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SubjectRe: [PATCH 2/2, v2] mm/migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable
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On Sun, Dec 2, 2012 at 7:12 AM, Ingo Molnar <mingo@kernel.org> 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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