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Date:   Thu, 27 Oct 2022 10:01:33 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     "Huang, Ying" <ying.huang@intel.com>
Cc:     Feng Tang <feng.tang@intel.com>,
        Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Waiman Long <longman@redhat.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "cgroups@vger.kernel.org" <cgroups@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Hansen, Dave" <dave.hansen@intel.com>,
        "Chen, Tim C" <tim.c.chen@intel.com>,
        "Yin, Fengwei" <fengwei.yin@intel.com>
Subject: Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion
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On Thu 27-10-22 15:39:00, Huang, Ying wrote:
> Michal Hocko <mhocko@suse.com> writes:
> 
> > On Thu 27-10-22 14:47:22, Huang, Ying wrote:
> >> Michal Hocko <mhocko@suse.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.

Isn't this something to be configured on the demotion topology side? Or
do you expect there will be per process/address space usecases? I mean
different processes running on the same topology, one requesting local
demotion while other ok with the whole demotion topology?
 
> >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?

mm->owner is a wrong direction. It doesn't have a strong meaning because
there is no one task explicitly responsible for the mm so there is no
real owner (our clone() semantic is just to permissive for that). The
memcg::owner is a crude and ugly hack and it should go away over time
rather than build new uses.

Besides that, and as I have already tried to explain, per task demotion
policy is what makes this whole thing expensive. So this better be a per
mm or per vma property. Whether it is a on/off knob like PR_[GS]ET_THP_DISABLE
or there are explicit requirements for fine grain control on the vma
level I dunno. I haven't seen those usecases yet and it is really easy
to overengineer this.

To be completely honest I would much rather wait for those usecases
before adding a more complex APIs.  PR_[GS]_DEMOTION_DISABLED sounds
like a reasonable first step. Should we have more fine grained
requirements wrt address space I would follow the MADV_{NO}HUGEPAGE
lead.

If we really need/want to give a fine grained control over demotion
nodemask then we would have to go with vma->mempolicy interface. In
any case a per process on/off knob sounds like a reasonable first step
before we learn more about real usecases.
-- 
Michal Hocko
SUSE Labs