linux-kernel - Re: [Patch v3 1/3] lib: Restrict cpumask_local

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Date:   Wed, 24 Jun 2020 16:38:39 -0400
From:   Nitesh Narayan Lal <nitesh@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        frederic@...nel.org, mtosatti@...hat.com, juri.lelli@...hat.com,
        abelits@...vell.com, bhelgaas@...gle.com,
        linux-pci@...r.kernel.org, rostedt@...dmis.org, mingo@...nel.org,
        peterz@...radead.org, tglx@...utronix.de, davem@...emloft.net,
        sfr@...b.auug.org.au, stephen@...workplumber.org,
        rppt@...ux.vnet.ibm.com, yuqi jin <jinyuqi@...wei.com>,
        Shaokun Zhang <zhangshaokun@...ilicon.com>
Subject: Re: [Patch v3 1/3] lib: Restrict cpumask_local_spread to houskeeping
 CPUs


On 6/24/20 3:26 PM, Andrew Morton wrote:
> On Tue, 23 Jun 2020 15:23:29 -0400 Nitesh Narayan Lal <nitesh@...hat.com> wrote:
>
>> From: Alex Belits <abelits@...vell.com>
>>
>> The current implementation of cpumask_local_spread() does not respect the
>> 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>
>>  
>>  /**
>>   * 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)
>>  {
>> -	int cpu;
>> +	int cpu, hk_flags;
>> +	const struct cpumask *mask;
>>  
>> +	hk_flags = HK_FLAG_DOMAIN | HK_FLAG_WQ;
>> +	mask = housekeeping_cpumask(hk_flags);
>>  	/* Wrap: we always want a cpu. */
>> -	i %= num_online_cpus();
>> +	i %= cpumask_weight(mask);
>>  
>>  	if (node == NUMA_NO_NODE) {
>> -		for_each_cpu(cpu, cpu_online_mask)
>> +		for_each_cpu(cpu, mask) {
>>  			if (i-- == 0)
>>  				return cpu;
>> +		}
>>  	} else {
>>  		/* NUMA first. */
>> -		for_each_cpu_and(cpu, cpumask_of_node(node), cpu_online_mask)
>> +		for_each_cpu_and(cpu, cpumask_of_node(node), mask) {
>>  			if (i-- == 0)
>>  				return cpu;
>> +		}
>>  
>> -		for_each_cpu(cpu, cpu_online_mask) {
>> +		for_each_cpu(cpu, mask) {
>>  			/* Skip NUMA nodes, done above. */
>>  			if (cpumask_test_cpu(cpu, cpumask_of_node(node)))
>>  				continue;
> Are you aware of these changes to cpu_local_spread()?
> https://lore.kernel.org/lkml/1582768688-2314-1-git-send-email-zhangshaokun@hisilicon.com/
>
> I don't see a lot of overlap but it would be nice for you folks to
> check each other's homework ;)

Sure, I will take a look.
Thanks

>
-- 
Nitesh



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