Re: [RFC PATCH V1 1/1] sched/numa: Enhance vma scanning logic

["Kalra, Ashish" <ashish.kalra@amd.com>]

Hello Mingwei, Sean,

Looking forward to your thoughts/feedback on the MMU invalidation 
notifier issues with SEV guests as mentioned below ?

Thanks,
Ashish

On 1/17/2023 10:43 PM, Bharata B Rao wrote:
> On 1/17/2023 8:29 PM, Mel Gorman wrote:
>> Note that the cc list is excessive for the topic.
> 
> (Wasn't sure about pruning the CC list mid-thread, hence continuing with it)
> 
> <snip>
> 
>>
>> This is a build-tested only prototype to illustrate how VMA could track
>> NUMA balancing state. It starts with applying the scan delay to every VMA
>> instead of every task to avoid scanning new or very short-lived VMAs. I
>> went back to my old notes on how I hoped to reduce excessive scanning in
>> NUMA balancing and it happened to be second on my list and straight-forward
>> to prototype in a few minutes.
> 
> While on the topic of improving NUMA balancer scanning relevancy, the following
> additional points may be worth noting:
> 
> Recently there have been reports about NUMA balancing induced scanning and
> subsequent MMU notifier invalidations causing problems in different scenarios.
> 
> 1. Currently NUMA balancing won't check at scan time, if a page (or a VMA )is
> not migratable since the page (or the address range) is pinned. It will go ahead
> with MMU invalidation notifications and changes the PTE protection to PAGE_NONE
> only to realize later that the pinned pages can't be migrated before reinstalling
> the original PTE.
> 
> This was found to cause issues to SEV guests whose pages are completely pinned.
> This was discussed here - https://lore.kernel.org/all/20220927000729.498292-1-Ashish.Kalra@amd.com/
> 
> We could probably use page_maybe_dma_pinned() to determine if the page is long
> term pinned and avoid MMU invalidation and protection change for such a page.
> However then we would have to do per-page invalidations (as against one time
> PMD range invalidation that is done currently) which is probably not desirable.
> 
> Also MMU invalidations are expected to be issued under sleepable context (mostly
> except in the OOM notification which uses nonblock verion, AFAICS). This makes it
> difficult to check the pinned state of the page prior to MMU invalidation. Some of
> this is discussed here: https://lore.kernel.org/linux-arm-kernel/YuEMkKY2RU%2F2KiZW@monolith.localdoman/
> 
> This current patchset where we attempt to restrict scanning to relevant VMAs may
> help the above case partially, but any ideas on addressing this issue
> comprehensively? It would have been ideal if we could identify such non-migratable
> pages (long term pinned) clearly and avoid them entirely from scanning and protection
> change.
> 
> 2. Applications that run on GPUs may like to avoid the NUMA balancing activity
> completely and they benefit from per-process enabling/disabling of NUMA balancing.
> The patchset (which has a different use case for per-process control) that helps
> this is here - https://lore.kernel.org/all/49ed07b1-e167-7f94-9986-8e86fb60bb09@nvidia.com/
> 
> Improvements to increase the relevant scanning can help this case to an extent
> but per-process NUMA balancing control should be a useful control to have.
> 
> Regards,
> Bharata.
>