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

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Date:   Thu, 27 Oct 2022 15:39:00 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Michal Hocko <mhocko@...e.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

Michal Hocko <mhocko@...e.com> writes:

> On Thu 27-10-22 14:47:22, Huang, Ying wrote:
>> Michal Hocko <mhocko@...e.com> writes:
> [...]
>> > I can imagine workloads which wouldn't like to get their memory demoted
>> > for some reason but wouldn't it be more practical to tell that
>> > explicitly (e.g. via prctl) rather than configuring cpusets/memory
>> > policies explicitly?
>> 
>> If my understanding were correct, prctl() configures the process or
>> thread.
>
> Not necessarily. There are properties which are per adddress space like
> PR_[GS]ET_THP_DISABLE. This could be very similar.
>
>> How can we get process/thread configuration at demotion time?
>
> As already pointed out in previous emails. You could hook into
> folio_check_references path, more specifically folio_referenced_one
> where you have all that you need already - all vmas mapping the page and
> then it is trivial to get the corresponding vm_mm. If at least one of
> them has the flag set then the demotion is not allowed (essentially the
> same model as VM_LOCKED).

Got it!  Thanks for detailed explanation.

One bit may be not sufficient.  For example, if we want to avoid or
control cross-socket demotion and still allow demoting to slow memory
nodes in local socket, we need to specify a node mask to exclude some
NUMA nodes from demotion targets.

>From overhead point of view, this appears similar as that of VMA/task
memory policy?  We can make mm->owner available for memory tiers
(CONFIG_NUMA && CONFIG_MIGRATION).  The advantage is that we don't need
to introduce new ABI.  I guess users may prefer to use `numactl` than a
new ABI?

Best Regards,
Huang, Ying