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From: Dave Hansen <>
To: Andrew Morton <>,
	Oscar Salvador <>
Cc: Dave Hansen <>,
	"Huang, Ying" <>,
	Abhishek Goel <>,
	Baolin Wang <>,,
Subject: Re: [PATCH v2] mm: Only re-generate demotion targets when a numa node changes its N_CPU state
Date: Fri, 11 Mar 2022 09:10:01 -0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 3/10/22 18:39, Andrew Morton wrote:
> On Thu, 10 Mar 2022 13:07:49 +0100 Oscar Salvador <> 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:


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.

  parent reply	other threads:[~2022-03-11 17:10 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-03-10 12:07 [PATCH v2] mm: Only re-generate demotion targets when a numa node changes its N_CPU state Oscar Salvador
2022-03-11  1:33 ` Baolin Wang
2022-03-11  2:24 ` Huang, Ying
2022-03-11  8:39   ` Oscar Salvador
2022-03-14  1:03     ` Huang, Ying
2022-03-14 15:13       ` Oscar Salvador
2022-03-14 15:20         ` Dave Hansen
2022-03-15  6:13           ` Oscar Salvador
2022-03-15  6:31             ` Huang, Ying
2022-03-11  2:39 ` Andrew Morton
2022-03-11  9:23   ` Oscar Salvador
2022-03-11 17:10   ` Dave Hansen [this message]
2022-03-14  9:09     ` Abhishek Goel
2022-03-11  5:06 ` Huang, Ying
2022-03-11  9:17   ` Oscar Salvador
2022-03-14  3:09     ` Huang, Ying

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