2014-10-04 1:39 GMT+09:00 Peter Hurley <pe...@hurleysoftware.com>: > On 10/03/2014 12:06 PM, Akinobu Mita wrote: >> 2014-10-03 23:27 GMT+09:00 Peter Hurley <pe...@hurleysoftware.com>: >>> On 10/02/2014 07:08 PM, Akinobu Mita wrote: >>>> 2014-10-03 7:03 GMT+09:00 Peter Hurley <pe...@hurleysoftware.com>: >>>>> 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. > > I think something may have gotten lost in translation. > > My "test" consists of doing my daily work (email, emacs, kernel builds, > web breaks, etc). > > I don't have a testsuite that validates a page allocator or records any > performance metrics (for TTM allocations under load, as an example). > > Without a unit test and performance metrics, my "test" is not really > positive affirmation of a correct implementation. > > >>>>> 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.szyprow...@samsung.com> >>>>> 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.szyprow...@samsung.com> >>>>> Signed-off-by: Kyungmin Park <kyungmin.p...@samsung.com> >>>>> CC: Michal Nazarewicz <min...@mina86.com> >>>>> Acked-by: Arnd Bergmann <a...@arndb.de> >>>>> >>>>> >>>>> 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).
kernel parameter 'cma=' is only available when CONFIG_DMA_CMA is enabled. cma=0 doesn't disable 1GB huge pages as far as I can see. So I prepare a patch which make default cma size zero on x86. >> 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? > > Again, since I don't know what you're using this for and > there are no existing mainline users, I can't really design this for > you. > > I'm just trying to do my best to come up with alternative solutions > that limit the impact to existing x86 configurations, while still > achieving your goals (without really knowing what those design > constraints are). Thanks a lot for your advise. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/