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Message-ID: <20140812204743.GA15496@laptop.dumpdata.com> Date: Tue, 12 Aug 2014 16:47:43 -0400 From: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> To: Mario Kleiner <mario.kleiner.de@...il.com> Cc: Jerome Glisse <j.glisse@...il.com>, Thomas Hellstrom <thellstrom@...are.com>, Dave Airlie <airlied@...hat.com>, kamal@...onical.com, ben@...adent.org.uk, LKML <linux-kernel@...r.kernel.org>, "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>, m.szyprowski@...sung.com Subject: Re: CONFIG_DMA_CMA causes ttm performance problems/hangs. On Tue, Aug 12, 2014 at 02:12:07PM +0200, Mario Kleiner wrote: > On 08/11/2014 05:17 PM, Jerome Glisse wrote: > >On Mon, Aug 11, 2014 at 12:11:21PM +0200, Thomas Hellstrom wrote: > >>On 08/10/2014 08:02 PM, Mario Kleiner wrote: > >>>On 08/10/2014 01:03 PM, Thomas Hellstrom wrote: > >>>>On 08/10/2014 05:11 AM, Mario Kleiner wrote: > >>>>>Resent this time without HTML formatting which lkml doesn't like. > >>>>>Sorry. > >>>>> > >>>>>On 08/09/2014 03:58 PM, Thomas Hellstrom wrote: > >>>>>>On 08/09/2014 03:33 PM, Konrad Rzeszutek Wilk wrote: > >>>>>>>On August 9, 2014 1:39:39 AM EDT, Thomas > >>>>>>>Hellstrom<thellstrom@...are.com> 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. > >>>>>>Then I feel inclined to say this is a DMA subsystem bug. Single page > >>>>>>allocations shouldn't get routed to CMA. > >>>>>> > >>>>>>/Thomas > >>>>>Yes, seems you're both right. I read through the code a bit more and > >>>>>indeed the TTM DMA pool allocates only one page during each > >>>>>dma_alloc_coherent() call, so it doesn't need CMA memory. The current > >>>>>allocators don't check for single page CMA allocations and therefore > >>>>>try to get it from the CMA area anyway, instead of skipping to the > >>>>>much cheaper fallback. > >>>>> > >>>>>So the callers of dma_alloc_from_contiguous() could need that little > >>>>>optimization of skipping it if only one page is requested. For > >>>>> > >>>>>dma_generic_alloc_coherent > >>>>><https://urldefense.proofpoint.com/v1/url?u=http://lxr.free-electrons.com/ident?i%3Ddma_generic_alloc_coherent&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=l5Ago9ekmVFZ3c4M6eauqrJWGwjf6fTb%2BP3CxbBFkVM%3D%0A&m=QQSN6uVpEiw6RuWLAfK%2FKWBFV5HspJUfDh4Y2mUz%2FH4%3D%0A&s=d1852625e2ab2ff07eb34a7f33fc1f55f7f13959912d5a6ce9316d23070ce939> > >>>>> > >>>>>andintel_alloc_coherent > >>>>><https://urldefense.proofpoint.com/v1/url?u=http://lxr.free-electrons.com/ident?i%3Dintel_alloc_coherent&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=l5Ago9ekmVFZ3c4M6eauqrJWGwjf6fTb%2BP3CxbBFkVM%3D%0A&m=QQSN6uVpEiw6RuWLAfK%2FKWBFV5HspJUfDh4Y2mUz%2FH4%3D%0A&s=82d587e9b6aeced5cf9a7caefa91bf47fba809f3522b7379d22e45a2d5d35ebd> > >>>>>this > >>>>>seems easy to do. Looking at the arm arch variants, e.g., > >>>>> > >>>>>https://urldefense.proofpoint.com/v1/url?u=http://lxr.free-electrons.com/source/arch/arm/mm/dma-mapping.c%23L1194&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=l5Ago9ekmVFZ3c4M6eauqrJWGwjf6fTb%2BP3CxbBFkVM%3D%0A&m=QQSN6uVpEiw6RuWLAfK%2FKWBFV5HspJUfDh4Y2mUz%2FH4%3D%0A&s=4c178257eab9b5d7ca650dedba76cf27abeb49ddc7aebb9433f52b6c8bb3bbac > >>>>> > >>>>> > >>>>>and > >>>>> > >>>>>https://urldefense.proofpoint.com/v1/url?u=http://lxr.free-electrons.com/source/arch/arm64/mm/dma-mapping.c%23L44&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=l5Ago9ekmVFZ3c4M6eauqrJWGwjf6fTb%2BP3CxbBFkVM%3D%0A&m=QQSN6uVpEiw6RuWLAfK%2FKWBFV5HspJUfDh4Y2mUz%2FH4%3D%0A&s=5f62f4cbe8cee1f1dd4cbba656354efe6867bcdc664cf90e9719e2f42a85de08 > >>>>> > >>>>> > >>>>>i'm not sure if it is that easily done, as there aren't any fallbacks > >>>>>for such a case and the code looks to me as if that's at least > >>>>>somewhat intentional. > >>>>> > >>>>>As far as TTM goes, one quick one-line fix to prevent it from using > >>>>>the CMA at least on SWIOTLB, NOMMU and Intel IOMMU (when using the > >>>>>above methods) would be to clear the __GFP_WAIT > >>>>><https://urldefense.proofpoint.com/v1/url?u=http://lxr.free-electrons.com/ident?i%3D__GFP_WAIT&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=l5Ago9ekmVFZ3c4M6eauqrJWGwjf6fTb%2BP3CxbBFkVM%3D%0A&m=QQSN6uVpEiw6RuWLAfK%2FKWBFV5HspJUfDh4Y2mUz%2FH4%3D%0A&s=d56d076770d3416264be6c9ea2829ac0d6951203696fa3ad04144f13307577bc> > >>>>>flag from the > >>>>>passed gfp_t flags. That would trigger the well working fallback. > >>>>>So, is > >>>>> > >>>>>__GFP_WAIT > >>>>><https://urldefense.proofpoint.com/v1/url?u=http://lxr.free-electrons.com/ident?i%3D__GFP_WAIT&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=l5Ago9ekmVFZ3c4M6eauqrJWGwjf6fTb%2BP3CxbBFkVM%3D%0A&m=QQSN6uVpEiw6RuWLAfK%2FKWBFV5HspJUfDh4Y2mUz%2FH4%3D%0A&s=d56d076770d3416264be6c9ea2829ac0d6951203696fa3ad04144f13307577bc> > >>>>>needed > >>>>>for those single page allocations that go through__ttm_dma_alloc_page > >>>>><https://urldefense.proofpoint.com/v1/url?u=http://lxr.free-electrons.com/ident?i%3D__ttm_dma_alloc_page&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=l5Ago9ekmVFZ3c4M6eauqrJWGwjf6fTb%2BP3CxbBFkVM%3D%0A&m=QQSN6uVpEiw6RuWLAfK%2FKWBFV5HspJUfDh4Y2mUz%2FH4%3D%0A&s=7898522bba274e4dcc332735fbcf0c96e48918f60c2ee8e9a3e9c73ab3487bd0>? > >>>>> > >>>>> > >>>>>It would be nice to have such a simple, non-intrusive one-line patch > >>>>>that we still could get into 3.17 and then backported to older stable > >>>>>kernels to avoid the same desktop hangs there if CMA is enabled. It > >>>>>would be also nice for actual users of CMA to not use up lots of CMA > >>>>>space for gpu's which don't need it. I think DMA_CMA was introduced > >>>>>around 3.12. > >>>>> > >>>>I don't think that's a good idea. Omitting __GFP_WAIT would cause > >>>>unnecessary memory allocation errors on systems under stress. > >>>>I think this should be filed as a DMA subsystem kernel bug / regression > >>>>and an appropriate solution should be worked out together with the DMA > >>>>subsystem maintainers and then backported. > >>>Ok, so it is needed. I'll file a bug report. > >>> > >>>>>The other problem is that probably TTM does not reuse pages from the > >>>>>DMA pool. If i trace the __ttm_dma_alloc_page > >>>>><https://urldefense.proofpoint.com/v1/url?u=http://lxr.free-electrons.com/ident?i%3D__ttm_dma_alloc_page&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=l5Ago9ekmVFZ3c4M6eauqrJWGwjf6fTb%2BP3CxbBFkVM%3D%0A&m=QQSN6uVpEiw6RuWLAfK%2FKWBFV5HspJUfDh4Y2mUz%2FH4%3D%0A&s=7898522bba274e4dcc332735fbcf0c96e48918f60c2ee8e9a3e9c73ab3487bd0> > >>>>>and > >>>>>__ttm_dma_free_page > >>>>><https://urldefense.proofpoint.com/v1/url?u=http://lxr.free-electrons.com/ident?i%3D__ttm_dma_alloc_page&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=l5Ago9ekmVFZ3c4M6eauqrJWGwjf6fTb%2BP3CxbBFkVM%3D%0A&m=QQSN6uVpEiw6RuWLAfK%2FKWBFV5HspJUfDh4Y2mUz%2FH4%3D%0A&s=7898522bba274e4dcc332735fbcf0c96e48918f60c2ee8e9a3e9c73ab3487bd0> > >>>>>calls for > >>>>>those single page allocs/frees, then over a 20 second interval of > >>>>>tracing and switching tabs in firefox, scrolling things around etc. i > >>>>>find about as many alloc's as i find free's, e.g., 1607 allocs vs. > >>>>>1648 frees. > >>>>This is because historically the pools have been designed to keep only > >>>>pages with nonstandard caching attributes since changing page caching > >>>>attributes have been very slow but the kernel page allocators have been > >>>>reasonably fast. > >>>> > >>>>/Thomas > >>>Ok. A bit more ftraceing showed my hang problem case goes through the > >>>"if (is_cached)" paths, so the pool doesn't recycle anything and i see > >>>it bouncing up and down by 4 pages all the time. > >>> > >>>But for the non-cached case, which i don't hit with my problem, could > >>>one of you look at line 954... > >>> > >>>https://urldefense.proofpoint.com/v1/url?u=http://lxr.free-electrons.com/source/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c%23L954&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=l5Ago9ekmVFZ3c4M6eauqrJWGwjf6fTb%2BP3CxbBFkVM%3D%0A&m=QQSN6uVpEiw6RuWLAfK%2FKWBFV5HspJUfDh4Y2mUz%2FH4%3D%0A&s=e15c51805d429ee6d8960d6b88035e9811a1cdbfbf13168eec2fbb2214b99c60 > >>> > >>> > >>>... and tell me why that unconditional npages = count; assignment > >>>makes sense? It seems to essentially disable all recycling for the dma > >>>pool whenever the pool isn't filled up to/beyond its maximum with free > >>>pages? When the pool is filled up, lots of stuff is recycled, but when > >>>it is already somewhat below capacity, it gets "punished" by not > >>>getting refilled? I'd just like to understand the logic behind that line. > >>> > >>>thanks, > >>>-mario > >>I'll happily forward that question to Konrad who wrote the code (or it > >>may even stem from the ordinary page pool code which IIRC has Dave > >>Airlie / Jerome Glisse as authors) > >This is effectively bogus code, i now wonder how it came to stay alive. > >Attached patch will fix that. > > Yes, that makes sense to me. Fwiw, > > Reviewed-by: Mario Kleiner <mario.kleiner.de@...il.com> What about testing? Did it make the issue less of a problem or did it disappear completely? Thank you. > > -mario > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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