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

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Message-ID: <8deb282a-07b7-2fe6-debc-db030a2fef64@redhat.com>
Date:   Wed, 24 Jun 2020 19:31:34 -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 ;)

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 valid
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.

>
>
-- 
Thanks
Nitesh



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