On Sat, Dec 01, 2012 at 09:15:38PM +0100, Ingo Molnar wrote:
>
> Note, with this optimization I went a farther than the
> boundaries of the migration code - it seemed worthwile to do and
> I've reviewed all the other users of page_lock_anon_vma() as
> well and none seemed to be modifying the list inside that lock.
>
> Please review this patch carefully - in particular the SMP races
> outlined in anon_vma_free() are exciting: I have updated the
> reasoning and it still appears to hold, but please double check
> the changes nevertheless ...
>
> Thanks,
>
> Ingo
>
> ------------------->
> From: Ingo Molnar <mi...@kernel.org>
> Date: Sat Dec 1 20:43:04 CET 2012
>
> 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.
>
> Cc: Linus Torvalds <torva...@linux-foundation.org>
> Cc: Andrew Morton <a...@linux-foundation.org>
> Cc: Peter Zijlstra <a.p.zijls...@chello.nl>
> Cc: Andrea Arcangeli <aarca...@redhat.com>
> Cc: Rik van Riel <r...@redhat.com>
> Cc: Mel Gorman <mgor...@suse.de>
> Cc: Hugh Dickins <hu...@google.com>
> Signed-off-by: Ingo Molnar <mi...@kernel.org>
> ---
> include/linux/rmap.h | 15 +++++++++++++--
> mm/huge_memory.c | 4 ++--
> mm/memory-failure.c | 4 ++--
> mm/migrate.c | 2 +-
> mm/rmap.c | 40 ++++++++++++++++++++--------------------
> 5 files changed, 38 insertions(+), 27 deletions(-)
>
> Index: linux/include/linux/rmap.h
> ===================================================================
> --- linux.orig/include/linux/rmap.h
> +++ linux/include/linux/rmap.h
> @@ -128,6 +128,17 @@ static inline void anon_vma_unlock(struc
> up_write(&anon_vma->root->rwsem);
> }
>
> +static inline void anon_vma_lock_read(struct anon_vma *anon_vma)
> +{
> + down_read(&anon_vma->root->rwsem);
> +}
> +
> +static inline void anon_vma_unlock_read(struct anon_vma *anon_vma)
> +{
> + up_read(&anon_vma->root->rwsem);
> +}
> +
> +
> /*
> * anon_vma helper functions.
> */
> @@ -220,8 +231,8 @@ int try_to_munlock(struct page *);
> /*
> * Called by memory-failure.c to kill processes.
> */
> -struct anon_vma *page_lock_anon_vma(struct page *page);
> -void page_unlock_anon_vma(struct anon_vma *anon_vma);
> +struct anon_vma *page_lock_anon_vma_read(struct page *page);
> +void page_unlock_anon_vma_read(struct anon_vma *anon_vma);
> int page_mapped_in_vma(struct page *page, struct vm_area_struct *vma);
>
> /*
> Index: linux/mm/huge_memory.c
> ===================================================================
> --- linux.orig/mm/huge_memory.c
> +++ linux/mm/huge_memory.c
> @@ -1645,7 +1645,7 @@ int split_huge_page(struct page *page)
> int ret = 1;
>
> BUG_ON(!PageAnon(page));
> - anon_vma = page_lock_anon_vma(page);
> + anon_vma = page_lock_anon_vma_read(page);
> if (!anon_vma)
> goto out;
> ret = 0;
> @@ -1658,7 +1658,7 @@ int split_huge_page(struct page *page)
>
> BUG_ON(PageCompound(page));
> out_unlock:
> - page_unlock_anon_vma(anon_vma);
> + page_unlock_anon_vma_read(anon_vma);
> out:
> return ret;
> }
> Index: linux/mm/memory-failure.c
> ===================================================================
> --- linux.orig/mm/memory-failure.c
> +++ linux/mm/memory-failure.c
> @@ -402,7 +402,7 @@ static void collect_procs_anon(struct pa
> struct anon_vma *av;
> pgoff_t pgoff;
>
> - av = page_lock_anon_vma(page);
> + av = page_lock_anon_vma_read(page);
> if (av == NULL) /* Not actually mapped anymore */
> return;
>
Probably no real benefit on this one. It takes the tasklist_lock just
after it which is a much heavier lock anyway. I don't think there is
anything wrong with this though.
