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

[Michal Hocko <mhocko@suse.com>]

On Thu 27-10-22 17:31:35, Huang, Ying wrote:
> Michal Hocko <mhocko@suse.com> writes:
> 
> > 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?
> 
> 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?
[...]
> > 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.
> 
> Yes.  Per-mm or per-vma property is much better than per-task property.
> Another possibility, how about add a new flag to set_mempolicy() system
> call to set the per-mm mempolicy?  `numactl` can use that by default.

Do you mean a flag to control whether the given policy is applied to a
task or mm?
-- 
Michal Hocko
SUSE Labs