linux-kernel - Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion

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Message-ID: <Y1+S/UkS1DNIwlp7@dhcp22.suse.cz>
Date:   Mon, 31 Oct 2022 10:18:53 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Feng Tang <feng.tang@...el.com>,
        Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Waiman Long <longman@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Chen, Tim C" <tim.c.chen@...el.com>,
        "Yin, Fengwei" <fengwei.yin@...el.com>
Subject: Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion

On Mon 31-10-22 16:51:11, Huang, Ying wrote:
> Michal Hocko <mhocko@...e.com> writes:
> 
> > On Fri 28-10-22 07:22:27, Huang, Ying wrote:
> >> Michal Hocko <mhocko@...e.com> writes:
> >> 
> >> > On Thu 27-10-22 17:31:35, Huang, Ying wrote:
> > [...]
> >> >> I think that it's possible for different processes have different
> >> >> requirements.
> >> >> 
> >> >> - Some processes don't care about where the memory is placed, prefer
> >> >>   local, then fall back to remote if no free space.
> >> >> 
> >> >> - Some processes want to avoid cross-socket traffic, bind to nodes of
> >> >>   local socket.
> >> >> 
> >> >> - Some processes want to avoid to use slow memory, bind to fast memory
> >> >>   node only.
> >> >
> >> > Yes, I do understand that. Do you have any specific examples in mind?
> >> > [...]
> >> 
> >> Sorry, I don't have specific examples.
> >
> > OK, then let's stop any complicated solution right here then. Let's
> > start simple with a per-mm flag to disable demotion of an address
> > space.
> 
> I'm not a big fan of per-mm flag.  Because we don't have users for that
> too and it needs to add ABI too.

OK, if there are no users for opt-out then let's jus document the
current limitations and be done with it.

> > Should there ever be a real demand for a more fine grained solution
> > let's go further but I do not think we want a half baked solution
> > without real usecases.
> 
> I'm OK to ignore per-task (and missing per-process) memory policy
> support for now.

I am against such a half baked solution.
-- 
Michal Hocko
SUSE Labs