Re: [PATCH 6/7] sched/fair: Remove on_null_domain() and redundant checks

[Pierre Gondois]

Hello Peter,

On 4/4/24 09:27, Peter Zijlstra wrote:
> On Wed, Apr 03, 2024 at 05:05:38PM +0200, Pierre Gondois wrote:
> > CPUs with a NULL sched domain are removed from the HKR_TYPE_SCHED
> > isolation mask. The two following checks are equialent:
> > - !housekeeping_runtime_test_cpu(cpu, HKR_TYPE_SCHED)
> > - on_null_domain(rq)
> >
> > Remove on_null_domain() and the redundant checks.
> >
> > Signed-off-by: Pierre Gondois <pierre.gondois@xxxxxxx>
> > ---
> >   kernel/sched/fair.c | 18 ++++++------------
> >   1 file changed, 6 insertions(+), 12 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 3e0f2a0f153f..9657c8f2176b 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -11830,11 +11830,6 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
> >   
> >   }
> >   
> > -static inline int on_null_domain(struct rq *rq)
> > -{
> > -	return unlikely(!rcu_dereference_sched(rq->sd));
> > -}
> > -
> >   #ifdef CONFIG_NO_HZ_COMMON
> >   /*
> >    * NOHZ idle load balancing (ILB) details:
> > @@ -12040,7 +12035,7 @@ void nohz_balance_exit_idle(struct rq *rq)
> >   	SCHED_WARN_ON(rq != this_rq());
> >   
> >   	/* If we're a completely isolated CPU, we don't play: */
> > -	if (on_null_domain(rq))
> > +	if (!housekeeping_runtime_test_cpu(cpu_of(rq), HKR_TYPE_SCHED))
> >   		return;
> >   
> >   	if (likely(!rq->nohz_tick_stopped))
>
> This seems broken, the whole null domain can happen with cpusets, but
> this housekeeping nonsense is predicated on CPU_ISOLATION and none of
> that is mandatory for CPUSETS.

ok right,
I will try to remove this implicit dependency,

Regards,
Pierre