Re: [PATCH v1 0/3] Speed up boot with faster linear map creation

[Ryan Roberts <ryan.roberts@arm.com>]

On 26/03/2024 10:14, 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 a large portion of the
> time it takes to create the linear map is issuing TLBIs. This series reworks the
> kernel pgtable generation code to significantly reduce the number of TLBIs. 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-rc1):
> 
>                | Apple M2 VM | Ampere Altra| Ampere Altra| Ampere Altra
>                | VM, 16G     | VM, 64G     | VM, 256G    | Metal, 512G
> ---------------|-------------|-------------|-------------|-------------
>                |   ms    (%) |   ms    (%) |   ms    (%) |    ms    (%)
> ---------------|-------------|-------------|-------------|-------------
> base           |  151   (0%) | 2191   (0%) | 8990   (0%) | 17443   (0%)
> no-cont-remap  |   77 (-49%) |  429 (-80%) | 1753 (-80%) |  3796 (-78%)
> no-alloc-remap |   77 (-49%) |  375 (-83%) | 1532 (-83%) |  3366 (-81%)
> lazy-unmap     |   63 (-58%) |  330 (-85%) | 1312 (-85%) |  2929 (-83%)

I've just appended an additional patch to this series. This takes us to a ~95%
reduction overall:

               | Apple M2 VM | Ampere Altra| Ampere Altra| Ampere Altra
               | VM, 16G     | VM, 64G     | VM, 256G    | Metal, 512G
---------------|-------------|-------------|-------------|-------------
               |   ms    (%) |   ms    (%) |   ms    (%) |    ms    (%)
---------------|-------------|-------------|-------------|-------------
base           |  151   (0%) | 2191   (0%) | 8990   (0%) | 17443   (0%)
no-cont-remap  |   77 (-49%) |  429 (-80%) | 1753 (-80%) |  3796 (-78%)
no-alloc-remap |   77 (-49%) |  375 (-83%) | 1532 (-83%) |  3366 (-81%)
lazy-unmap     |   63 (-58%) |  330 (-85%) | 1312 (-85%) |  2929 (-83%)
batch-barriers |   11 (-93%) |   61 (-97%) |  261 (-97%) |   837 (-95%)

Don't believe the intermediate block-based pgtable idea will now be neccessary
so I don't intend to persue that. It might be that we choose to drop the middle
two patchs; I'm keen to hear opinions.

Thanks,
Ryan


> 
> This series applies on top of v6.9-rc1. All mm selftests pass. I haven't yet
> tested all VA size configs (although I don't anticipate any issues); I'll do
> this as part of followup.
> 
> Thanks,
> Ryan
> 
> 
> Ryan Roberts (3):
>   arm64: mm: Don't remap pgtables per- cont(pte|pmd) block
>   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 |   4 -
>  arch/arm64/kernel/cpufeature.c   |  10 +-
>  arch/arm64/mm/fixmap.c           |  11 +
>  arch/arm64/mm/mmu.c              | 364 +++++++++++++++++++++++--------
>  include/linux/pgtable.h          |   8 +
>  7 files changed, 307 insertions(+), 103 deletions(-)
> 
> --
> 2.25.1
>