CONFIG_DMA_CMA causes ttm performance problems/hangs.

From: Mario Kleiner
Date: Fri Aug 08 2014 - 13:43:00 EST

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 | }

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