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Subject: Re: [PATCH v2 1/4] sched/isolation: API to get housekeeping online
 CPUs
To:     Bjorn Helgaas <helgaas@kernel.org>
Cc:     linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
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References: <20200924204759.GA2342589@bjorn-Precision-5520>
From:   Nitesh Narayan Lal <nitesh@redhat.com>
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Message-ID: <c19c4bc8-05d4-42a2-9469-2de79919c808@redhat.com>
Date:   Thu, 24 Sep 2020 17:52:19 -0400
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On 9/24/20 4:47 PM, Bjorn Helgaas wrote:
> On Wed, Sep 23, 2020 at 02:11:23PM -0400, Nitesh Narayan Lal wrote:
>> Introduce a new API hk_num_online_cpus(), that can be used to
>> retrieve the number of online housekeeping CPUs that are meant to handle
>> managed IRQ jobs.
>>
>> This API is introduced for the drivers that were previously relying only
>> on num_online_cpus() to determine the number of MSIX vectors to create.
>> In an RT environment with large isolated but fewer housekeeping CPUs thi=
s
>> was leading to a situation where an attempt to move all of the vectors
>> corresponding to isolated CPUs to housekeeping CPUs were failing due to
>> per CPU vector limit.
>>
>> Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
>> ---
>>  include/linux/sched/isolation.h | 13 +++++++++++++
>>  1 file changed, 13 insertions(+)
>>
>> diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isola=
tion.h
>> index cc9f393e2a70..2e96b626e02e 100644
>> --- a/include/linux/sched/isolation.h
>> +++ b/include/linux/sched/isolation.h
>> @@ -57,4 +57,17 @@ static inline bool housekeeping_cpu(int cpu, enum hk_=
flags flags)
>>  =09return true;
>>  }
>> =20
>> +static inline unsigned int hk_num_online_cpus(void)
>> +{
>> +#ifdef CONFIG_CPU_ISOLATION
>> +=09const struct cpumask *hk_mask;
>> +
>> +=09if (static_branch_unlikely(&housekeeping_overridden)) {
>> +=09=09hk_mask =3D housekeeping_cpumask(HK_FLAG_MANAGED_IRQ);
>> +=09=09return cpumask_weight(hk_mask);
>> +=09}
>> +#endif
>> +=09return cpumask_weight(cpu_online_mask);
> Just curious: why is this not
>
>   #ifdef CONFIG_CPU_ISOLATION
>   ...
>   #endif
>     return num_online_cpus();

I think doing an atomic read is better than a bitmap operation.
Thanks for pointing this out.

>
>> +}
>> +
>>  #endif /* _LINUX_SCHED_ISOLATION_H */
>> --=20
>> 2.18.2
>>
--=20
Nitesh


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