead of 10ms even though there wasn't any
> > > actual time stolen. The second task is getting charged for less time
> > > than it ran, even though it didn't deserve it.
> > > This can result in tasks getting more run time than they should
> > > actually
> > > get.
> > >
> > > So, we instead don't make future updates pay back past excess stolen
> > > time.
> > >
> > > Signed-off-by: Suleiman Souhlal <suleiman@google.com>
> > > ---
> > > v2:
> > > - Slightly changed to simply moving one line up instead of adding
> > >   new variable.
> > >
> > > v1:
> > > https://lore.kernel.org/lkml/20240806111157.1336532-1-suleiman@google.com
> > > ---
> > >  kernel/sched/core.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index f3951e4a55e5..6c34de8b3fbb 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -730,11 +730,11 @@ static void update_rq_clock_task(struct rq *rq,
> > > s64 delta)
> > >         if (static_key_false((&paravirt_steal_rq_enabled))) {
> > >                 steal = paravirt_steal_clock(cpu_of(rq));
> > >                 steal -= rq->prev_steal_time_rq;
> > > +               rq->prev_steal_time_rq += steal;
> >
> > The above two lines are essentially:
> >
> >         steal -= prev;
> >         prev += steal;
> >
> > It's like one of those clever ways of exchanging two variables with
> > three XOR operations. I don't like it :)
> >
> > Ultimately, you're just setting rq->prev_steal_time_rq to the latest
> > value that you just read from paravirt_steal_clock(). And then setting
> > 'steal' to the delta between the new reading and the previous one.
> >
> > The code above is *far* from obvious. At the very least it wants a
> > comment, but I'd rather see it refactored so that it doesn't need one.
> >
> >     u64 abs_steal_now = paravirt_steal_clock(cpu_of(rq));
> >     steal = abs_steal_now - rq->prev_steal_time_rq;
> >     rq->prev_steal_time_rq = abs_steal_now;
>
> That is what v1 did:
> https://lore.kernel.org/lkml/20240806111157.1336532-1-suleiman@google.com/
>
> It is also more obvious to me, but the feedback I received was that
> the way in the current iteration is better.
>
> I don't feel strongly about it, and I'd be ok with either version applied.

Fair enough. Not really a hill anyone should choose to die on, I
suppose.

> >
> > I'm still not utterly convinced this is the right thing to do, though.
> > It means you will constantly undermeasure the accounting of steal time
> > as you discard the excess each time.
> >
> > The underlying bug here is that we are sampling the steal time and the
> > time slice at *different* times. This update_rq_clock_task() function
> > could be called with a calculated 'delta' argument... and then
> > experience a large amount of steal time *before* calling
> > paravirt_steal_clock(), which is how we end up in the situation where
> > the calculated steal time exceeds the running time of the previous
> > task.
> >
> > Which task *should* that steal time be accounted to? At the moment it
> > ends up being accounted to the next task to run — which seems to make
> > sense to me. In the situation I just described, we can consider the
> > time stolen in update_rq_clock_task() itself to have been stolen from
> > the *incoming* task, not the *outgoing* one. But that seems to be what
> > you're objecting to?
>
> This is a good description of the problem, except that the time stolen
> in update_rq_clock_task() itself is actually being stolen from the
> outgoing task. This is because we are still trying to calculate how long
> it ran for (update_curr()), and time hasn't started ticking for the
> incoming task yet. We haven't set the incoming task's exec_start with the
> new clock_task time yet.
>
> So, in my opinion, it's wrong to give that time to the incoming task.

That makes sense. That steal time is actually stolen from *neither*
task, since it's after the 'end' timestamp of the outgoing task, and
before the 'start' timestamp of the incoming task.

So where *should* it be accounted?

Or is it actually correct to drop it completely?

If you can make a coherent case for the fact that dropping it is really
the right thing to do (not *just* that it doesn't belong to the
outgoing task, which is the case you make in your existing commit
message), then I suppose I'm OK with your patch as-is.
>

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