On 8/21/20 1:47 PM, Song Bao Hua (Barry Song) wrote:
>
>
>> -Original Message-
>> From: Song Bao Hua (Barry Song)
>> Sent: Saturday, August 22, 2020 7:27 AM
>> To: 'Mike Kravetz' ; h...@lst.de;
>> m.szyprow...@samsung.com; robin.mur...@arm.com; w...@kernel.org;
>> ganapatrao.kulka...@cavium.com; catalin.mari...@arm.com;
>> a...@linux-foundation.org
>> Cc: iommu@lists.linux-foundation.org; linux-arm-ker...@lists.infradead.org;
>> linux-ker...@vger.kernel.org; Zengtao (B) ;
>> huangdaode ; Linuxarm
>> Subject: RE: [PATCH v7 0/3] make dma_alloc_coherent NUMA-aware by
>> per-NUMA CMA
>>
>>
>>
>>> -Original Message-
>>> From: Mike Kravetz [mailto:mike.krav...@oracle.com]
>>> Sent: Saturday, August 22, 2020 5:53 AM
>>> To: Song Bao Hua (Barry Song) ; h...@lst.de;
>>> m.szyprow...@samsung.com; robin.mur...@arm.com; w...@kernel.org;
>>> ganapatrao.kulka...@cavium.com; catalin.mari...@arm.com;
>>> a...@linux-foundation.org
>>> Cc: iommu@lists.linux-foundation.org; linux-arm-ker...@lists.infradead.org;
>>> linux-ker...@vger.kernel.org; Zengtao (B) ;
>>> huangdaode ; Linuxarm
>>
>>> Subject: Re: [PATCH v7 0/3] make dma_alloc_coherent NUMA-aware by
>>> per-NUMA CMA
>>>
>>> Hi Barry,
>>> Sorry for jumping in so late.
>>>
>>> On 8/21/20 4:33 AM, Barry Song wrote:
>>>>
>>>> with per-numa CMA, smmu will get memory from local numa node to save
>>> command
>>>> queues and page tables. that means dma_unmap latency will be shrunk
>>> much.
>>>
>>> Since per-node CMA areas for hugetlb was introduced, I have been thinking
>>> about the limited number of CMA areas. In most configurations, I believe
>>> it is limited to 7. And, IIRC it is not something that can be changed at
>>> runtime, you need to reconfig and rebuild to increase the number. In
>> contrast
>>> some configs have NODES_SHIFT set to 10. I wasn't too worried because of
>>> the limited hugetlb use case. However, this series is adding another user
>>> of per-node CMA areas.
>>>
>>> With more users, should try to sync up number of CMA areas and number of
>>> nodes? Or, perhaps I am worrying about nothing?
>>
>> Hi Mike,
>> The current limitation is 8. If the server has 4 nodes and we enable both
>> pernuma
>> CMA and hugetlb, the last node will fail to get one cma area as the default
>> global cma area will take 1 of 8. So users need to change menuconfig.
>> If the server has 8 nodes, we enable one of pernuma cma and hugetlb, one
>> node
>> will fail to get cma.
>>
>> We may set the default number of CMA areas as 8+MAX_NODES(if hugetlb
>> enabled) +
>> MAX_NODES(if pernuma cma enabled) if we don't expect users to change
>> config, but
>> right now hugetlb has not an option in Kconfig to enable or disable like
>> pernuma cma
>> has DMA_PERNUMA_CMA.
>
> I would prefer we make some changes like:
>
> config CMA_AREAS
> int "Maximum count of the CMA areas"
> depends on CMA
> + default 19 if NUMA
> default 7
> help
> CMA allows to create CMA areas for particular purpose, mainly,
> used as device private area. This parameter sets the maximum
> number of CMA area in the system.
>
> - If unsure, leave the default value "7".
> + If unsure, leave the default value "7" or "19" if NUMA is used.
>
> 1+ CONFIG_CMA_AREAS should be quite enough for almost all servers in the
> markets.
>
> If 2 numa nodes, and both hugetlb cma and pernuma cma is enabled, we need 2*2
> + 1 = 5
> If 4 numa nodes, and both hugetlb cma and pernuma cma is enabled, we need 2*4
> + 1 = 9-> default ARM64 config.
> If 8 numa nodes, and both hugetlb cma and pernuma cma is enabled, we need 2*8
> + 1 = 17
>
> The default value is supporting the most common case and is not going to
> support those servers
> with NODES_SHIFT=10, they can make their own config just like users need to
> increase CMA_AREAS
> if they add many cma areas in device tree in a system even without NUMA.
>
> How do you think, mike?
I'm OK with that. I really did not want to sidetrach this series. It is
just something I thought about when looking at the hugetlb code. My 'to do'
list includes looking at a way to make the number of CMA areas dynamic.
--
Mike Kravetz
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