On Tue, Aug 22, 2017 at 10:25:42AM +0100, Juri Lelli wrote:
> Hi,
>
> On 22/08/17 17:30, Byungchul Park wrote:
> > It would be better to try to check other siblings first if
> > SD_PREFER_SIBLING is flaged when pushing tasks - migration.
> >
> > Signed-off-by: Byungchul Park <[email protected]>
> > Suggested-by: Peter Zijlstra <[email protected]>
> ^
> This has to come before your SoB.
Thank you, I will.
> > ---
> > kernel/sched/deadline.c | 80
> > ++++++++++++++++++++++++++++++++++++++++++++-----
> > 1 file changed, 73 insertions(+), 7 deletions(-)
> >
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index 0223694..b6b3855 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -1319,12 +1319,35 @@ static struct task_struct
> > *pick_earliest_pushable_dl_task(struct rq *rq, int cpu
> >
> > static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask_dl);
> >
> > +/*
> > + * Find the first cpu in: mask & sd & ~prefer
> > + */
> > +static int find_cpu(const struct cpumask *mask,
> > + const struct sched_domain *sd,
> > + const struct sched_domain *prefer)
> > +{
> > + const struct cpumask *sds = sched_domain_span(sd);
> > + const struct cpumask *ps = prefer ? sched_domain_span(prefer) : NULL;
> > + int cpu;
> > +
> > + for_each_cpu(cpu, mask) {
> > + if (!cpumask_test_cpu(cpu, sds))
> > + continue;
> > + if (ps && cpumask_test_cpu(cpu, ps))
> > + continue;
> > + break;
> > + }
> > +
> > + return cpu;
> > +}
> > +
> > static int find_later_rq(struct task_struct *task)
> > {
> > - struct sched_domain *sd;
> > + struct sched_domain *sd, *prefer = NULL;
> > struct cpumask *later_mask =
> > this_cpu_cpumask_var_ptr(local_cpu_mask_dl);
> > int this_cpu = smp_processor_id();
> > int cpu = task_cpu(task);
> > + int fallback_cpu = -1;
> >
> > /* Make sure the mask is initialized first */
> > if (unlikely(!later_mask))
> > @@ -1376,15 +1399,35 @@ static int find_later_rq(struct task_struct *task)
> > return this_cpu;
> > }
> >
> > - best_cpu = cpumask_first_and(later_mask,
> > - sched_domain_span(sd));
> > /*
> > - * Last chance: if a cpu being in both later_mask
> > - * and current sd span is valid, that becomes our
> > - * choice. Of course, the latest possible cpu is
> > - * already under consideration through later_mask.
> > + * If a cpu being in later_mask & current sd &
> > + * ~prefer sd is valid, that becomes our choice.
> > + * Of course, the latest possible cpu is already
> > + * under consideration through later_mask.
> > */
> > + best_cpu = find_cpu(later_mask, sd, prefer);
> > +
> > if (best_cpu < nr_cpu_ids) {
> > + /*
> > + * If current domain is SD_PREFER_SIBLING
> > + * flaged, we have to try to check other
> > + * siblings first.
> > + */
> > + if (sd->flags & SD_PREFER_SIBLING) {
> > + prefer = sd;
> > +
> > + /*
> > + * fallback_cpu should be one
> > + * in the closest domain among
> > + * SD_PREFER_SIBLING domains,
> > + * in case that more than one
> > + * SD_PREFER_SIBLING domains
> > + * exist in the hierachy.
> > + */
> > + if (fallback_cpu == -1)
> > + fallback_cpu = best_cpu;
> > + continue;
> > + }
> > rcu_read_unlock();
> > return best_cpu;
> > }
> > @@ -1393,6 +1436,29 @@ static int find_later_rq(struct task_struct *task)
> > rcu_read_unlock();
> >
> > /*
> > + * If fallback_cpu is valid, all our guesses failed *except* for
> > + * SD_PREFER_SIBLING domain. Now, we can return the fallback cpu.
> > + *
> > + * XXX: Consider the following example, 4 cores SMT2 system:
> > + *
> > + * LLC [0 - 7]
> > + * SMT [0 1][2 3][4 5][6 7]
> > + * o x o x x x x x
> > + *
> > + * where 'o': occupied and 'x': empty.
> > + *
> > + * A wakeup on cpu0 will exclude cpu1 and choose cpu3, since
> > + * cpu1 is in a SD_PREFER_SIBLING sd and cpu3 is not. However,
> > + * in this case, we have to choose cpu4 for better work, instead
>
> ... in this case cpu4 would have been a better choice, since cpu3 is a
> (SMT) thread of an already loaded core.
Thank you, I will.
> > + * of cpu3 that is fully loaded.
> > + *
> > + * We have to do the best if possible, but choose the second
> > + * best here since that is too expensive to adopt.
> > + */
>
> I'd also modify this last paragraph with something like:
>
> "Doing it 'right' is difficult and expensive. The current solution is an
> acceptable approximation."
Thank you, I will.
> Apart from these minor points, patch looks ok to me.
>
> Acked-by: Juri Lelli <[email protected]>
>
> Best,
>
> - Juri
