linux-kernel - Re: [PATCH V2 1/3] sched/numa: Apply the scan delay to every vma instead of tasks

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Date:   Sat, 4 Feb 2023 22:49:10 +0530
From:   Raghavendra K T <raghavendra.kt@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>, rppt@...nel.org,
        Bharata B Rao <bharata@....com>,
        Disha Talreja <dishaa.talreja@....com>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH V2 1/3] sched/numa: Apply the scan delay to every vma
 instead of tasks

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@...hsingularity.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@...hsingularity.net>
>> Signed-off-by: Raghavendra K T <raghavendra.kt@....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...."