Re: [PATCH 2/2, v2] mm/migration: Make rmap_walk_anon() andtry_to_unmap_anon() more scalable

[Michel Lespinasse]

On Sun, Dec 2, 2012 at 7:12 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> Subject: [PATCH] mm/rmap, migration: Make rmap_walk_anon() and
>  try_to_unmap_anon() more scalable
>
> rmap_walk_anon() and try_to_unmap_anon() appears to be too
> careful about locking the anon vma: while it needs protection
> against anon vma list modifications, it does not need exclusive
> access to the list itself.
>
> Transforming this exclusive lock to a read-locked rwsem removes
> a global lock from the hot path of page-migration intense
> threaded workloads which can cause pathological performance like
> this:
>
>     96.43%        process 0  [kernel.kallsyms]  [k] perf_trace_sched_switch
>                   |
>                   --- perf_trace_sched_switch
>                       __schedule
>                       schedule
>                       schedule_preempt_disabled
>                       __mutex_lock_common.isra.6
>                       __mutex_lock_slowpath
>                       mutex_lock
>                      |
>                      |--50.61%-- rmap_walk
>                      |          move_to_new_page
>                      |          migrate_pages
>                      |          migrate_misplaced_page
>                      |          __do_numa_page.isra.69
>                      |          handle_pte_fault
>                      |          handle_mm_fault
>                      |          __do_page_fault
>                      |          do_page_fault
>                      |          page_fault
>                      |          __memset_sse2
>                      |          |
>                      |           --100.00%-- worker_thread
>                      |                     |
>                      |                      --100.00%-- start_thread
>                      |
>                       --49.39%-- page_lock_anon_vma
>                                 try_to_unmap_anon
>                                 try_to_unmap
>                                 migrate_pages
>                                 migrate_misplaced_page
>                                 __do_numa_page.isra.69
>                                 handle_pte_fault
>                                 handle_mm_fault
>                                 __do_page_fault
>                                 do_page_fault
>                                 page_fault
>                                 __memset_sse2
>                                 |
>                                  --100.00%-- worker_thread
>                                            start_thread
>
> With this change applied the profile is now nicely flat
> and there's no anon-vma related scheduling/blocking.

Wouldn't the same reasoning apply to i_mmap_mutex ? Should we make
that a rwsem as well ? I take it that Ingo's test case does not show
this, but i_mmap_mutex's role with file pages is actually quite
similar to the anon_vma lock with anon pages...

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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