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

[Huang, Ying]

Michal Hocko <mhocko@xxxxxxxx> writes:

> On Thu 27-10-22 17:31:35, Huang, Ying wrote:
>> Michal Hocko <mhocko@xxxxxxxx> writes:
>> 
>> > On Thu 27-10-22 15:39:00, Huang, Ying wrote:
>> >> Michal Hocko <mhocko@xxxxxxxx> writes:
>> >> 
>> >> > On Thu 27-10-22 14:47:22, Huang, Ying wrote:
>> >> >> Michal Hocko <mhocko@xxxxxxxx> 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?
> [...]

Sorry, I don't have specific examples.

>> > 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?

Yes.  That is the idea.

Best Regards,
Huang, Ying