> @@ -423,7 +423,7 @@ static void collect_procs_anon(struct pa
> }
> }
> read_unlock(&tasklist_lock);
> - page_unlock_anon_vma(av);
> + page_unlock_anon_vma_read(av);
> }
>
> /*
> Index: linux/mm/migrate.c
> ===================================================================
> --- linux.orig/mm/migrate.c
> +++ linux/mm/migrate.c
> @@ -751,7 +751,7 @@ static int __unmap_and_move(struct page
> */
> if (PageAnon(page)) {
> /*
> - * Only page_lock_anon_vma() understands the subtleties of
> + * Only page_lock_anon_vma_read() understands the subtleties of
> * getting a hold on an anon_vma from outside one of its mms.
> */
> anon_vma = page_get_anon_vma(page);
> Index: linux/mm/rmap.c
> ===================================================================
> --- linux.orig/mm/rmap.c
> +++ linux/mm/rmap.c
> @@ -87,18 +87,18 @@ static inline void anon_vma_free(struct
> VM_BUG_ON(atomic_read(&anon_vma->refcount));
>
> /*
> - * Synchronize against page_lock_anon_vma() such that
> + * Synchronize against page_lock_anon_vma_read() such that
> * we can safely hold the lock without the anon_vma getting
> * freed.
> *
> * Relies on the full mb implied by the atomic_dec_and_test() from
> * put_anon_vma() against the acquire barrier implied by
> - * mutex_trylock() from page_lock_anon_vma(). This orders:
> + * down_read_trylock() from page_lock_anon_vma_read(). This orders:
> *
> - * page_lock_anon_vma() VS put_anon_vma()
> - * mutex_trylock() atomic_dec_and_test()
> + * page_lock_anon_vma_read() VS put_anon_vma()
> + * down_read_trylock() atomic_dec_and_test()
> * LOCK MB
> - * atomic_read() mutex_is_locked()
> + * atomic_read() rwsem_is_locked()
> *
> * LOCK should suffice since the actual taking of the lock must
> * happen _before_ what follows.
> @@ -146,7 +146,7 @@ static void anon_vma_chain_link(struct v
> * allocate a new one.
> *
> * Anon-vma allocations are very subtle, because we may have
> - * optimistically looked up an anon_vma in page_lock_anon_vma()
> + * optimistically looked up an anon_vma in page_lock_anon_vma_read()
> * and that may actually touch the spinlock even in the newly
> * allocated vma (it depends on RCU to make sure that the
> * anon_vma isn't actually destroyed).
> @@ -442,7 +442,7 @@ out:
> * atomic op -- the trylock. If we fail the trylock, we fall back to getting
> a
> * reference like with page_get_anon_vma() and then block on the mutex.
> */
> -struct anon_vma *page_lock_anon_vma(struct page *page)
> +struct anon_vma *page_lock_anon_vma_read(struct page *page)
> {
> struct anon_vma *anon_vma = NULL;
> struct anon_vma *root_anon_vma;
> @@ -457,14 +457,14 @@ struct anon_vma *page_lock_anon_vma(stru
>
> anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
> root_anon_vma = ACCESS_ONCE(anon_vma->root);
> - if (down_write_trylock(&root_anon_vma->rwsem)) {
> + if (down_read_trylock(&root_anon_vma->rwsem)) {
> /*
> * If the page is still mapped, then this anon_vma is still
> * its anon_vma, and holding the mutex ensures that it will
> * not go away, see anon_vma_free().
