linux-kernel - Re: [PATCH v5] numa: make node_to_cpumask_map() NUMA_NO

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Message-ID: <20190917093655.GA1872@dhcp22.suse.cz>
Date:   Tue, 17 Sep 2019 11:36:55 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>, catalin.marinas@....com,
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Subject: Re: [PATCH v5] numa: make node_to_cpumask_map() NUMA_NO_NODE aware

On Tue 17-09-19 14:20:11, Yunsheng Lin wrote:
> On 2019/9/17 13:28, Michael Ellerman wrote:
> > Yunsheng Lin <linyunsheng@...wei.com> writes:
[...]
> >> But we cannot really copy the page allocator logic. Simply because the
> >> page allocator doesn't enforce the near node affinity. It just picks it
> >> up as a preferred node but then it is free to fallback to any other numa
> >> node. This is not the case here and node_to_cpumask_map will only restrict
> >> to the particular node's cpus which would have really non deterministic
> >> behavior depending on where the code is executed. So in fact we really
> >> want to return cpu_online_mask for NUMA_NO_NODE.
> >>
> >> Some arches were already NUMA_NO_NODE aware, but they return cpu_all_mask,
> >> which should be identical with cpu_online_mask when those arches do not
> >> support cpu hotplug, this patch also changes them to return cpu_online_mask
> >> in order to be consistent and use NUMA_NO_NODE instead of "-1".
> > 
> > Except some of those arches *do* support CPU hotplug, powerpc and sparc
> > at least. So switching from cpu_all_mask to cpu_online_mask is a
> > meaningful change.
> 
> Yes, thanks for pointing out.
> 
> > 
> > That doesn't mean it's wrong, but you need to explain why it's the right
> > change.
> 
> How about adding the below to the commit log:
> Even if some of the arches do support CPU hotplug, it does not make sense
> to return the cpu that has been hotplugged.
>
> Any suggestion?

Again, for the third time, I believe. Make it a separate patch please.
There is absolutely no reason to conflate those two things.
-- 
Michal Hocko
SUSE Labs