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

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Date:   Tue, 23 Jun 2020 11:21:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nitesh Narayan Lal <nitesh@...hat.com>
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 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.
Still when you're there, at the very least you can fix the horrible
style:


> +		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
>