> */
> if (!page_mapped(page)) {
> - up_write(&root_anon_vma->rwsem);
> + up_read(&root_anon_vma->rwsem);
> anon_vma = NULL;
> }
> goto out;
> @@ -484,7 +484,7 @@ struct anon_vma *page_lock_anon_vma(stru
>
> /* we pinned the anon_vma, its safe to sleep */
> rcu_read_unlock();
> - anon_vma_lock(anon_vma);
> + anon_vma_lock_read(anon_vma);
>
> if (atomic_dec_and_test(&anon_vma->refcount)) {
> /*
> @@ -492,7 +492,7 @@ struct anon_vma *page_lock_anon_vma(stru
> * and bail -- can't simply use put_anon_vma() because
> * we'll deadlock on the anon_vma_lock() recursion.
> */
> - anon_vma_unlock(anon_vma);
> + anon_vma_unlock_read(anon_vma);
> __put_anon_vma(anon_vma);
> anon_vma = NULL;
> }
> @@ -504,9 +504,9 @@ out:
> return anon_vma;
> }
>
> -void page_unlock_anon_vma(struct anon_vma *anon_vma)
> +void page_unlock_anon_vma_read(struct anon_vma *anon_vma)
> {
> - anon_vma_unlock(anon_vma);
> + anon_vma_unlock_read(anon_vma);
> }
>
> /*
> @@ -732,7 +732,7 @@ static int page_referenced_anon(struct p
> struct anon_vma_chain *avc;
> int referenced = 0;
>
> - anon_vma = page_lock_anon_vma(page);
> + anon_vma = page_lock_anon_vma_read(page);
> if (!anon_vma)
> return referenced;
>
This is a slightly trickier one as this path is called from reclaim. It does
open the possibility that reclaim can stall something like a parallel fork
or anything that requires the anon_vma rwsem for a period of time. I very
severely doubt it'll really be a problem but keep an eye out for bug reports
related to delayed mmap/fork/anything_needing_write_lock during page reclaim.
> @@ -754,7 +754,7 @@ static int page_referenced_anon(struct p
> break;
> }
>
> - page_unlock_anon_vma(anon_vma);
> + page_unlock_anon_vma_read(anon_vma);
> return referenced;
> }
>
> @@ -1474,7 +1474,7 @@ static int try_to_unmap_anon(struct page
> struct anon_vma_chain *avc;
> int ret = SWAP_AGAIN;
>
> - anon_vma = page_lock_anon_vma(page);
> + anon_vma = page_lock_anon_vma_read(page);
> if (!anon_vma)
> return ret;
>
> @@ -1501,7 +1501,7 @@ static int try_to_unmap_anon(struct page
> break;
> }
>
> - page_unlock_anon_vma(anon_vma);
> + page_unlock_anon_vma_read(anon_vma);
> return ret;
> }
>
> @@ -1696,7 +1696,7 @@ static int rmap_walk_anon(struct page *p
> int ret = SWAP_AGAIN;
>
> /*
> - * Note: remove_migration_ptes() cannot use page_lock_anon_vma()
> + * Note: remove_migration_ptes() cannot use page_lock_anon_vma_read()
> * because that depends on page_mapped(); but not all its usages
> * are holding mmap_sem. Users without mmap_sem are required to
> * take a reference count to prevent the anon_vma disappearing
> @@ -1704,7 +1704,7 @@ static int rmap_walk_anon(struct page *p
> anon_vma = page_anon_vma(page);
> if (!anon_vma)
> return ret;
> - anon_vma_lock(anon_vma);
> + anon_vma_lock_read(anon_vma);
> anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root, pgoff, pgoff) {
> struct vm_area_struct *vma = avc->vma;
> unsigned long address = vma_address(page, vma);
> @@ -1712,7 +1712,7 @@ static int rmap_walk_anon(struct page *p
> if (ret != SWAP_AGAIN)
> break;
> }
> - anon_vma_unlock(anon_vma);
> + anon_vma_unlock_read(anon_vma);
> return ret;
> }
>
I can't think of any reason why this would not work. Good stuff!
Acked-by: Mel Gorman <mgor...@suse.de>
--
Mel Gorman
SUSE Labs
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