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Date: Tue, 9 Apr 2024 13:51:38 +0200
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Subject: Re: [PATCH v2 0/4] Speed up boot with faster linear map creation
From: David Hildenbrand <david@redhat.com>
To: Itaru Kitayama <itaru.kitayama@linux.dev>,
 Ryan Roberts <ryan.roberts@arm.com>
Cc: Catalin Marinas <Catalin.Marinas@arm.com>, Will Deacon <will@kernel.org>,
 Mark Rutland <Mark.Rutland@arm.com>, Ard Biesheuvel <ardb@kernel.org>,
 Donald Dutile <ddutile@redhat.com>, Eric Chanudet <echanude@redhat.com>,
 Linux ARM <linux-arm-kernel@lists.infradead.org>,
 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <20240404143308.2224141-1-ryan.roberts@arm.com>
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On 09.04.24 13:29, David Hildenbrand wrote:
> On 09.04.24 13:22, David Hildenbrand wrote:
>> On 09.04.24 12:13, Itaru Kitayama wrote:
>>>
>>>
>>>> On Apr 9, 2024, at 19:04, Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>
>>>> On 09/04/2024 01:10, Itaru Kitayama wrote:
>>>>> Hi Ryan,
>>>>>
>>>>>> On Apr 8, 2024, at 16:30, Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>>>
>>>>>> On 06/04/2024 11:31, Itaru Kitayama wrote:
>>>>>>> Hi Ryan,
>>>>>>>
>>>>>>> On Sat, Apr 06, 2024 at 09:32:34AM +0100, Ryan Roberts wrote:
>>>>>>>> Hi Itaru,
>>>>>>>>
>>>>>>>> On 05/04/2024 08:39, Itaru Kitayama wrote:
>>>>>>>>> On Thu, Apr 04, 2024 at 03:33:04PM +0100, Ryan Roberts wrote:
>>>>>>>>>> Hi All,
>>>>>>>>>>
>>>>>>>>>> It turns out that creating the linear map can take a significant proportion of
>>>>>>>>>> the total boot time, especially when rodata=full. And most of the time is spent
>>>>>>>>>> waiting on superfluous tlb invalidation and memory barriers. This series reworks
>>>>>>>>>> the kernel pgtable generation code to significantly reduce the number of those
>>>>>>>>>> TLBIs, ISBs and DSBs. See each patch for details.
>>>>>>>>>>
>>>>>>>>>> The below shows the execution time of map_mem() across a couple of different
>>>>>>>>>> systems with different RAM configurations. We measure after applying each patch
>>>>>>>>>> and show the improvement relative to base (v6.9-rc2):
>>>>>>>>>>
>>>>>>>>>>                   | Apple M2 VM | Ampere Altra| Ampere Altra| Ampere Altra
>>>>>>>>>>                   | VM, 16G     | VM, 64G     | VM, 256G    | Metal, 512G
>>>>>>>>>> ---------------|-------------|-------------|-------------|-------------
>>>>>>>>>>                   |   ms    (%) |   ms    (%) |   ms    (%) |    ms    (%)
>>>>>>>>>> ---------------|-------------|-------------|-------------|-------------
>>>>>>>>>> base           |  153   (0%) | 2227   (0%) | 8798   (0%) | 17442   (0%)
>>>>>>>>>> no-cont-remap  |   77 (-49%) |  431 (-81%) | 1727 (-80%) |  3796 (-78%)
>>>>>>>>>> batch-barriers |   13 (-92%) |  162 (-93%) |  655 (-93%) |  1656 (-91%)
>>>>>>>>>> no-alloc-remap |   11 (-93%) |  109 (-95%) |  449 (-95%) |  1257 (-93%)
>>>>>>>>>> lazy-unmap     |    6 (-96%) |   61 (-97%) |  257 (-97%) |   838 (-95%)
>>>>>>>>>>
>>>>>>>>>> This series applies on top of v6.9-rc2. All mm selftests pass. I've compile and
>>>>>>>>>> boot tested various PAGE_SIZE and VA size configs.
>>>>>>>>>>
>>>>>>>>>> ---
