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

[Ard Biesheuvel]

Hi Ryan,

On Tue, 26 Mar 2024 at 12:15, Ryan Roberts <ryan.roberts@xxxxxxx> 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%)
>
> 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.
>

These are very nice results!

Before digging into the details: do we still have a strong case for
supporting contiguous PTEs and PMDs in these routines?