Re: EEVDF and NUMA balancing

[Julia Lawall]

On Wed, 20 Dec 2023, Vincent Guittot wrote:

> On Tue, 19 Dec 2023 at 18:51, Julia Lawall <julia.lawall@xxxxxxxx> wrote:
> >
> > > > One CPU has 2 threads, and the others have one.  The one with two threads
> > > > is returned as the busiest one.  But nothing happens, because both of them
> > > > prefer the socket that they are on.
> > >
> > > This explains way load_balance uses migrate_util and not migrate_task.
> > > One CPU with 2 threads can be overloaded
> > >
> > > ok, so it seems that your 1st problem is that you have 2 threads on
> > > the same CPU whereas you should have an idle core in this numa node.
> > > All cores are sharing the same LLC, aren't they ?
> >
> > Sorry, not following this.
> >
> > Socket 1 has N-1 threads, and thus an idle CPU.
> > Socket 2 has N+1 threads, and thus one CPU with two threads.
> >
> > Socket 1 tries to steal from that one CPU with two threads, but that
> > fails, because both threads prefer being on Socket 2.
> >
> > Since most (or all?) of the threads on Socket 2 perfer being on Socket 2.
> > the only hope for Socket 1 to fill in its idle core is active balancing.
> > But active balancing is not triggered because of migrate_util and because
> > CPU_NEWLY_IDLE prevents the failure counter from ebing increased.
>
>  CPU_NEWLY_IDLE load_balance doesn't aims to do active load balance so
> you should focus on the CPU_NEWLY_IDLE load_balance

I'm still perplexed why a core that has been idle for 1 second or more is
considered to be newly idle.

>
> >
> > The part that I am currently missing to understand is that when I convert
> > CPU_NEWLY_IDLE to CPU_IDLE, it typically picks a CPU with only one thread
> > as busiest.  I have the impression that the fbq_type intervenes to cause
>
> find_busiest_queue skips rqs which only have threads preferring being
> in there. So it selects another rq with a thread that doesn't prefer
> its current node.
>
> do you know what is the value of env->fbq_type ?

I have seen one trace in which it is all.  There are 33 tasks on one
socket, and they are all considered to have a preference for that socket.

But I have another trace in which it is regular.  There are 33 tasks on
the socket, but only 32 have a preference.

>
> need_active_balance() probably needs a new condition for the numa case
> where the busiest queue can't be selected and we have to trigger an
> active load_balance on a rq with only 1 thread but that is not running
> on its preferred node. Something like the untested below :
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e5da5eaab6ce..de1474191488 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11150,6 +11150,24 @@ imbalanced_active_balance(struct lb_env *env)
>         return 0;
>  }
>
> +static inline bool
> +numa_active_balance(struct lb_env *env)
> +{
> +       struct sched_domain *sd = env->sd;
> +
> +       /*
> +        * We tried to migrate only a !numa task or a task on wrong node but
> +        * the busiest queue with such task has only 1 running task. Previous
> +        * attempt has failed so force the migration of such task.
> +        */
> +       if ((env->fbq_type < all) &&
> +           (env->src_rq->cfs.h_nr_running == 1) &&
> +           (sd->nr_balance_failed > 0))

The last condition will still be a problem because of CPU_NEWLY_IDLE.  The
nr_balance_failed counter doesn't get incremented very often.

julia

> +               return 1;
> +
> +       return 0;
> +}
> +
>  static int need_active_balance(struct lb_env *env)
>  {
>         struct sched_domain *sd = env->sd;
> @@ -11176,6 +11194,9 @@ static int need_active_balance(struct lb_env *env)
>         if (env->migration_type == migrate_misfit)
>                 return 1;
>
> +       if (numa_active_balance(env))
> +               return 1;
> +
>         return 0;
>  }
>
>
> > it to avoid the CPU with two threads that already prefer Socket 2.  But I
> > don't know at the moment why that is the case.  In any case, it's fine to
> > active balance from a CPU with only one thread, because Socket 2 will
> > even itself out afterwards.
> >
> > >
> > > You should not have more than 1 thread per CPU when there are N+1
> > > threads on a node with N cores / 2N CPUs.
> >
> > Hmm, I think there is a miscommunication about cores and CPUs.  The
> > machine has two sockets with 16 physical cores each, and thus 32
> > hyperthreads.  There are 64 threads running.
>
> Ok, I have been confused by what you wrote previously:
> " The context is that there are 2N threads running on 2N cores, one thread
> gets NUMA balanced to the other socket, leaving N+1 threads on one socket
> and N-1 threads on the other socket."
>
> I have assumed that there were N cores and 2N CPUs per socket as you
> mentioned Intel Xeon 6130 in the commit message . My previous emails
> don't apply at all with N CPUs per socket and the group_overloaded is
> correct.
>
>
>
> >
> > julia
> >
> > > This will enable the
> > > load_balance to try to migrate a task instead of some util(ization)
> > > and you should reach the active load balance.
> > >
> > > >
> > > > > In theory you should have the
> > > > > local "group_has_spare" and the busiest "group_fully_busy" (at most).
> > > > > This means that no group should be overloaded and load_balance should
> > > > > not try to migrate utli but only task
> > > >
> > > > I didn't collect information about the groups.  I will look into that.
> > > >
> > > > julia
> > > >
> > > > >
> > > > >
> > > > > >
> > > > > > and changing the above test to:
> > > > > >
> > > > > >         if ((env->migration_type == migrate_task || env->migration_type == migrate_util) &&
> > > > > >             (sd->nr_balance_failed > sd->cache_nice_tries+2))
> > > > > >
> > > > > > seems to solve the problem.
> > > > > >
> > > > > > I will test this on more applications.  But let me know if the above
> > > > > > solution seems completely inappropriate.  Maybe it violates some other
> > > > > > constraints.
> > > > > >
> > > > > > I have no idea why this problem became more visible with EEVDF.  It seems
> > > > > > to have to do with the time slices all turning out to be the same.  I got
> > > > > > the same behavior in 6.5 by overwriting the timeslice calculation to
> > > > > > always return 1.  But I don't see the connection between the timeslice and
> > > > > > the behavior of the idle task.
> > > > > >
> > > > > > thanks,
> > > > > > julia
> > > > >
> > >
>