Hi. 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? /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 > http://lxr.free-electrons.com/source/drivers/base/dma-contiguous.c#L339) > that takes forever. > > 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? > > 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: > http://lxr.free-electrons.com/source/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c#L884 > > However i'm not familiar enough with memory management, so likely > greater minds here have much better ideas on how to deal with this? > > 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 > http://lists.freedesktop.org/mailman/listinfo/dri-devel _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