>>>>>>>>>>
>>>>>>>>>> Changes since v1 [1]
>>>>>>>>>> ====================
>>>>>>>>>>
>>>>>>>>>>      - Added Tested-by tags (thanks to Eric and Itaru)
>>>>>>>>>>      - Renamed ___set_pte() -> __set_pte_nosync() (per Ard)
>>>>>>>>>>      - Reordered patches (biggest impact & least controversial first)
>>>>>>>>>>      - Reordered alloc/map/unmap functions in mmu.c to aid reader
>>>>>>>>>>      - pte_clear() -> __pte_clear() in clear_fixmap_nosync()
>>>>>>>>>>      - Reverted generic p4d_index() which caused x86 build error. Replaced with
>>>>>>>>>>        unconditional p4d_index() define under arm64.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> [1] https://lore.kernel.org/linux-arm-kernel/20240326101448.3453626-1-ryan.roberts@arm.com/<https://lore.kernel.org/linux-arm-kernel/20240326101448.3453626-1-ryan.roberts@arm.com/>
>>>>>>>>>>
>>>>>>>>>> Thanks,
>>>>>>>>>> Ryan
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Ryan Roberts (4):
>>>>>>>>>>      arm64: mm: Don't remap pgtables per-cont(pte|pmd) block
>>>>>>>>>>      arm64: mm: Batch dsb and isb when populating pgtables
>>>>>>>>>>      arm64: mm: Don't remap pgtables for allocate vs populate
>>>>>>>>>>      arm64: mm: Lazily clear pte table mappings from fixmap
>>>>>>>>>>
>>>>>>>>>> arch/arm64/include/asm/fixmap.h  |   5 +-
>>>>>>>>>> arch/arm64/include/asm/mmu.h     |   8 +
>>>>>>>>>> arch/arm64/include/asm/pgtable.h |  13 +-
>>>>>>>>>> arch/arm64/kernel/cpufeature.c   |  10 +-
>>>>>>>>>> arch/arm64/mm/fixmap.c           |  11 +
>>>>>>>>>> arch/arm64/mm/mmu.c              | 377 +++++++++++++++++++++++--------
>>>>>>>>>> 6 files changed, 319 insertions(+), 105 deletions(-)
>>>>>>>>>>
>>>>>>>>>> --
>>>>>>>>>> 2.25.1
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> I've build and boot tested the v2 on FVP, base is taken from your
>>>>>>>>> linux-rr repo. Running run_vmtests.sh on v2 left some gup longterm not oks, would you take a look at it? The mm ksefltests used is from your linux-rr repo too.
>>>>>>>>
>>>>>>>> Thanks for taking a look at this.
>>>>>>>>
>>>>>>>> I can't reproduce your issue unfortunately; steps as follows on Apple M2 VM:
>>>>>>>>
>>>>>>>> Config: arm64 defconfig + the following:
>>>>>>>>
>>>>>>>> # Squashfs for snaps, xfs for large file folios.
>>>>>>>> ./scripts/config --enable CONFIG_SQUASHFS_LZ4
>>>>>>>> ./scripts/config --enable CONFIG_SQUASHFS_LZO
>>>>>>>> ./scripts/config --enable CONFIG_SQUASHFS_XZ
>>>>>>>> ./scripts/config --enable CONFIG_SQUASHFS_ZSTD
>>>>>>>> ./scripts/config --enable CONFIG_XFS_FS
>>>>>>>>
>>>>>>>> # For general mm debug.
>>>>>>>> ./scripts/config --enable CONFIG_DEBUG_VM
>>>>>>>> ./scripts/config --enable CONFIG_DEBUG_VM_MAPLE_TREE
>>>>>>>> ./scripts/config --enable CONFIG_DEBUG_VM_RB
>>>>>>>> ./scripts/config --enable CONFIG_DEBUG_VM_PGFLAGS
>>>>>>>> ./scripts/config --enable CONFIG_DEBUG_VM_PGTABLE
>>>>>>>> ./scripts/config --enable CONFIG_PAGE_TABLE_CHECK
>>>>>>>>
>>>>>>>> # For mm selftests.
>>>>>>>> ./scripts/config --enable CONFIG_USERFAULTFD
>>>>>>>> ./scripts/config --enable CONFIG_TEST_VMALLOC
>>>>>>>> ./scripts/config --enable CONFIG_GUP_TEST
>>>>>>>>
>>>>>>>> Running on VM with 12G memory, split across 2 (emulated) NUMA nodes (needed by
