linux-kernel - Re: [CHANGE 1/2] sched/isolation: Make use of more than one housekeeping cpu

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Message-ID: <4a8d54e7-fa29-4ce4-9023-3cdffa0807e6@linux.ibm.com>
Date: Thu, 13 Feb 2025 10:14:04 +0530
From: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
To: Phil Auld <pauld@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Juri Lelli
 <juri.lelli@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Waiman Long <longman@...hat.com>, linux-kernel@...r.kernel.org,
        Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
Subject: Re: [CHANGE 1/2] sched/isolation: Make use of more than one
 housekeeping cpu

Hi Phil Auld,

On 11/02/25 19:31, Phil Auld wrote:
> The exising code uses housekeeping_any_cpu() to select a cpu for
> a given housekeeping task. However, this often ends up calling
> cpumask_any_and() which is defined as cpumask_first_and() which has
> the effect of alyways using the first cpu among those available.
> 
> The same applies when multiple NUMA nodes are involved. In that
> case the first cpu in the local node is chosen which does provide
> a bit of spreading but with multiple HK cpus per node the same
> issues arise.
> 
> Spread the HK work out by having housekeeping_any_cpu() and
> sched_numa_find_closest() use cpumask_any_and_distribute()
> instead of cpumask_any_and().
> 

Got the overall intent of the patch for better load distribution on
housekeeping tasks. However, one potential drawback could be that by
spreading HK work across multiple CPUs might reduce the time that
some cores can spend in deeper idle states which can be beneficial for
power-sensitive systems.

Thoughts?

Thanks,
Madadi Vineeth Reddy

> Signed-off-by: Phil Auld <pauld@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Frederic Weisbecker <frederic@...nel.org>
> Cc: Waiman Long <longman@...hat.com>
> Cc: linux-kernel@...r.kernel.org
> ---
>  kernel/sched/isolation.c | 2 +-
>  kernel/sched/topology.c  | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index 81bc8b329ef1..93b038d48900 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -40,7 +40,7 @@ int housekeeping_any_cpu(enum hk_type type)
>  			if (cpu < nr_cpu_ids)
>  				return cpu;
>  
> -			cpu = cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask);
> +			cpu = cpumask_any_and_distribute(housekeeping.cpumasks[type], cpu_online_mask);
>  			if (likely(cpu < nr_cpu_ids))
>  				return cpu;
>  			/*
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index c49aea8c1025..94133f843485 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2101,7 +2101,7 @@ int sched_numa_find_closest(const struct cpumask *cpus, int cpu)
>  	for (i = 0; i < sched_domains_numa_levels; i++) {
>  		if (!masks[i][j])
>  			break;
> -		cpu = cpumask_any_and(cpus, masks[i][j]);
> +		cpu = cpumask_any_and_distribute(cpus, masks[i][j]);
>  		if (cpu < nr_cpu_ids) {
>  			found = cpu;
>  			break;