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

[Ryan Roberts]

On 27/03/2024 11:06, Itaru Kitayama wrote:
> On Tue, Mar 26, 2024 at 10:14:45AM +0000, 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%)
>>
>> 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.
> 
> The series was applied cleanly on top of v6.9-rc1+ of Linus's master
> branch, and boots fine on M1 VM with 14GB of memory.
> 
> Just out of curiosity, how did you measure the boot time and obtain the
> breakdown of the execution times of each phase?

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 495b732d5af3..8a9d47115784 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -792,7 +792,14 @@ static void __init create_idmap(void)

 void __init paging_init(void)
 {
+       u64 start, end;
+
+       start = __arch_counter_get_cntvct();
        map_mem(swapper_pg_dir);
+       end = __arch_counter_get_cntvct();
+
+       pr_err("map_mem: time=%llu us\n",
+               ((end - start) * 1000000) / arch_timer_get_cntfrq());

        memblock_allow_resize();

> 
> Tested-by: Itaru Kitayama <itaru.kitayama at fujitsu.com>

Thanks!

> 
> Thanks,
> Itaru.
> 
>>
>> 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
>>