Date:   Mon, 19 Sep 2022 13:59:39 +0200
From:   Michael Nazzareno Trimarchi <michael@...rulasolutions.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Mike Rapoport <rppt@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Correlation CMA size and FORCE_MAX_ZONEORDER

Hi David


On Mon, Sep 19, 2022 at 1:28 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 19.09.22 13:17, Michael Nazzareno Trimarchi wrote:
> > Hi
> >
> > On Mon, Sep 19, 2022 at 1:03 PM David Hildenbrand <david@...hat.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@...hat.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@...hat.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 ?

Wait we have:
- CMA kconfig alignment that in most config we have not considered
natural dma alignment but is put to 1Mb in a lot of embedded
- We have CMA_SIZE, CMA_SIZE_PERCENTAGE etc. Those seems that are not
effect if ZONEORDER is not reasonable and without
HUGETLB_PAGE
- etc

Changing MAX_ZONEORDER is ok and yes if you have an IOT device that
you know about your CMA allocation, it makes no sense to have
it 32MB for a 128Mb device. What I point out is that I need to figure
it out because in Kconfig there is no mention of it. Should it be
added there?

Michael

>
> --
> Thanks,
>
> David / dhildenb
>

--
Michael Nazzareno Trimarchi
Co-Founder & Chief Executive Officer
M. +39 347 913 2170
michael@...rulasolutions.com
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