Re: CONFIG_DMA_CMA causes ttm performance problems/hangs.

[Konrad Rzeszutek Wilk]

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.
>
>/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.

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:
>>
>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?
>>

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   |                          }
>>
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