Re: EEVDF and NUMA balancing

Vincent Guittot
2023-12-22T16:42:18Z

On Fri, 22 Dec 2023 at 17:29, Julia Lawall <julia.lawall@inria.fr> wrote:
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> On Fri, 22 Dec 2023, Julia Lawall wrote:
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> >
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> > On Fri, 22 Dec 2023, Vincent Guittot wrote:
> >
> > > On Fri, 22 Dec 2023 at 16:00, Julia Lawall <julia.lawall@inria.fr> wrote:
> > > >
> > > >
> > > >
> > > > On Fri, 22 Dec 2023, Vincent Guittot wrote:
> > > >
> > > > > On Thu, 21 Dec 2023 at 19:20, Julia Lawall <julia.lawall@inria.fr> wrote:
> > > > > >
> > > > > >
> > > > > >
> > > > > > On Wed, 20 Dec 2023, Vincent Guittot wrote:
> > > > > >
> > > > > > > On Tue, 19 Dec 2023 at 18:51, Julia Lawall <julia.lawall@inria.fr> 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.
> > > > >
> > > > > CPU_NEWLY_IDLE load balance is called when the scheduler was
> > > > > scheduling something that just migrated or went back to sleep and
> > > > > doesn't have anything to schedule so it tries to pull a task from
> > > > > somewhere else.
> > > > >
> > > > > But you should still have some CPU_IDLE load balance according to your
> > > > > description where one CPU of the socket remains idle and those will
> > > > > increase the nr_balance_failed
> > > >
> > > > This happens. But not often.
> > > >
> > > > > I'm surprised that you have mainly CPU_NEWLY_IDLE. Do you know the reason ?
> > > >
> > > > No. They come from do_idle calling the scheduler. I will look into why
> > > > this happens so often.
> > >
> > > Hmm, the CPU was idle and received a need resched which triggered the
> > > scheduler but there was nothing to schedule so it goes back to idle
> > > after running a newly_idle _load_balance.
> >
> > I spent quite some time thinking the same until I saw the following code
> > in do_idle:
> >
> > preempt_set_need_resched();
> >
> > So I have the impression that do_idle sets need resched itself.
>
> But of course that code is only executed if need_resched is true. But I

Yes, that is your root cause. something, most probably in interrupt
context, wakes up your CPU and expect to wake up a thread

> don't know who would be setting need resched on each clock tick.

that can be a timer, interrupt, ipi, rcu ...
a trace should give you some hints

>
> julia
>
> >
> > julia
> >
> > >
> > > >
> > > > >
> > > > > >
> > > > > > >
> > > > > > > >
> > > > > > > > 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.
> > > > >
> > > > > With env->fbq_type == all, load_balance and find_busiest_queue should
> > > > > be able to select the actual busiest queue with 2 threads.
> > > >
> > > > That's what it does. But nothing can be stolen because there is no active
> > > > balancing.
> > >
> > > My patch below should enable to pull a task from the 1st idle load
> > > balance that fails
> > >
> > > >
> > > > >
> > > > > But then I imagine that can_migrate/ migrate_degrades_locality
> > > > > prevents to detach the task
> > > >
> > > > Exactly.
> > > >
> > > > julia
> > > >
> > > > > >
> > > > > > 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.
> > > > >
> > > > > It waits for at least 1 failed CPU_IDLE load_balance
> > > > >
> > > > > >
> > > > > > 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
> > > > > > > > > > >
> > > > > > > > >
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> > > > >
> > >
> >