Re: [PATCH 2/2] page table iterators

[Nick Piggin <nickpiggin@yahoo.com.au>]

Nick Piggin wrote:

> Haven't yet pulled out a pre-4-level kernel to see how 3-level compares
> I guess I'll do that now.
> 

Close.
Before 4level: 119.5us, after folded walkers: 132.8us

I think most of this is now coming from clear_page_range, rather
than the actual traversing of the page tables (because they should
be completely folded by now):

before:
   4089 total                                      0.0017
    753 kmap_atomic                                4.7358
    682 do_wp_page                                 0.6713
    680 do_page_fault                              0.4561
    261 zap_pte_range                              0.3625
    176 copy_page_range                            0.2133
    159 pte_alloc_one                              1.5743
    145 clear_page_tables                          0.4866

after:
   4307 total                                      0.0018
    676 kmap_atomic                                4.2516
    665 do_page_fault                              0.4472
    615 do_wp_page                                 0.6225
    550 clear_page_range                           0.9982
    262 zap_pte_range                              0.4870

I think the additional work done by clear_page_range (versus
clear_page_tables) justifies the extra cost, even for 3-level
architectures.