linux-kernel - Re: [Patch v2 1/3] lib: Restrict cpumask_local

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Message-ID: <9b499cd8-e311-db5b-4261-0b3f355c8c89@redhat.com>
Date:   Tue, 23 Jun 2020 09:18:37 -0400
From:   Nitesh Narayan Lal <nitesh@...hat.com>
To:     Peter Zijlstra <peterz@...radead.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,
        tglx@...utronix.de, davem@...emloft.net, akpm@...ux-foundation.org,
        sfr@...b.auug.org.au, stephen@...workplumber.org,
        rppt@...ux.vnet.ibm.com
Subject: Re: [Patch v2 1/3] lib: Restrict cpumask_local_spread to houskeeping
 CPUs


On 6/23/20 5:21 AM, Peter Zijlstra wrote:
> On Mon, Jun 22, 2020 at 07:45:08PM -0400, Nitesh Narayan Lal 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.
>>
>> Signed-off-by: Alex Belits <abelits@...vell.com>
>> Signed-off-by: Nitesh Narayan Lal <nitesh@...hat.com>
>> ---
>>  lib/cpumask.c | 43 +++++++++++++++++++++++++------------------
>>  1 file changed, 25 insertions(+), 18 deletions(-)
>>
>> diff --git a/lib/cpumask.c b/lib/cpumask.c
>> index fb22fb266f93..cc4311a8c079 100644
>> --- 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
>> @@ -205,28 +206,34 @@ void __init free_bootmem_cpumask_var(cpumask_var_t mask)
>>   */
>>  unsigned int cpumask_local_spread(unsigned int i, int node)
>>  {
>> -	int cpu;
>> +	int cpu, m, n, hk_flags;
>> +	const struct cpumask *mask;
>>  
>> +	hk_flags = HK_FLAG_DOMAIN | HK_FLAG_WQ;
>> +	mask = housekeeping_cpumask(hk_flags);
>> +	m = cpumask_weight(mask);
>>  	/* Wrap: we always want a cpu. */
>> -	i %= num_online_cpus();
>> +	n = i % m;
>> +	while (m-- > 0) {
> I are confuzled. What do we need this outer loop for?
>
> Why isn't something like:
>
> 	i %= cpumask_weight(mask);
>
> good enough? That voids having to touch the test.

Makes sense.
Thanks

> Still when you're there, at the very least you can fix the horrible
> style:

Sure.

>
>
>> +		if (node == NUMA_NO_NODE) {
>> +			for_each_cpu(cpu, mask)
>> +				if (n-- == 0)
>> +					return cpu;
> { }
>
>> +		} else {
>> +			/* NUMA first. */
>> +			for_each_cpu_and(cpu, cpumask_of_node(node), mask)
>> +				if (n-- == 0)
>> +					return cpu;
> { }
>
>>  
>> +			for_each_cpu(cpu, mask) {
>> +				/* Skip NUMA nodes, done above. */
>> +				if (cpumask_test_cpu(cpu,
>> +						     cpumask_of_node(node)))
>> +					continue;
> No linebreak please.
>
>>  
>> +				if (n-- == 0)
>> +					return cpu;
>> +			}
>>  		}
>>  	}
>>  	BUG();
>> -- 
>> 2.18.4
>>
-- 
Nitesh



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