by Raghavendra K T

[permalink] [raw]

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

On 1/27/2023 3:47 PM, Mel Gorman wrote:
> On Wed, Jan 25, 2023 at 12:48:16AM +0530, Raghavendra K T wrote:
>> looks like we have to additionally handle numab initialization in
>> vm_area_dup() code path. something like below fixed it (copied pasted
>> from tty):
>>
>
> Yep, it wasn't even boot tested. Better approach is something like this,
> still not actually tested
>
> include/linux/mm.h | 9 +++++++++
> include/linux/mm_types.h | 7 +++++++
> kernel/fork.c | 2 ++
> kernel/sched/fair.c | 17 +++++++++++++++++
> 4 files changed, 35 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 8f857163ac89..481f90dc1983 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -612,6 +612,14 @@ struct vm_operations_struct {
> unsigned long addr);
> };
>
> +#ifdef CONFIG_NUMA_BALANCING
> +#define vma_numab_init(vma) do { (vma)->numab = NULL; } while (0)
> +#define vma_numab_free(vma) do { kfree((vma)->numab); } while (0)
> +#else
> +static inline void vma_numab_init(struct vm_area_struct *vma) {}
> +static inline void vma_numab_free(struct vm_area_struct *vma) {}
> +#endif /* CONFIG_NUMA_BALANCING */
> +
> static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
> {
> static const struct vm_operations_struct dummy_vm_ops = {};
> @@ -620,6 +628,7 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
> vma->vm_mm = mm;
> vma->vm_ops = &dummy_vm_ops;
> INIT_LIST_HEAD(&vma->anon_vma_chain);
> + vma_numab_init(vma);
> }
>
> static inline void vma_set_anonymous(struct vm_area_struct *vma)
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 9757067c3053..43ce363d5124 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -526,6 +526,10 @@ struct anon_vma_name {
> char name[];
> };
>
> +struct vma_numab {
> + unsigned long next_scan;
> +};
> +
> /*
> * This struct describes a virtual memory area. There is one of these
> * per VM-area/task. A VM area is any part of the process virtual memory
> @@ -593,6 +597,9 @@ struct vm_area_struct {
> #endif
> #ifdef CONFIG_NUMA
> struct mempolicy *vm_policy; /* NUMA policy for the VMA */
> +#endif
> +#ifdef CONFIG_NUMA_BALANCING
> + struct vma_numab *numab; /* NUMA Balancing state */
> #endif
> struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
> } __randomize_layout;
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 9f7fe3541897..5a2e8c3cc410 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -474,6 +474,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
> */
> *new = data_race(*orig);
> INIT_LIST_HEAD(&new->anon_vma_chain);
> + vma_numab_init(new);
> dup_anon_vma_name(orig, new);
> }
> return new;
> @@ -481,6 +482,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
>
> void vm_area_free(struct vm_area_struct *vma)
> {
> + vma_numab_free(vma);
> free_anon_vma_name(vma);
> kmem_cache_free(vm_area_cachep, vma);
> }
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c36aa54ae071..6a1cffdfc76b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3027,6 +3027,23 @@ static void task_numa_work(struct callback_head *work)
> if (!vma_is_accessible(vma))
> continue;
>
> + /* Initialise new per-VMA NUMAB state. */
> + if (!vma->numab) {
> + vma->numab = kzalloc(sizeof(struct vma_numab), GFP_KERNEL);
> + if (!vma->numab)
> + continue;
> +
> + vma->numab->next_scan = now +
> + msecs_to_jiffies(sysctl_numa_balancing_scan_delay);
> + }
> +
> + /*
> + * After the first scan is complete, delay the balancing scan
> + * for new VMAs.
> + */
> + if (mm->numa_scan_seq && time_before(jiffies, vma->numab->next_scan))
> + continue;
> +
> do {
> start = max(start, vma->vm_start);
> end = ALIGN(start + (pages << PAGE_SHIFT), HPAGE_SIZE);

Thank you Mel. This looks better now.
Yes we would have moved to mm.h eventually to avoid #if clutter.

Also for PATCH2 function common to memory.c and huge_memory.c would
need the same to handle hugetlb vma as suggested by you.
Working on gathering numbers and PID clear logic now. will post V2 soon.

Thanks
- Raghu

2023-02-21 00:38:22

by Ashish Kalra

[permalink] [raw]

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

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/[email protected]/
>
> 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%[email protected]/
>
> 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/[email protected]/
>
> 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.
>