linux-kernel - Re: [PATCH] mm/vmalloc: fix numa spreading for large hash tables

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Date:   Thu, 14 Oct 2021 16:50:16 +0800
From:   Chen Wandun <chenwandun@...wei.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     <npiggin@...il.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <edumazet@...gle.com>,
        <wangkefeng.wang@...wei.com>, <guohanjun@...wei.com>
Subject: Re: [PATCH] mm/vmalloc: fix numa spreading for large hash tables



在 2021/9/29 6:33, Andrew Morton 写道:
> On Tue, 28 Sep 2021 20:10:40 +0800 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>
> 
> This seems like it could cause significant performance regressions in
> some situations?
Yes，I indeed will cause some performance regressions, I will send a 
optimization patch based on this patch.
> 
> If "yes" then wouldn't a cc:stable be appropriate?  And some (perhaps
> handwavy) quantification of the slowdown would help people understand
> why we're recommending a backport.
> 
> If "no" then why the heck do we have that feature in there anyway ;)
> 
> .
>