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

[Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>]

On 10/27/22 11:25 PM, Yang Shi wrote:
> On Thu, Oct 27, 2022 at 12:12 AM Feng Tang <feng.tang@intel.com> wrote:
>>
>> On Thu, Oct 27, 2022 at 01:57:52AM +0800, Yang Shi wrote:
>>> On Wed, Oct 26, 2022 at 8:59 AM Michal Hocko <mhocko@suse.com> wrote:
>> [...]
>>>>>> This all can get quite expensive so the primary question is, does the
>>>>>> existing behavior generates any real issues or is this more of an
>>>>>> correctness exercise? I mean it certainly is not great to demote to an
>>>>>> incompatible numa node but are there any reasonable configurations when
>>>>>> the demotion target node is explicitly excluded from memory
>>>>>> policy/cpuset?
>>>>>
>>>>> We haven't got customer report on this, but there are quite some customers
>>>>> use cpuset to bind some specific memory nodes to a docker (You've helped
>>>>> us solve a OOM issue in such cases), so I think it's practical to respect
>>>>> the cpuset semantics as much as we can.
>>>>
>>>> Yes, it is definitely better to respect cpusets and all local memory
>>>> policies. There is no dispute there. The thing is whether this is really
>>>> worth it. How often would cpusets (or policies in general) go actively
>>>> against demotion nodes (i.e. exclude those nodes from their allowes node
>>>> mask)?
>>>>
>>>> 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?
>>>>
>>>>> Your concern about the expensive cost makes sense! Some raw ideas are:
>>>>> * if the shrink_folio_list is called by kswapd, the folios come from
>>>>>   the same per-memcg lruvec, so only one check is enough
>>>>> * if not from kswapd, like called form madvise or DAMON code, we can
>>>>>   save a memcg cache, and if the next folio's memcg is same as the
>>>>>   cache, we reuse its result. And due to the locality, the real
>>>>>   check is rarely performed.
>>>>
>>>> memcg is not the expensive part of the thing. You need to get from page
>>>> -> all vmas::vm_policy -> mm -> task::mempolicy
>>>
>>> Yeah, on the same page with Michal. Figuring out mempolicy from page
>>> seems quite expensive and the correctness can't be guranteed since the
>>> mempolicy could be set per-thread and the mm->task depends on
>>> CONFIG_MEMCG so it doesn't work for !CONFIG_MEMCG.
>>
>> Yes, you are right. Our "working" psudo code for mem policy looks like
>> what Michal mentioned, and it can't work for all cases, but try to
>> enforce it whenever possible:
>>
>> static bool  __check_mpol_demotion(struct folio *folio, struct vm_area_struct *vma,
>>                 unsigned long addr, void *arg)
>> {
>>         bool *skip_demotion = arg;
>>         struct mempolicy *mpol;
>>         int nid, dnid;
>>         bool ret = true;
>>
>>         mpol = __get_vma_policy(vma, addr);
>>         if (!mpol) {
>>                 struct task_struct *task;
>>                 if (vma->vm_mm)
>>                         task = vma->vm_mm->owner;
> 
> But this task may not be the task you want IIUC. For example, the
> process has two threads, A and B. They have different mempolicy. The
> vmscan is trying to demote a page belonging to thread A, but the task
> may point to thread B, so you actually get the wrong mempolicy IIUC.
> 

But if we swap out this page and fault back in via thread B the page would
get allocated as per thread B mempolicy. So if we demote based on thread B
policy are we breaking anything? 

-aneesh