linux-kernel - Re: [PATCH 1/1] mm: only dispaly online cpus of the numa node

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Date:   Mon, 28 Aug 2017 15:13:28 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Zhen Lei <thunder.leizhen@...wei.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-mm <linux-mm@...ck.org>, Zefan Li <lizefan@...wei.com>,
        Xinwei Hu <huxinwei@...wei.com>,
        Tianhong Ding <dingtianhong@...wei.com>,
        Hanjun Guo <guohanjun@...wei.com>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH 1/1] mm: only dispaly online cpus of the numa node

On Fri 25-08-17 18:34:33, Will Deacon wrote:
> On Thu, Aug 24, 2017 at 10:32:26AM +0200, Michal Hocko wrote:
> > It seems this has slipped through cracks. Let's CC arm64 guys
> > 
> > On Tue 20-06-17 20:43:28, Zhen Lei wrote:
> > > When I executed numactl -H(which read /sys/devices/system/node/nodeX/cpumap
> > > and display cpumask_of_node for each node), but I got different result on
> > > X86 and arm64. For each numa node, the former only displayed online CPUs,
> > > and the latter displayed all possible CPUs. Unfortunately, both Linux
> > > documentation and numactl manual have not described it clear.
> > > 
> > > I sent a mail to ask for help, and Michal Hocko <mhocko@...nel.org> replied
> > > that he preferred to print online cpus because it doesn't really make much
> > > sense to bind anything on offline nodes.
> > 
> > Yes printing offline CPUs is just confusing and more so when the
> > behavior is not consistent over architectures. I believe that x86
> > behavior is the more appropriate one because it is more logical to dump
> > the NUMA topology and use it for affinity setting than adding one
> > additional step to check the cpu state to achieve the same.
> > 
> > It is true that the online/offline state might change at any time so the
> > above might be tricky on its own but if we should at least make the
> > behavior consistent.
> > 
> > > Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> > 
> > Acked-by: Michal Hocko <mhocko@...e.com>
> 
> The concept looks find to me, but shouldn't we use cpumask_var_t and
> alloc/free_cpumask_var?

This will be safer but both callers of node_read_cpumap are shallow
stack so I am not sure a stack is a limiting factor here.

Zhen Lei, would you care to update that part please?

-- 
Michal Hocko
SUSE Labs