CONFIG_DMA_CMA causes ttm performance problems/hangs. - dri-devel - freedesktop.org experimental mailing list

8 Aug 2014


      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....
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   |                          }