On 08/09/2014 03:33 PM, Konrad Rzeszutek Wilk wrote: > On August 9, 2014 1:39:39 AM EDT, Thomas Hellstrom <thellstrom@xxxxxxxxxx> wrote: >> Hi. >> > Hey Thomas! > >> IIRC I don't think the TTM DMA pool allocates coherent pages more than >> one page at a time, and _if that's true_ it's pretty unnecessary for >> the >> dma subsystem to route those allocations to CMA. Maybe Konrad could >> shed >> some light over this? > It should allocate in batches and keep them in the TTM DMA pool for some time to be reused. > > The pages that it gets are in 4kb granularity though. Then I feel inclined to say this is a DMA subsystem bug. Single page allocations shouldn't get routed to CMA. /Thomas >> /Thomas >> >> >> On 08/08/2014 07:42 PM, Mario Kleiner wrote: >>> Hi all, >>> >>> there is a rather severe performance problem i accidentally found >> when >>> trying to give Linux 3.16.0 a final test on a x86_64 MacBookPro under >>> Ubuntu 14.04 LTS with nouveau as graphics driver. >>> >>> I was lazy and just installed the Ubuntu precompiled mainline kernel. >>> That kernel happens to have CONFIG_DMA_CMA=y set, with a default CMA >>> (contiguous memory allocator) size of 64 MB. Older Ubuntu kernels >>> weren't compiled with CMA, so i only observed this on 3.16, but >>> previous kernels would likely be affected too. >>> >>> After a few minutes of regular desktop use like switching workspaces, >>> scrolling text in a terminal window, Firefox with multiple tabs open, >>> Thunderbird etc. (tested with KDE/Kwin, with/without desktop >>> composition), i get chunky desktop updates, then multi-second >> freezes, >>> after a few minutes the desktop hangs for over a minute on almost any >>> GUI action like switching windows etc. --> Unuseable. >>> >>> ftrace'ing shows the culprit being this callchain (typical good/bad >>> example ftrace snippets at the end of this mail): >>> >>> ...ttm dma coherent memory allocations, e.g., from >>> __ttm_dma_alloc_page() ... --> dma_alloc_coherent() --> platform >>> specific hooks ... -> dma_generic_alloc_coherent() [on x86_64] --> >>> dma_alloc_from_contiguous() >>> >>> dma_alloc_from_contiguous() is a no-op without CONFIG_DMA_CMA, or >> when >>> the machine is booted with kernel boot cmdline parameter "cma=0", so >>> it triggers the fast alloc_pages_node() fallback at least on x86_64. >>> >>> With CMA, this function becomes progressively more slow with every >>> minute of desktop use, e.g., runtimes going up from < 0.3 usecs to >>> hundreds or thousands of microseconds (before it gives up and >>> alloc_pages_node() fallback is used), so this causes the >>> multi-second/minute hangs of the desktop. >>> >>> So it seems ttm memory allocations quickly fragment and/or exhaust >> the >>> CMA memory area, and dma_alloc_from_contiguous() tries very hard to >>> find a fitting hole big enough to satisfy allocations with a retry >>> loop (see >>> >> https://urldefense.proofpoint.com/v1/url?u=http://lxr.free-electrons.com/source/drivers/base/dma-contiguous.c%23L339&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=l5Ago9ekmVFZ3c4M6eauqrJWGwjf6fTb%2BP3CxbBFkVM%3D%0A&m=6cy0madhpBCtEyOKu95ucqhzU%2FjAHPP7ODVTc47UYQs%3D%0A&s=42356aad2ff181236f4704283dc058fdd7b7e213cdea7378665094b35ee0dfdf) >>> that takes forever. > I am curious why it does not end up using the pool. As in use the TTM DMA pool to pick pages instead of allocating (and freeing) new ones? > >>> This is not good, also not for other devices which actually need a >>> non-fragmented CMA for DMA, so what to do? I doubt most current gpus >>> still need physically contiguous dma memory, maybe with exception of >>> some embedded gpus? > Oh. If I understood you correctly - the CMA ends up giving huge chunks of contiguous area. But if the sizes are 4kb I wonder why it would do that? > > The modern GPUs on x86 can deal with scatter gather and as you surmise don't need