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    SubjectRe: [PATCH 17/17] mm: filemap: Avoid unnecessary barries and waitqueue lookup in unlock_page fastpath
    On Mon, May 05, 2014 at 12:50:54PM +0200, Jan Kara wrote:
    > On Thu 01-05-14 09:44:48, Mel Gorman wrote:
    > > From: Nick Piggin <npiggin@suse.de>
    > >
    > > This patch introduces a new page flag for 64-bit capable machines,
    > > PG_waiters, to signal there are processes waiting on PG_lock and uses it to
    > > avoid memory barriers and waitqueue hash lookup in the unlock_page fastpath.
    > >
    > > This adds a few branches to the fast path but avoids bouncing a dirty
    > > cache line between CPUs. 32-bit machines always take the slow path but the
    > > primary motivation for this patch is large machines so I do not think that
    > > is a concern.
    > ...
    > > /*
    > > diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
    > > index 7d50f79..fb83fe0 100644
    > > --- a/kernel/sched/wait.c
    > > +++ b/kernel/sched/wait.c
    > > @@ -304,8 +304,7 @@ int wake_bit_function(wait_queue_t *wait, unsigned mode, int sync, void *arg)
    > > = container_of(wait, struct wait_bit_queue, wait);
    > >
    > > if (wait_bit->key.flags != key->flags ||
    > > - wait_bit->key.bit_nr != key->bit_nr ||
    > > - test_bit(key->bit_nr, key->flags))
    > > + wait_bit->key.bit_nr != key->bit_nr)
    > > return 0;
    > > else
    > > return autoremove_wake_function(wait, mode, sync, key);
    > This change seems to be really unrelated? And it would deserve a comment
    > on its own I'd think so maybe split that in a separate patch?
    >

    Without it processes can sleep forever on the lock bit and hang due to
    races between when the PG_waiters is set and cleared. I'll investigate
    if this can be done a better way.

    > > diff --git a/mm/filemap.c b/mm/filemap.c
    > > index c60ed0f..93e4385 100644
    > > --- a/mm/filemap.c
    > > +++ b/mm/filemap.c
    > > +int __wait_on_page_locked_killable(struct page *page)
    > > +{
    > > + int ret = 0;
    > > + wait_queue_head_t *wq = page_waitqueue(page);
    > > + DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);
    > > +
    > > + if (!test_bit(PG_locked, &page->flags))
    > > + return 0;
    > > + do {
    > > + prepare_to_wait(wq, &wait.wait, TASK_KILLABLE);
    > > + if (!PageWaiters(page))
    > > + SetPageWaiters(page);
    > > + if (likely(PageLocked(page)))
    > > + ret = sleep_on_page_killable(page);
    > > + finish_wait(wq, &wait.wait);
    > > + } while (PageLocked(page) && !ret);
    > So I'm somewhat wondering why this is the only page waiting variant that
    > does finish_wait() inside the loop. Everyone else does it outside the while
    > loop which seems sufficient to me even in this case...
    >

    No reason. The finish_wait can always be outside. I'll fix it up.
    Thanks.

    --
    Mel Gorman
    SUSE Labs


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