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Message-ID: <e91bcc83-37c8-dcca-e088-8b3fcd737b2c@arm.com>
Date:   Tue, 8 Mar 2022 17:03:07 +0100
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From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
Subject: Re: [PATCH v3] topology: make core_mask include at least
 cluster_siblings
To:     Vincent Guittot <vincent.guittot@linaro.org>,
        Will Deacon <will@kernel.org>
Cc:     Darren Hart <darren@os.amperecomputing.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Arm <linux-arm-kernel@lists.infradead.org>,
        Sudeep Holla <sudeep.holla@arm.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Catalin Marinas <Catalin.Marinas@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Barry Song <song.bao.hua@hisilicon.com>,
        Valentin Schneider <Valentin.Schneider@arm.com>,
        "D . Scott Phillips" <scott@os.amperecomputing.com>,
        Ilkka Koskinen <ilkka@os.amperecomputing.com>,
        stable@vger.kernel.org
References: <f1deaeabfd31fdf512ff6502f38186ef842c2b1f.1646413117.git.darren@os.amperecomputing.com>
 <20220308103012.GA31267@willie-the-truck>
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On 08/03/2022 12:04, Vincent Guittot wrote:
> On Tue, 8 Mar 2022 at 11:30, Will Deacon <will@kernel.org> wrote:

[...]

>>> ---
>>> v1: Drop MC level if coregroup weight == 1
>>> v2: New sd topo in arch/arm64/kernel/smp.c
>>> v3: No new topo, extend core_mask to cluster_siblings
>>>
>>>  drivers/base/arch_topology.c | 8 ++++++++
>>>  1 file changed, 8 insertions(+)
>>>
>>> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
>>> index 976154140f0b..a96f45db928b 100644
>>> --- a/drivers/base/arch_topology.c
>>> +++ b/drivers/base/arch_topology.c
>>> @@ -628,6 +628,14 @@ const struct cpumask *cpu_coregroup_mask(int cpu)
>>>                       core_mask = &cpu_topology[cpu].llc_sibling;
>>>       }
>>>
>>> +     /*
>>> +      * For systems with no shared cpu-side LLC but with clusters defined,
>>> +      * extend core_mask to cluster_siblings. The sched domain builder will
>>> +      * then remove MC as redundant with CLS if SCHED_CLUSTER is enabled.

IMHO, if core_mask weight is 1, MC will be removed/degenerated anyway.

This is what I get on my Ampere Altra (I guess I don't have the ACPI
changes which would let to a CLS sched domain):

# cat /sys/kernel/debug/sched/domains/cpu0/domain*/name
DIE
NUMA
root@oss-altra01:~# zcat /proc/config.gz | grep SCHED_CLUSTER
CONFIG_SCHED_CLUSTER=y

>>> +      */
>>> +     if (cpumask_subset(core_mask, &cpu_topology[cpu].cluster_sibling))
>>> +             core_mask = &cpu_topology[cpu].cluster_sibling;
>>> +
>>
>> Sudeep, Vincent, are you happy with this now?
> 
> I would not say that I'm happy because this solution skews the core
> cpu mask in order to abuse the scheduler so that it will remove a
> wrong but useless level when it will build its domains.
> But this works so as long as the maintainer are happy, I'm fine

I do not have any better idea than this tweak here either in case the
platform can't provide a cleaner setup.

Maybe the following is easier to read but then we use
'&cpu_topology[cpu].llc_sibling' in cpu_coregroup_mask() already ...

@@ -617,6 +617,7 @@ EXPORT_SYMBOL_GPL(cpu_topology);
 const struct cpumask *cpu_coregroup_mask(int cpu)
 {
        const cpumask_t *core_mask = cpumask_of_node(cpu_to_node(cpu));
+       const cpumask_t *cluster_mask = cpu_clustergroup_mask(cpu);

        /* Find the smaller of NUMA, core or LLC siblings */
        if (cpumask_subset(&cpu_topology[cpu].core_sibling, core_mask)) {
@@ -628,6 +629,9 @@ const struct cpumask *cpu_coregroup_mask(int cpu)
                        core_mask = &cpu_topology[cpu].llc_sibling;
        }

+       if (cpumask_subset(core_mask, cluster_mask))
+               core_mask = cluster_mask;
+
        return core_mask;
 }

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>