Re: CONFIG_DMA_CMA causes ttm performance problems/hangs.

From: Thomas Hellstrom
Date: Sat Aug 09 2014 - 01:40:14 EST

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 | }
>
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