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SubjectRe: [PATCH V2 1/3] sched/numa: Apply the scan delay to every vma instead of tasks
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On 2/3/2023 3:54 PM, Peter Zijlstra wrote:
> On Wed, Feb 01, 2023 at 01:32:20PM +0530, Raghavendra K T wrote:
>> From: Mel Gorman <mgorman@techsingularity.net>
>>
>> Avoid scanning new or very short-lived VMAs.
>>
>> (Raghavendra: Add initialization in vm_area_dup())
>
> Given this is a performance centric patch -- some sort of qualification
> / justification would be much appreciated.
>

Thank you Peter for the review.
Sure will add more detailed result in cover and summary for the patch
commit message.

> Also, perhaps explain the rationale for the actual heuristics chosen.
>

Sure will add more detail in the V3

>> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
>> Signed-off-by: Raghavendra K T <raghavendra.kt@amd.com>
>> ---
>> 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 974ccca609d2..74d9df1d8982 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -611,6 +611,14 @@ 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 */
>
> I'm tripping over the inconsistency of macros and functions here. I'd
> suggest making both cases functions.
>
>

Sure will do that

>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index 500e536796ca..e84f95a77321 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -435,6 +435,10 @@ struct anon_vma_name {
>> char name[];
>> };
>>
>> +struct vma_numab {
>> + unsigned long next_scan;
>> +};
>
> I'm not sure what a numab is; contraction of new-kebab, something else?
>
> While I appreciate short names, they'd ideally also make sense. If we
> cannot come up with a better one, perhaps elucidate the reader with a
> comment.

Agree.. How about vma_nuamb vma_numab_state or vma_numab_info as
abbreviation for vma_numa_balancing_info /state?

>
>> +
>> /*
>> * 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
>> @@ -504,6 +508,9 @@ struct vm_area_struct {
>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index e4a0b8bd941c..060b241ce3c5 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -3015,6 +3015,23 @@ 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;
>
> I think I sorta see why, but I'm thinking it would be good to include
> more of the why in that comment.

Sure. Will add something in the lines of.. "scanning the VMA's of short
lived tasks add more overhead than benefit...."

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