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Subject: Re: [Patch v3 1/3] lib: Restrict cpumask_local_spread to houskeeping
 CPUs
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        frederic@kernel.org, mtosatti@redhat.com, juri.lelli@redhat.com,
        abelits@marvell.com, bhelgaas@google.com,
        linux-pci@vger.kernel.org, rostedt@goodmis.org, mingo@kernel.org,
        peterz@infradead.org, tglx@linutronix.de, davem@davemloft.net,
        sfr@canb.auug.org.au, stephen@networkplumber.org,
        rppt@linux.vnet.ibm.com, yuqi jin <jinyuqi@huawei.com>,
        Shaokun Zhang <zhangshaokun@hisilicon.com>
References: <20200623192331.215557-1-nitesh@redhat.com>
 <20200623192331.215557-2-nitesh@redhat.com>
 <20200624122647.766bec7760d9197ba71a58c4@linux-foundation.org>
From:   Nitesh Narayan Lal <nitesh@redhat.com>
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Date:   Wed, 24 Jun 2020 19:31:34 -0400
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On 6/24/20 3:26 PM, Andrew Morton wrote:
> On Tue, 23 Jun 2020 15:23:29 -0400 Nitesh Narayan Lal <nitesh@redhat.com>=
 wrote:
>
>> From: Alex Belits <abelits@marvell.com>
>>
>> The current implementation of cpumask_local_spread() does not respect th=
e
>> isolated CPUs, i.e., even if a CPU has been isolated for Real-Time task,
>> it will return it to the caller for pinning of its IRQ threads. Having
>> these unwanted IRQ threads on an isolated CPU adds up to a latency
>> overhead.
>>
>> Restrict the CPUs that are returned for spreading IRQs only to the
>> available housekeeping CPUs.
>>
>> ...
>>
>> --- a/lib/cpumask.c
>> +++ b/lib/cpumask.c
>> @@ -6,6 +6,7 @@
>>  #include <linux/export.h>
>>  #include <linux/memblock.h>
>>  #include <linux/numa.h>
>> +#include <linux/sched/isolation.h>
>> =20
>>  /**
>>   * cpumask_next - get the next cpu in a cpumask
>> @@ -205,22 +206,27 @@ void __init free_bootmem_cpumask_var(cpumask_var_t=
 mask)
>>   */
>>  unsigned int cpumask_local_spread(unsigned int i, int node)
>>  {
>> -=09int cpu;
>> +=09int cpu, hk_flags;
>> +=09const struct cpumask *mask;
>> =20
>> +=09hk_flags =3D HK_FLAG_DOMAIN | HK_FLAG_WQ;
>> +=09mask =3D housekeeping_cpumask(hk_flags);
>>  =09/* Wrap: we always want a cpu. */
>> -=09i %=3D num_online_cpus();
>> +=09i %=3D cpumask_weight(mask);
>> =20
>>  =09if (node =3D=3D NUMA_NO_NODE) {
>> -=09=09for_each_cpu(cpu, cpu_online_mask)
>> +=09=09for_each_cpu(cpu, mask) {
>>  =09=09=09if (i-- =3D=3D 0)
>>  =09=09=09=09return cpu;
>> +=09=09}
>>  =09} else {
>>  =09=09/* NUMA first. */
>> -=09=09for_each_cpu_and(cpu, cpumask_of_node(node), cpu_online_mask)
>> +=09=09for_each_cpu_and(cpu, cpumask_of_node(node), mask) {
>>  =09=09=09if (i-- =3D=3D 0)
>>  =09=09=09=09return cpu;
>> +=09=09}
>> =20
>> -=09=09for_each_cpu(cpu, cpu_online_mask) {
>> +=09=09for_each_cpu(cpu, mask) {
>>  =09=09=09/* Skip NUMA nodes, done above. */
>>  =09=09=09if (cpumask_test_cpu(cpu, cpumask_of_node(node)))
>>  =09=09=09=09continue;
> Are you aware of these changes to cpu_local_spread()?
> https://lore.kernel.org/lkml/1582768688-2314-1-git-send-email-zhangshaoku=
n@hisilicon.com/
>
> I don't see a lot of overlap but it would be nice for you folks to
> check each other's homework ;)

I took a look at the patch and as you said there is not much overlap.
The idea of keeping isolated CPUs untouched for RT environments will be val=
id
for the optimization that Shaokun is suggesting as well.
I am not sure about the current state of the patch-set but I will certainly=
 keep
an eye on it.

>
>
--=20
Thanks
Nitesh


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