linux-kernel - Re: [RFC PATCH v3 0/8] mm: workingset reporting

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Date: Wed, 27 Mar 2024 15:53:39 -0700
From: Yuanchu Xie <yuanchu@...gle.com>
To: Gregory Price <gregory.price@...verge.com>
Cc: David Hildenbrand <david@...hat.com>, "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>, 
	Khalid Aziz <khalid.aziz@...cle.com>, Henry Huang <henry.hj@...group.com>, 
	Yu Zhao <yuzhao@...gle.com>, Dan Williams <dan.j.williams@...el.com>, 
	Huang Ying <ying.huang@...el.com>, Wei Xu <weixugc@...gle.com>, 
	David Rientjes <rientjes@...gle.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	Shuah Khan <shuah@...nel.org>, Yosry Ahmed <yosryahmed@...gle.com>, 
	Matthew Wilcox <willy@...radead.org>, Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>, 
	Kairui Song <kasong@...cent.com>, "Michael S. Tsirkin" <mst@...hat.com>, 
	Vasily Averin <vasily.averin@...ux.dev>, Nhat Pham <nphamcs@...il.com>, 
	Miaohe Lin <linmiaohe@...wei.com>, Qi Zheng <zhengqi.arch@...edance.com>, 
	Abel Wu <wuyun.abel@...edance.com>, "Vishal Moola (Oracle)" <vishal.moola@...il.com>, 
	Kefeng Wang <wangkefeng.wang@...wei.com>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, cgroups@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [RFC PATCH v3 0/8] mm: workingset reporting

On Wed, Mar 27, 2024 at 2:44 PM Gregory Price
<gregory.price@...verge.com> wrote:
>
> On Wed, Mar 27, 2024 at 02:30:59PM -0700, Yuanchu Xie wrote:
> > I realize this does not generalize well to hotness information, but I
> > lack the intuition for an abstraction that presents hotness in a useful
> > way. Based on a recent proposal for move_phys_pages[2], it seems like
> > userspace tiering software would like to move specific physical pages,
> > instead of informing the kernel "move x number of hot pages to y
> > device". Please advise.
> >
> > [2]
> > https://lore.kernel.org/lkml/20240319172609.332900-1-gregory.price@memverge.com/
> >
>
> Please note that this proposed interface (move_phys_pages) is very
> unlikely to be received upstream due to side channel concerns. Instead,
> it's more likely that the tiering component will expose a "promote X
> pages from tier A to tier B", and the kernel component would then
> use/consume hotness information to determine which pages to promote.

I see that mm/memory-tiers.c only has support for demotion. What kind
of hotness information do devices typically provide? The OCP proposal
is not very specific about this.
A list of hot pages with configurable threshold?
Access frequency for all pages at configured granularity?
Is there a way to tell which NUMA node is accessing them, for page promotion?
>
> (Just as one example, there are many more realistic designs)
>
> So if there is a way to expose workingset data to the mm/memory_tiers.c
> component instead of via sysfs/cgroup - that is preferable.

Appreciate the feedback. The data in its current form might be useful
to inform demotion decisions, but for promotion, are you aware of any
recent developments? I would like to encode hotness as workingset data
as well.
>
> The 'move_phys_pages' interface is more of an experimental interface to
> test the effectiveness of this approach without having to plumb out the
> entire system.  Definitely anything userland interface should not be
> designed to generate physical address information for consumption unless
> it is hard-locked behind admin caps.
>
> Regards,
> Gregory