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

[Michal Hocko]

On Mon 31-10-22 22:09:15, Feng Tang wrote:
> On Mon, Oct 31, 2022 at 04:40:15PM +0800, Michal Hocko wrote:
> > On Fri 28-10-22 07:22:27, Huang, Ying wrote:
> > > Michal Hocko <mhocko@xxxxxxxx> 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.
> > 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.
> 
> Yes, the concern about the high cost for mempolicy from you and Yang is
> valid. 
> 
> How about the cpuset part?

Cpusets fall into the same bucket as per task mempolicies wrt costs. Geting a
cpuset requires knowing all tasks associated with a page. Or am I just
missing any magic? And no memcg->cpuset association is not a proper
solution at all.

> We've got bug reports from different channels
> about using cpuset+docker to control meomry placement in memory tiering
> system, leading to 2 commits solving them:
> 
> 2685027fca38 ("cgroup/cpuset: Remove cpus_allowed/mems_allowed setup in
> cpuset_init_smp()") 
> https://lore.kernel.org/all/20220419020958.40419-1-feng.tang@xxxxxxxxx/
> 
> 8ca1b5a49885 ("mm/page_alloc: detect allocation forbidden by cpuset and
> bail out early")
> https://lore.kernel.org/all/1632481657-68112-1-git-send-email-feng.tang@xxxxxxxxx/
> 
> >From these bug reports, I think it's reasonable to say there are quite
> some real world users using cpuset+docker+memory-tiering-system.

I don't think anybody is questioning existence of those usecases. The
primary question is whether any of them really require any non-trivial
(read nodemask aware) demotion policies. In other words do we know of
cpuset policy setups where demotion fallbacks are (partially) excluded?
-- 
Michal Hocko
SUSE Labs