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Message-ID: <9d6f8c72-8bcb-39ce-baa8-00e42e5e97da@huawei.com>
Date:   Thu, 14 Oct 2021 16:50:16 +0800
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Subject: Re: [PATCH] mm/vmalloc: fix numa spreading for large hash tables
To:     Andrew Morton <akpm@linux-foundation.org>
CC:     <npiggin@gmail.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>, <edumazet@google.com>,
        <wangkefeng.wang@huawei.com>, <guohanjun@huawei.com>
References: <20210928121040.2547407-1-chenwandun@huawei.com>
 <20210928153302.f87537b5faac8637c3c5eb53@linux-foundation.org>
From:   Chen Wandun <chenwandun@huawei.com>
In-Reply-To: <20210928153302.f87537b5faac8637c3c5eb53@linux-foundation.org>
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在 2021/9/29 6:33, Andrew Morton 写道:
> On Tue, 28 Sep 2021 20:10:40 +0800 Chen Wandun <chenwandun@huawei.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@google.com>
>> Signed-off-by: Chen Wandun <chenwandun@huawei.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 ;)
> 
> .
>