linux-kernel - Re: [PATCH v2] mm: Only re-generate demotion targets when a numa node changes its N

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Date:   Fri, 11 Mar 2022 09:10:01 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Abhishek Goel <huntbag@...ux.vnet.ibm.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: Only re-generate demotion targets when a numa node
 changes its N_CPU state

On 3/10/22 18:39, Andrew Morton wrote:
> On Thu, 10 Mar 2022 13:07:49 +0100 Oscar Salvador <osalvador@...e.de> wrote:
>> We do already have two CPU callbacks (vmstat_cpu_online() and vmstat_cpu_dead())
>> that check exactly that, so get rid of the CPU callbacks in
>> migrate_on_reclaim_init() and only call set_migration_target_nodes() from
>> vmstat_cpu_{dead,online}() whenever a numa node change its N_CPU state.
> What I'm not getting here (as so often happens) is a sense of how badly
> this affects our users.  Does anyone actually hotplug frequently enough
> to care?

I asked Abhishek about this a bit here:

> https://lore.kernel.org/all/4e8067e1-0574-c9d2-9d6c-d676d32071bd@linux.vnet.ibm.com/

It sounded to me like there are ppc users who convert their systems from
SMT=1 to SMT=8.  I'd guess that they want to do this as a side-channel
mitigation because ppc has been dealing with the same basic issues as
those of us over in x86 land.  The increase in time (20s->36s) would be
noticeable and probably slightly annoying to a human waiting on it.

I'd love to hear more details on this from Abhishek, like whether end
users do this as opposed to IBM's kernel developers.  But, it does sound
deserving of a stable@ tag to me.