Messages in this thread | | | Date | Thu, 24 Aug 2017 09:52:31 +0900 | From | Byungchul Park <> | Subject | Re: [PATCH v9 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq() |
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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 <byungchul.park@lge.com> > > Suggested-by: Peter Zijlstra <peterz@infradead.org> > ^ > 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 <juri.lelli@arm.com> > > Best, > > - Juri
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