Messages in this thread | | | Date | Mon, 19 Sep 2022 13:28:19 +0200 | From | David Hildenbrand <> | Subject | Re: Correlation CMA size and FORCE_MAX_ZONEORDER |
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On 19.09.22 13:17, Michael Nazzareno Trimarchi wrote: > Hi > > On Mon, Sep 19, 2022 at 1:03 PM David Hildenbrand <david@redhat.com> wrote: >> >> On 19.09.22 11:57, Michael Nazzareno Trimarchi wrote: >>> Hi >>> >>> On Mon, Sep 19, 2022 at 11:31 AM David Hildenbrand <david@redhat.com> wrote: >>>> >>>> On 19.09.22 11:17, Michael Nazzareno Trimarchi wrote: >>>>> Hi David >>>>> >>>>> On Mon, Sep 19, 2022 at 10:38 AM David Hildenbrand <david@redhat.com> wrote: >>>>>> >>>>>> On 15.09.22 23:36, Michael Nazzareno Trimarchi wrote: >>>>>>> Hi all >>>>>> >>>>>> Hi, >>>>>> >>>>>>> >>>>>>> Working on a small device with 128MB of memory and using imx_v6_v7 >>>>>>> defconfig I found that CMA_SIZE_MBYTES, CMA_SIZE_PERCENTAGE >>>>>>> are not respected. The calculation done does not allow the requested >>>>>>> size. I think that this should be somehow documented and described but >>>>>>> I did not >>>>>>> find the documentation. Does it work this way? >>>>>>> >>>>>>> With CMA_SIZE of 8MB I need to have FORCE_MAX_ZONEORDER=12 if I have >>>>>>> the default FORCE_MAX_ZONEORDER=14 the min size is 32Mb >>>>>> >>>>>> The underlying constraint is that CMA regions require a certain minimum >>>>>> alignment+size. They cannot be arbitrarily in size. >>>>>> >>>>>> CMA_MIN_ALIGNMENT_BYTES expresses that, and corresponds in upstream >>>>>> kernels to the size of a single pageblock. >>>>>> >>>>>> In previous kernels, it used to be the size of the largest buddy >>>>>> allocation granularity (derived from MAX_ORDER, derived from >>>>>> FORCE_MAX_ZONEORDER). >>>>>> >>>>>> On upstream kernels, the FORCE_MAX_ZONEORDER constraint should no longer >>>>>> apply. On most archs, the minimum alignment+size should be 2 MiB >>>>>> (x86-64, aarch64 with 4k base pages) -- the size of a single pageblock. >>>>>> >>>>>> So far the theory. Are you still running into this limitation on >>>>>> upstream kernels? >>>>>> >>>>> >>>>> I can run 6-rc2 on my board. I test again but according to it, if I >>>>> put 4M as CMA in cma=4M in boot >>>>> parameters, the result is 32Mb of CMA. Apart of that seems that >>>>> process lime tiny membench can not even start >>>>> to mblock memory >>>>> >>>> >>>> The CMA alignemnt change went into v5.19. If "cma=4M" still gives you > >>>> 4M, can you post /proc/meminfo and the early console output? >>>> >>> >>> cat /proc/cmdline >>> cma=4M mtdparts=gpmi-nand:4m(nandboot),1m(env),24m(kernel),1m(nanddtb),-(rootfs) >>> root=ubi0:root rw ubi.mtd=ro >>> otfs rootfstype=ubifs rootwait=1 >>> # cat /proc/meminfo >>> MemTotal: 109560 kB >>> MemFree: 56084 kB >>> MemAvailable: 56820 kB >>> Buffers: 0 kB >>> Cached: 39680 kB >>> SwapCached: 0 kB >>> Active: 44 kB >>> Inactive: 644 kB >>> Active(anon): 44 kB >>> Inactive(anon): 644 kB >>> Active(file): 0 kB >>> Inactive(file): 0 kB >>> Unevictable: 39596 kB >>> Mlocked: 0 kB >>> HighTotal: 0 kB >>> HighFree: 0 kB >>> LowTotal: 109560 kB >>> LowFree: 56084 kB >>> SwapTotal: 0 kB >>> SwapFree: 0 kB >>> Dirty: 0 kB >>> Writeback: 0 kB >>> AnonPages: 628 kB >>> Mapped: 1480 kB >>> Shmem: 84 kB >>> KReclaimable: 4268 kB >>> Slab: 8456 kB >>> SReclaimable: 4268 kB >>> SUnreclaim: 4188 kB >>> KernelStack: 392 kB >>> PageTables: 88 kB >>> NFS_Unstable: 0 kB >>> Bounce: 0 kB >>> WritebackTmp: 0 kB >>> CommitLimit: 54780 kB >>> Committed_AS: 1876 kB >>> VmallocTotal: 901120 kB >>> VmallocUsed: 2776 kB >>> VmallocChunk: 0 kB >>> Percpu: 72 kB >>> CmaTotal: 32768 kB >>> CmaFree: 32484 kB >>> # uname -a >>> Linux buildroot 6.0.0-rc5 #20 SMP Mon Sep 19 11:51:26 CEST 2022 armv7l GNU/Linux >>> # >>> >>> Then here https://pastebin.com/6MUB2VBM dmesg >>> >>> CONFIG_ARM_MODULE_PLTS=y >>> CONFIG_FORCE_MAX_ZONEORDER=14 >>> CONFIG_ALIGNMENT_TRAP=y >>> ... >>> CONFIG_CMA >>> CONFIG_CMA_AREAS=7 >>> ... >>> >>> CONFIG_CMA_SIZE_MBYTES=8 >>> CONFIG_CMA_SIZE_SEL_MBYTES=y >>> >> >> Thanks! >> >> I assume that in your setup, the pageblock size depends on MAX_ORDER >> and, therefore, FORCE_MAX_ZONEORDER. >> >> This should be the case especially if CONFIG_HUGETLB_PAGE is not defined >> (include/linux/pageblock-flags.h). >> >> In contrast to what I remember, the pageblock size does not seem to >> depend on the THP size (weird) as well. >> >> >> So, yes, that limitation is still in effect for some kernel configs. >> >> One could make the pageblock size configurable (similar to >> CONFIG_HUGETLB_PAGE_SIZE_VARIABLE) or simply default to a smaller >> pageblock size as default with CONFIG_CMA and !CONFIG_HUGETLB_PAGE. >> >> I imagine something reasonable might be to set the pageblock size to >> 2MiB without CONFIG_HUGETLB_PAGE but with CONFIG_CMA. >> > > I don't think making more configuration options makes things clear.
Yes, in an ideal case it should be automatic.
> When we enable some configuration > we can force down the configuration. You need to explain clearly how > you envision it. FORCE_MAX_ZONEORDER > for me is the largest allocation that you can get from a zone (ex CMA > one). Any request allocation that is align to the
FORCE_MAX_ZONEORDER is just a way to increase/decrease the maximum allocation size of the buddy in general.
> CMA align and can fit inside a region should be allowed > > What am I missing?
I think that the issue is that the CMA alignments nowadays depend on the pageblock size. And the pageblock size depends on *some* configurations on the maximum allocation size of the buddy.
Documenting the interaction between FORCE_MAX_ZONEORDER and CMA size alignment is not trivial.
For example, with CONFIG_HUGETLB_PAGE there might not be such an interaction. With CONFIG_HUGETLB_PAGE, there clearly is one.
Let me phrase it this way: is it good enough in you setup to get 32mb vs 8mb or do you want/need to reduce it without adjusting FORCE_MAX_ZONEORDER ?
-- Thanks,
David / dhildenb
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