Messages in this thread | | | Date | Mon, 12 Dec 2022 09:53:45 -0800 | From | Ricardo Neri <> | Subject | Re: [PATCH v2 1/7] sched/fair: Generalize asym_packing logic for SMT local sched group |
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On Tue, Dec 06, 2022 at 06:22:41PM +0100, Dietmar Eggemann wrote: > On 22/11/2022 21:35, Ricardo Neri wrote: > > When balancing load between two physical cores, an idle destination CPU can > > help another core only if all of its SMT siblings are also idle. Otherwise, > > there is not increase in throughput. It does not matter whether the other > > core is composed of SMT siblings. > > > > Simply check if there are any tasks running on the local group and the > > other core has exactly one busy CPU before proceeding. Let > > find_busiest_group() handle the case of more than one busy CPU. This is > > true for SMT2, SMT4, SMT8, etc. > > [...]
Thank you very much for your feedback, Dietmar!
> > > Changes since v1: > > * Reworded commit message and inline comments for clarity. > > * Stated that this changeset does not impact STM4 or SMT8. > > --- > > kernel/sched/fair.c | 29 +++++++++-------------------- > > 1 file changed, 9 insertions(+), 20 deletions(-) > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index e4a0b8bd941c..18c672ff39ef 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -8900,12 +8900,10 @@ static bool asym_smt_can_pull_tasks(int dst_cpu, struct sd_lb_stats *sds, > > struct sched_group *sg) > > I'm not sure why you change asym_smt_can_pull_tasks() together with > removing SD_ASYM_PACKING from SMT level (patch 5/7)?
In x86 we have SD_ASYM_PACKING at the MC, CLS* and, before my patches, SMT sched domains.
> > update_sg_lb_stats() > > ... && env->sd->flags & SD_ASYM_PACKING && .. && sched_asym() > ^^^^^^^^^^^^ > sched_asym() > > if ((sds->local->flags & SD_SHARE_CPUCAPACITY) || > (group->flags & SD_SHARE_CPUCAPACITY)) > return asym_smt_can_pull_tasks() > ^^^^^^^^^^^^^^^^^^^^^^^^^ > > So x86 won't have a sched domain with SD_SHARE_CPUCAPACITY and > SD_ASYM_PACKING anymore. So sched_asym() would call sched_asym_prefer() > directly on MC. What do I miss here?
asym_smt_can_pull_tasks() is used above the SMT level *and* when either the local or sg sched groups are composed of CPUs that are SMT siblings.
In fact, asym_smt_can_pull_tasks() can only be called above the SMT level. This is because the flags of a sched_group in a sched_domain are equal to the flags of the child sched_domain. Since SMT is the lowest sched_domain, its groups' flags are 0.
sched_asym() calls sched_asym_prefer() directly if balancing at the SMT level and, at higher domains, if the child domain is not SMT.
This meets the requirement of Power7, where SMT siblings have different priorities; and of x86, where physical cores have different priorities.
Thanks and BR, Ricardo
* The target of these patches is Intel hybrid processors, on which cluster scheduling is disabled - cabdc3a8475b ("sched,x86: Don't use cluster topology for x86 hybrid CPUs"). Also, I have not observed topologies in which CPUs of the same cluster have different priorities.
We are also looking into re-enabling cluster scheduling on hybrid topologies.
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