Re: [PATCH v3 0/5] enhance DMA CMA on x86

[Akinobu Mita]

2014-10-03 23:27 GMT+09:00 Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>:
> On 10/02/2014 07:08 PM, Akinobu Mita wrote:
>> 2014-10-03 7:03 GMT+09:00 Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>:
>>> On 10/02/2014 12:41 PM, Konrad Rzeszutek Wilk wrote:
>>>> On Tue, Sep 30, 2014 at 09:49:54PM -0400, Peter Hurley wrote:
>>>>> On 09/30/2014 07:45 PM, Thomas Gleixner wrote:
>>
>>>>> Which is different than if the plan is to ship production units for x86;
>>>>> then a general purpose solution will be required.
>>>>>
>>>>> As to the good design of a general purpose solution for allocating and
>>>>> mapping huge order pages, you are certainly more qualified to help Akinobu
>>>>> than I am.
>>>
>>> What Akinobu's patches intend to support is:
>>>
>>>         phys_addr = dma_alloc_coherent(dev, 64 * 1024 * 1024, &bus_addr, GFP_KERNEL);
>>>
>>> which raises three issues:
>>>
>>> 1. Where do coherent blocks of this size come from?
>>> 2. How to prevent fragmentation of these reserved blocks over time by
>>>    existing DMA users?
>>> 3. Is this support generically required across all iommu implementations on x86?
>>>
>>> Questions 1 and 2 are non-trivial, in the general case, otherwise the page
>>> allocator would already do this. Simply dropping in the contiguous memory
>>> allocator doesn't work because CMA does not have the same policy and performance
>>> as the page allocator, and is already causing performance regressions even
>>> in the absence of huge page allocations.
>>
>> Could you take a look at the patches I sent?  Can they fix these issues?
>> https://lkml.org/lkml/2014/9/28/110
>>
>> With these patches, normal alloc_pages() is used for allocation first
>> and dma_alloc_from_contiguous() is used as a fallback.
>
> Sure, I can test these patches this weekend.
> Where are the unit tests?

Thanks a lot.  I would like to know whether the performance regression
you see will disappear or not with these patches as if CONFIG_DMA_CMA is
disabled.

>>> So that's why I raised question 3; is making the necessary compromises to support
>>> 64MB coherent DMA allocations across all x86 iommu implementations actually
>>> required?
>>>
>>> Prior to Akinobu's patches, the use of CMA by x86 iommu configurations was
>>> designed to be limited to testing configurations, as the introductory
>>> commit states:
>>>
>>> commit 0a2b9a6ea93650b8a00f9fd5ee8fdd25671e2df6
>>> Author: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
>>> Date:   Thu Dec 29 13:09:51 2011 +0100
>>>
>>>     X86: integrate CMA with DMA-mapping subsystem
>>>
>>>     This patch adds support for CMA to dma-mapping subsystem for x86
>>>     architecture that uses common pci-dma/pci-nommu implementation. This
>>>     allows to test CMA on KVM/QEMU and a lot of common x86 boxes.
>>>
>>>     Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
>>>     Signed-off-by: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
>>>     CC: Michal Nazarewicz <mina86@xxxxxxxxxx>
>>>     Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
>>>
>>>
>>> Which brings me to my suggestion: if support for huge coherent DMA is
>>> required only for a special test platform, then could not this support
>>> be specific to a new iommu configuration, namely iommu=cma, which would
>>> get initialized much the same way that iommu=calgary is now.
>>>
>>> The code for such a iommu configuration would mostly duplicate
>>> arch/x86/kernel/pci-swiotlb.c and the CMA support would get removed from
>>> the other x86 iommu implementations.
>>
>> I'm not sure I read correctly, though.  Can boot option 'cma=0' also
>> help avoiding CMA from IOMMU implementation?
>
> Maybe, but that's not an appropriate solution for distro kernels.
>
> Nor does this address configurations that want a really large CMA so
> 1GB huge pages can be allocated (not for DMA though).

Now I see the point of iommu=cma you suggested.  But what should we do
when CONFIG_SWIOTLB is disabled, especially for x86_32?
Should we just introduce yet another flag to tell not using DMA_CMA
instead of adding new swiotlb-like iommu implementation?
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