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Subject: Re: [PATCH] mm/khugepaged: sched to numa node when collapse huge page
To:     Yang Shi <shy828301@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20220311090119.2412738-1-maobibo@loongson.cn>
 <CAHbLzkoiP+uRYGDm+FC_zg-LkPbTMFQ-wSzGMh0RPr-XP4_ciw@mail.gmail.com>
From:   maobibo <maobibo@loongson.cn>
Message-ID: <2b8b8349-d0fe-7a86-1591-21a7684a0f45@loongson.cn>
Date:   Mon, 14 Mar 2022 08:46:19 +0800
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Hi Yang,

thanks for reviewing my patch. I reply inline.

On 03/12/2022 03:08 AM, Yang Shi wrote:
> On Fri, Mar 11, 2022 at 1:01 AM Bibo Mao <maobibo@loongson.cn> wrote:
>>
>> collapse huge page is slow, specially when khugepaged daemon runs
>> on different numa node with that of huge page. It suffers from
>> huge page copying across nodes, also cache is not used for target
>> node. With this patch, khugepaged daemon switches to the same numa
>> node with huge page. It saves copying time and makes use of local
>> cache better.
>>
>> Signed-off-by: Bibo Mao <maobibo@loongson.cn>
>> ---
>>  mm/khugepaged.c | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 131492fd1148..460c285dc974 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -116,6 +116,7 @@ struct khugepaged_scan {
>>         struct list_head mm_head;
>>         struct mm_slot *mm_slot;
>>         unsigned long address;
>> +       int node;
>>  };
>>
>>  static struct khugepaged_scan khugepaged_scan = {
>> @@ -1066,6 +1067,7 @@ static void collapse_huge_page(struct mm_struct *mm,
>>         struct vm_area_struct *vma;
>>         struct mmu_notifier_range range;
>>         gfp_t gfp;
>> +       const struct cpumask *cpumask;
>>
>>         VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>>
>> @@ -1079,6 +1081,13 @@ static void collapse_huge_page(struct mm_struct *mm,
>>          * that. We will recheck the vma after taking it again in write mode.
>>          */
>>         mmap_read_unlock(mm);
>> +
>> +       /* sched to specified node before huage page memory copy */
>> +       cpumask = cpumask_of_node(node);
>> +       if ((khugepaged_scan.node != node) && !cpumask_empty(cpumask)) {
>> +               set_cpus_allowed_ptr(current, cpumask);
>> +               khugepaged_scan.node = node;
> 
> What if khugepaged was scheduled to the other nodes after this, but
> khugepaged_scan.node still equals to node? It seems possible to me
> IIUC.
khugepaged will not schedule to the other nodes after function 
set_cpus_allowed_ptr with my understanding. Or node is not necessary
to record and we can simply use task_node(current) like this:
 +       /* sched to specified node before huage page memory copy */
 +       cpumask = cpumask_of_node(node);
 +       if ((task_node(current) != node) && !cpumask_empty(cpumask))
 +               set_cpus_allowed_ptr(current, cpumask);
 
> 
> TBH I'm not quite sure if migrating khugepaged is really worth it for
> everyone or not. The worst case is the locality of base pages are not
> obvious, for example, the base pages may be across all nodes, so you
> always get cross nodes memory copy. And khugepaged may get slower if
> cpu is contentious.
target node is calculated from src node of base pages. If base pages
are across all nodes, target node is the most one from that of base pages.
Most time if THP is used, memory footprint for workload is large, it
is deserved for other architectures, however I have no such binary workload
on other architectures.

> 
> In addition, I saw MIPS has its own copy_user_highpage(), is it a
> contributing factor too?
copy_user_highpage is similar with function copy on mips64, it is only
different on mips32. It has nothing to do with this, else it will be big
issue.

regards
bibo,mao

> 
>> +       }
>>         new_page = khugepaged_alloc_page(hpage, gfp, node);
>>         if (!new_page) {
>>                 result = SCAN_ALLOC_HUGE_PAGE_FAIL;
>> @@ -2380,6 +2389,7 @@ int start_stop_khugepaged(void)
>>                 kthread_stop(khugepaged_thread);
>>                 khugepaged_thread = NULL;
>>         }
>> +       khugepaged_scan.node = NUMA_NO_NODE;
>>         set_recommended_min_free_kbytes();
>>  fail:
>>         mutex_unlock(&khugepaged_mutex);
>> --
>> 2.31.1
>>
>>