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Date:   Fri, 15 Oct 2021 17:11:25 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Chen Wandun <chenwandun@...wei.com>,
        Shakeel Butt <shakeelb@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Eric Dumazet <edumazet@...gle.com>, guohanjun@...wei.com,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>
Subject: Re: [PATCH] mm/vmalloc: fix numa spreading for large hash tables

Excerpts from Chen Wandun's message of October 15, 2021 12:31 pm:
> 
> 
> 在 2021/10/15 9:34, Nicholas Piggin 写道:
>> Excerpts from Chen Wandun's message of October 14, 2021 6:59 pm:
>>>
>>>
>>> 在 2021/10/14 5:46, Shakeel Butt 写道:
>>>> On Tue, Sep 28, 2021 at 5:03 AM Chen Wandun <chenwandun@...wei.com> wrote:
>>>>>
>>>>> Eric Dumazet reported a strange numa spreading info in [1], and found
>>>>> commit 121e6f3258fe ("mm/vmalloc: hugepage vmalloc mappings") introduced
>>>>> this issue [2].
>>>>>
>>>>> Dig into the difference before and after this patch, page allocation has
>>>>> some difference:
>>>>>
>>>>> before:
>>>>> alloc_large_system_hash
>>>>>       __vmalloc
>>>>>           __vmalloc_node(..., NUMA_NO_NODE, ...)
>>>>>               __vmalloc_node_range
>>>>>                   __vmalloc_area_node
>>>>>                       alloc_page /* because NUMA_NO_NODE, so choose alloc_page branch */
>>>>>                           alloc_pages_current
>>>>>                               alloc_page_interleave /* can be proved by print policy mode */
>>>>>
>>>>> after:
>>>>> alloc_large_system_hash
>>>>>       __vmalloc
>>>>>           __vmalloc_node(..., NUMA_NO_NODE, ...)
>>>>>               __vmalloc_node_range
>>>>>                   __vmalloc_area_node
>>>>>                       alloc_pages_node /* choose nid by nuam_mem_id() */
>>>>>                           __alloc_pages_node(nid, ....)
>>>>>
>>>>> So after commit 121e6f3258fe ("mm/vmalloc: hugepage vmalloc mappings"),
>>>>> it will allocate memory in current node instead of interleaving allocate
>>>>> memory.
>>>>>
>>>>> [1]
>>>>> https://lore.kernel.org/linux-mm/CANn89iL6AAyWhfxdHO+jaT075iOa3XcYn9k6JJc7JR2XYn6k_Q@mail.gmail.com/
>>>>>
>>>>> [2]
>>>>> https://lore.kernel.org/linux-mm/CANn89iLofTR=AK-QOZY87RdUZENCZUT4O6a0hvhu3_EwRMerOg@mail.gmail.com/
>>>>>
>>>>> Fixes: 121e6f3258fe ("mm/vmalloc: hugepage vmalloc mappings")
>>>>> Reported-by: Eric Dumazet <edumazet@...gle.com>
>>>>> Signed-off-by: Chen Wandun <chenwandun@...wei.com>
>>>>> ---
>>>>>    mm/vmalloc.c | 33 ++++++++++++++++++++++++++-------
>>>>>    1 file changed, 26 insertions(+), 7 deletions(-)
>>>>>
>>>>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>>>>> index f884706c5280..48e717626e94 100644
>>>>> --- a/mm/vmalloc.c
>>>>> +++ b/mm/vmalloc.c
>>>>> @@ -2823,6 +2823,8 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
>>>>>                   unsigned int order, unsigned int nr_pages, struct page **pages)
>>>>>    {
>>>>>           unsigned int nr_allocated = 0;
>>>>> +       struct page *page;
>>>>> +       int i;
>>>>>
>>>>>           /*
>>>>>            * For order-0 pages we make use of bulk allocator, if
>>>>> @@ -2833,6 +2835,7 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
>>>>>           if (!order) {
>>>>
>>>> Can you please replace the above with if (!order && nid != NUMA_NO_NODE)?
>>>>
>>>>>                   while (nr_allocated < nr_pages) {
>>>>>                           unsigned int nr, nr_pages_request;
>>>>> +                       page = NULL;
>>>>>
>>>>>                           /*
>>>>>                            * A maximum allowed request is hard-coded and is 100
>>>>> @@ -2842,9 +2845,23 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
>>>>>                            */
>>>>>                           nr_pages_request = min(100U, nr_pages - nr_allocated);
>>>>>
>>>>
>>>> Undo the following change in this if block.
>>>
>>> Yes, It seem like more simpler as you suggested, But it still have
>>> performance regression, I plan to change the following to consider
>>> both mempolcy and alloc_pages_bulk.
>> 
>> Thanks for finding and debugging this. These APIs are a maze of twisty
>> little passages, all alike so I could be as confused as I was when I
>> wrote that patch, but doesn't a minimal fix look something like this?
> 
> Yes, I sent a patch,it looks like as you show, besides it also
> contains some performance optimization.
> 
> [PATCH] mm/vmalloc: introduce alloc_pages_bulk_array_mempolicy to 
> accelerate memory allocation

Okay. It would be better to do it as two patches. First the minimal fix 
so it can be backported easily and have the Fixes: tag pointed at my 
commit. Then the performance optimization.

Thanks,
Nick

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