Messages in this thread Patch in this message | | | Date | Wed, 9 Nov 2022 11:42:20 +0100 | Subject | Re: [PATCH v2 8/9] sched/fair: Detect capacity inversion | From | Dietmar Eggemann <> |
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- Qais Yousef <qais.yousef@arm.com> + Qais Yousef <qyousef@layalina.io>
On 04/08/2022 16:36, Qais Yousef wrote:
I was surprised to see these capacity inversion patches in v2. They were not part of v1 so I never review them (even internally).
> Check each performance domain to see if thermal pressure is causing its
I guess that's `PELT thermal pressure` instead of `instantaneous thermal pressure`. IMHO an important detail here to understand the patch.
> capacity to be lower than another performance domain. ^^^^^^^ s/another/next lower (CPU capacity) level/ ?
I assume that is the definition of `capacity inversion`? IMHO it appeared the first time in your discussion with Xuewen and Lukasz:
https://lkml.kernel.org/r/20220503144352.lxduzhl6jq6xdhw2@airbuntu
> We assume that each performance domain has CPUs with the same > capacities, which is similar to an assumption made in energy_model.c > > We also assume that thermal pressure impacts all CPUs in a performance > domain equally. > > If there're multiple performance domains with the same capacity_orig, we
Not aware of such a system. At least it wouldn't make much sense. Not sure if EAS would correctly work on such a system.
> will trigger a capacity inversion if the domain is under thermal > pressure. > > The new cpu_in_capacity_inversion() should help users to know when > information about capacity_orig are not reliable and can opt in to use > the inverted capacity as the 'actual' capacity_orig. > > Signed-off-by: Qais Yousef <qais.yousef@arm.com> > --- > kernel/sched/fair.c | 63 +++++++++++++++++++++++++++++++++++++++++--- > kernel/sched/sched.h | 19 +++++++++++++ > 2 files changed, 79 insertions(+), 3 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 59ba7106ddc6..cb32dc9a057f 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -8659,16 +8659,73 @@ static unsigned long scale_rt_capacity(int cpu) > > static void update_cpu_capacity(struct sched_domain *sd, int cpu) > { > + unsigned long capacity_orig = arch_scale_cpu_capacity(cpu); > unsigned long capacity = scale_rt_capacity(cpu); > struct sched_group *sdg = sd->groups; > + struct rq *rq = cpu_rq(cpu); > > - cpu_rq(cpu)->cpu_capacity_orig = arch_scale_cpu_capacity(cpu); > + rq->cpu_capacity_orig = capacity_orig; > > if (!capacity) > capacity = 1; > > - cpu_rq(cpu)->cpu_capacity = capacity; > - trace_sched_cpu_capacity_tp(cpu_rq(cpu)); > + rq->cpu_capacity = capacity; > + > + /* > + * Detect if the performance domain is in capacity inversion state. > + * > + * Capacity inversion happens when another perf domain with equal or > + * lower capacity_orig_of() ends up having higher capacity than this > + * domain after subtracting thermal pressure. > + * > + * We only take into account thermal pressure in this detection as it's > + * the only metric that actually results in *real* reduction of > + * capacity due to performance points (OPPs) being dropped/become > + * unreachable due to thermal throttling. > + * > + * We assume: > + * * That all cpus in a perf domain have the same capacity_orig > + * (same uArch). > + * * Thermal pressure will impact all cpus in this perf domain > + * equally. > + */ > + if (static_branch_unlikely(&sched_asym_cpucapacity)) {
This should be sched_energy_enabled(). Performance Domains (PDs) are an EAS thing.
> + unsigned long inv_cap = capacity_orig - thermal_load_avg(rq);
rcu_read_lock()
> + struct perf_domain *pd = rcu_dereference(rq->rd->pd);
rcu_read_unlock()
It's called from build_sched_domains() too. I assume static_branch_unlikely(&sched_asym_cpucapacity) hides this issue so far.
> + > + rq->cpu_capacity_inverted = 0; > + > + for (; pd; pd = pd->next) { > + struct cpumask *pd_span = perf_domain_span(pd); > + unsigned long pd_cap_orig, pd_cap; > + > + cpu = cpumask_any(pd_span); > + pd_cap_orig = arch_scale_cpu_capacity(cpu); > + > + if (capacity_orig < pd_cap_orig) > + continue; > + > + /* > + * handle the case of multiple perf domains have the > + * same capacity_orig but one of them is under higher
Like I said above, I'm not aware of such an EAS system.
> + * thermal pressure. We record it as capacity > + * inversion. > + */ > + if (capacity_orig == pd_cap_orig) { > + pd_cap = pd_cap_orig - thermal_load_avg(cpu_rq(cpu)); > + > + if (pd_cap > inv_cap) { > + rq->cpu_capacity_inverted = inv_cap; > + break; > + }
In case `capacity_orig == pd_cap_orig` and cpumask_test_cpu(cpu_of(rq), pd_span) the code can set rq->cpu_capacity_inverted = inv_cap erroneously since thermal_load_avg(rq) can return different values for inv_cap and pd_cap.
So even on a classical big little system, this condition can set rq->cpu_capacity_inverted for a CPU in the little or big cluster.
thermal_load_avg(rq) would have to stay constant for all CPUs within the PD to avoid this.
This is one example of the `thermal pressure` is per PD (or Frequency Domain) in Thermal but per-CPU in the task scheduler.
> + } else if (pd_cap_orig > inv_cap) { > + rq->cpu_capacity_inverted = inv_cap; > + break; > + } > + } > + } > + > + trace_sched_cpu_capacity_tp(rq);
> sdg->sgc->capacity = capacity; > sdg->sgc->min_capacity = capacity;
[...]
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