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Date:   Tue, 5 Nov 2019 17:33:59 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Shaokun Zhang <zhangshaokun@hisilicon.com>,
        linux-kernel@vger.kernel.org, yuqi jin <jinyuqi@huawei.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Paul Burton <paul.burton@mips.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Anshuman Khandual <anshuman.khandual@arm.com>
Subject: Re: [PATCH v2] lib: optimize cpumask_local_spread()
Message-Id: <20191105173359.39052327cf221d9c4b26b783@linux-foundation.org>
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        <20191105070141.GF22672@dhcp22.suse.cz>
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On Tue, 5 Nov 2019 08:01:41 +0100 Michal Hocko <mhocko@kernel.org> wrote:

> On Mon 04-11-19 18:27:48, Shaokun Zhang wrote:
> > From: yuqi jin <jinyuqi@huawei.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

The changelog had

: When Parameter Server workload is tested using NIC device on Huawei
: Kunpeng 920 SoC:
: Without the patch, the performance is 22W QPS;
: Added this patch, the performance become better and it is 26W QPS.

> [...]
> > +/**
> > + * 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).

Yes, that's big.  From a quick peek I suspect we could get by using an
array of unsigned shorts here but that might be fragile over time even
if it works now?

Perhaps we could make it a statically allocated array and protect the
entire thing with a spin_lock_irqsave()?  It's not a frequently called
function.