contiguous physical contiguous areas. >>> My naive approach would be to add a new gfp_t flag a la >>> ___GFP_AVOIDCMA, and make callers of dma_alloc_from_contiguous() >>> refrain from doing so if they have some fallback for getting memory. >>> And then add that flag to ttm's ttm_dma_populate() gfp_flags, e.g., >>> around here: >>> >> https://urldefense.proofpoint.com/v1/url?u=http://lxr.free-electrons.com/source/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c%23L884&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=l5Ago9ekmVFZ3c4M6eauqrJWGwjf6fTb%2BP3CxbBFkVM%3D%0A&m=6cy0madhpBCtEyOKu95ucqhzU%2FjAHPP7ODVTc47UYQs%3D%0A&s=0c2a37c8bac57e0ab7333a9580eb5114e09566d1d34ab43be7a80de8316bdcdd >>> However i'm not familiar enough with memory management, so likely >>> greater minds here have much better ideas on how to deal with this? >>> > That is a bit of hack to deal with CMA being slow. > > Hmm. Let's first figure out why TTM DMA pool is not reusing pages. >>> thanks, >>> -mario >>> >>> Typical snippet from an example trace of a badly stalling desktop >> with >>> CMA (alloc_pages_node() fallback may have been missing in this traces >>> ftrace_filter settings): >>> >>> 1) | ttm_dma_pool_get_pages >>> [ttm]() { >>> 1) | ttm_dma_page_pool_fill_locked [ttm]() { >>> 1) | ttm_dma_pool_alloc_new_pages [ttm]() { >>> 1) | __ttm_dma_alloc_page [ttm]() { >>> 1) | dma_generic_alloc_coherent() { >>> 1) ! 1873.071 us | dma_alloc_from_contiguous(); >>> 1) ! 1874.292 us | } >>> 1) ! 1875.400 us | } >>> 1) | __ttm_dma_alloc_page [ttm]() { >>> 1) | dma_generic_alloc_coherent() { >>> 1) ! 1868.372 us | dma_alloc_from_contiguous(); >>> 1) ! 1869.586 us | } >>> 1) ! 1870.053 us | } >>> 1) | __ttm_dma_alloc_page [ttm]() { >>> 1) | dma_generic_alloc_coherent() { >>> 1) ! 1871.085 us | dma_alloc_from_contiguous(); >>> 1) ! 1872.240 us | } >>> 1) ! 1872.669 us | } >>> 1) | __ttm_dma_alloc_page [ttm]() { >>> 1) | dma_generic_alloc_coherent() { >>> 1) ! 1888.934 us | dma_alloc_from_contiguous(); >>> 1) ! 1890.179 us | } >>> 1) ! 1890.608 us | } >>> 1) 0.048 us | ttm_set_pages_caching [ttm](); >>> 1) ! 7511.000 us | } >>> 1) ! 7511.306 us | } >>> 1) ! 7511.623 us | } >>> >>> The good case (with cma=0 kernel cmdline, so >>> dma_alloc_from_contiguous() no-ops,) >>> >>> 0) | ttm_dma_pool_get_pages >>> [ttm]() { >>> 0) | ttm_dma_page_pool_fill_locked [ttm]() { >>> 0) | ttm_dma_pool_alloc_new_pages [ttm]() { >>> 0) | __ttm_dma_alloc_page [ttm]() { >>> 0) | dma_generic_alloc_coherent() { >>> 0) 0.171 us | dma_alloc_from_contiguous(); >>> 0) 0.849 us | __alloc_pages_nodemask(); >>> 0) 3.029 us | } >>> 0) 3.882 us | } >>> 0) | __ttm_dma_alloc_page [ttm]() { >>> 0) | dma_generic_alloc_coherent() { >>> 0) 0.037 us | dma_alloc_from_contiguous(); >>> 0) 0.163 us | __alloc_pages_nodemask(); >>> 0) 1.408 us | } >>> 0) 1.719 us | } >>> 0) | __ttm_dma_alloc_page [ttm]() { >>> 0) | dma_generic_alloc_coherent() { >>> 0) 0.035 us | dma_alloc_from_contiguous(); >>> 0) 0.153 us | __alloc_pages_nodemask(); >>> 0) 1.454 us | } >>> 0) 1.720 us | } >>> 0) | __ttm_dma_alloc_page [ttm]() { >>> 0) | dma_generic_alloc_coherent() { >>> 0) 0.036 us | dma_alloc_from_contiguous(); >>> 0) 0.112 us | __alloc_pages_nodemask(); >>> 0) 1.211 us | } >>> 0) 1.541 us | } >>> 0) 0.035 us | ttm_set_pages_caching [ttm](); >>> 0) + 10.902 us | } >>> 0) + 11.577 us | } >>> 0) + 11.988 us | } >>> >>> _______________________________________________ >>> dri-devel mailing list >>> dri-devel@xxxxxxxxxxxxxxxxxxxxx >>> https://urldefense.proofpoint.com/v1/url?u=http://lists.freedesktop.org/mailman/listinfo/dri-devel&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=l5Ago9ekmVFZ3c4M6eauqrJWGwjf6fTb%2BP3CxbBFkVM%3D%0A&m=6cy0madhpBCtEyOKu95ucqhzU%2FjAHPP7ODVTc47UYQs%3D%0A&s=d2636419e1f7f56c0d270e29ffe6ab6c6e29249876a578d70d973058f9411831 > _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