>>>>>>>> some mm selftests), with kernel command line to reserve hugetlbs and other
>>>>>>>> features required by some mm selftests:
>>>>>>>>
>>>>>>>> "
>>>>>>>> transparent_hugepage=madvise earlycon root=/dev/vda2 secretmem.enable
>>>>>>>> hugepagesz=1G hugepages=0:2,1:2 hugepagesz=32M hugepages=0:2,1:2
>>>>>>>> default_hugepagesz=2M hugepages=0:64,1:64 hugepagesz=64K hugepages=0:2,1:2
>>>>>>>> "
>>>>>>>>
>>>>>>>> Ubuntu userspace running off XFS rootfs. Build and run mm selftests from same
>>>>>>>> git tree.
>>>>>>>>
>>>>>>>>
>>>>>>>> Although I don't think any of this config should make a difference to gup_longterm.
>>>>>>>>
>>>>>>>> Looks like your errors are all "ftruncate() failed". I've seen this problem on
>>>>>>>> our CI system. There it is due to running the tests from NFS file system. What
>>>>>>>> filesystem are you using? Perhaps you are sharing into the FVP using 9p? That
>>>>>>>> might also be problematic.
>>>>>>>
>>>>>>> That was it. This time I booted up the kernel including your series on
>>>>>>> QEMU on my M1 and executed the gup_longterm program without the ftruncate
>>>>>>> failures. When testing your kernel on FVP, I was executing the script from the FVP's host filesystem using 9p.
>>>>>>
>>>>>> I'm not sure exactly what the root cause is. Perhaps there isn't enough space on
>>>>>> the disk? It might be worth enhancing the error log to provide the errno in
>>>>>> tools/testing/selftests/mm/gup_longterm.c.
>>>>>>
>>>>>
>>>>> Attached is the strace’d gup_longterm executiong log on your
>>>>> pgtable-boot-speedup-v2 kernel.
>>>>
>>>> Sorry are you saying that it only fails with the pgtable-boot-speedup-v2 patch
>>>> set applied? I thought we previously concluded that it was independent of that?
>>>> I was under the impression that it was filesystem related and not something that
>>>> I was planning to investigate.
>>>
>>> No, irrespective of the kernel, if using 9p on FVP the test program fails.
>>> It is indeed 9p filesystem related, as I switched to using NFS all the issues are gone.
>>
>> Did it never work on 9p? If so, we might have to SKIP that test.
>>
>> openat(AT_FDCWD, "gup_longterm.c_tmpfile_BLboOt", O_RDWR|O_CREAT|O_EXCL, 0600) = 3
>> unlinkat(AT_FDCWD, "gup_longterm.c_tmpfile_BLboOt", 0) = 0
>> fstatfs(3, 0xffffe505a840)              = -1 EOPNOTSUPP (Operation not supported)
>> ftruncate(3, 4096)                      = -1 ENOENT (No such file or directory)
> 
> Note: I'm wondering if the unlinkat here is the problem that makes
> ftruncate() with 9p result in weird errors (e.g., the hypervisor
> unlinked the file and cannot reopen it for the fstatfs/ftruncate. ...
> which gives us weird errors here).
> 
> Then, we should lookup the fs type in run_with_local_tmpfile() before
> the unlink() and simply skip the test if it is 9p.

The unlink with 9p most certainly was a known issue in the past:

https://gitlab.com/qemu-project/qemu/-/issues/103

Maybe it's still an issue with older hypervisors (QEMU?)? Or it was 
never completely resolved?

According to https://bugs.launchpad.net/qemu/+bug/1336794, QEMU v5.2.0 
should contain a fix that is supposed to work with never kernels.

-- 
Cheers,

David / dhildenb