linux-kernel - Re: [PATCH v2 0/1] arm: topology: parse the topology from the dt

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Message-ID: <CAG7+-3OMkC_tYEKeXW6MGrsTbEsf6iUG24-9TYJGL-Kh6Dr83A@mail.gmail.com>
Date:   Fri, 23 Apr 2021 14:25:25 +0800
From:   Ruifeng Zhang <ruifeng.zhang0110@...il.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        linux@...linux.org.uk, sudeep.holla@....com,
        Greg KH <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>, a.p.zijlstra@...llo.nl,
        mingo@...nel.org, ruifeng.zhang1@...soc.com, nianfu.bai@...soc.com,
        linux-arm-kernel@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/1] arm: topology: parse the topology from the dt

Dietmar Eggemann <dietmar.eggemann@....com> 于2021年4月20日周二 上午5:27写道：
>
> On 19/04/2021 04:55, Ruifeng Zhang wrote:
> > Dietmar Eggemann <dietmar.eggemann@....com> 于2021年4月17日周六 上午1:00写道：
> >>
> >> On 16/04/2021 13:04, Ruifeng Zhang wrote:
> >>> Dietmar Eggemann <dietmar.eggemann@....com> 于2021年4月16日周五 下午6:39写道：
> >>>>
> >>>> On 16/04/2021 11:32, Valentin Schneider wrote:
> >>>>> On 16/04/21 15:47, Ruifeng Zhang wrote:
>
> [...]
>
> >> I'm afraid that this is now a much weaker case to get this into
> >> mainline.
> >
> > But it's still a problem and it's not break the original logic ( parse
> > topology from MPIDR or parse capacity ), only add the support for
> > parse topology from DT.
> > I think it should still be merged into the mainline. If don't, the
> > DynamIQ SoC has some issue in sched and cpufreq.
>
> IMHO, not necessarily. Your DynamIQ SoC is one cluster with 8 CPUs. It's
> subdivided into 2 Frequency Domains (FDs).
>
> CFS Energy-Aware-Scheduling (EAS, find_energy_efficient_cpu()) and
> Capacity-Aware-Scheduling (CAS, select_idle_sibling() ->
> select_idle_capacity()) work correctly even in case you only have an MC
> sched domain (sd).
> No matter which sd (MC, DIE) the sd_asym_cpucapacity is, we always
> iterate over all CPUs. Per Performance Domains (i.e. FDs) in EAS and
> over sched_domain_span(sd) in CAS.
>
> CFS load-balancing (in case your system is `over-utilized`) might work
> slightly different due to the missing DIE sd but not inevitably worse.
>
> Do you have benchmarks or testcases in mind which convince you that
> Phantom Domains is something you would need? BTW, they are called
> Phantom since they let you use uarch and/or max CPU frequency domain to
> fake real topology (like LLC) boundaries.

I'm researching the impact of all CPUs in the same cluster, such as
DVFS. If there is any progress in the future, I hope to keep
communicating with you. Thank you very much.
>
> [...]
>
> > Why do you keep the logic of topology_parse_cpu_capacity in arm
> > get_coretype_capacity function? The capacity-dmips-mhz will be parsed
> > by drivers/base/arch_topology.c as following:
> > parse_dt_topology
> >     parse_cluster
> >         parse_core
> >             get_cpu_for_node
> >                 topology_parse_cpu_capacity
>
> I think we still need it for systems out there w/o cpu-map in dt, like
> my arm32 TC2 with mainline vexpress-v2p-ca15_a7.dts.
>
> It's called twice on each CPU in case I add the cpu-map dt entry though.