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

[Suleiman Souhlal <ssouhlal@freebsd.org>]

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. 

> 
> 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.

> 
> In
> https://lore.kernel.org/all/20240522001817.619072-22-dwmw2@infradead.org/
> I put a limit on the amount of steal time carried forward from one
> timeslice to the next, as it was misbehaving when a bad hypervisor
> reported negative steal time. But I don't think the limit should be
> zero.
> 
> Of course, *ideally* we'd be able to sample the time and steal time
> *simultaneously*, with a single sched_clock_cpu_and_steal() function so
> that we don't have to deal with this slop between readings. Then none
> of this would be necessary. But that seems hard.

I agree that that would be ideal.

Thanks,
-- Suleiman