linux-kernel - Re: [PATCH v2] lib: optimize cpumask_local

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Date:   Tue, 5 Nov 2019 08:01:41 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Shaokun Zhang <zhangshaokun@...ilicon.com>
Cc:     linux-kernel@...r.kernel.org, yuqi jin <jinyuqi@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Paul Burton <paul.burton@...s.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v2] lib: optimize cpumask_local_spread()

On Mon 04-11-19 18:27:48, Shaokun Zhang wrote:
> From: yuqi jin <jinyuqi@...wei.com>
> 
> In the multi-processor and NUMA system, I/O device may have many numa
> nodes belonging to multiple cpus. When we get a local numa, it is
> better to find the node closest to the local numa node, instead
> of choosing any online cpu immediately.
> 
> For the current code, it only considers the local NUMA node and it
> doesn't compute the distances between different NUMA nodes for the
> non-local NUMA nodes. Let's optimize it and find the nearest node
> through NUMA distance. The performance will be better if it return
> the nearest node than the random node.

Numbers please

[...]
> +/**
> + * cpumask_local_spread - select the i'th cpu with local numa cpu's first
> + * @i: index number
> + * @node: local numa_node
> + *
> + * This function selects an online CPU according to a numa aware policy;
> + * local cpus are returned first, followed by the nearest non-local ones,
> + * then it wraps around.
> + *
> + * It's not very efficient, but useful for setup.
> + */
> +unsigned int cpumask_local_spread(unsigned int i, int node)
> +{
> +	int node_dist[MAX_NUMNODES] = {0};
> +	bool used[MAX_NUMNODES] = {0};

Ugh. This might be a lot of stack space. Some distro kernels use large
NODE_SHIFT (e.g 10 so this would be 4kB of stack space just for the
node_dist).

> +	int cpu, j, id;
-- 
Michal Hocko
SUSE Labs