On Wed, Dec 21, 2016 at 09:09:31AM +0100, Peter Zijlstra wrote:
> On Tue, Dec 20, 2016 at 10:02:46AM -0800, Linus Torvalds wrote:
> > On Tue, Dec 20, 2016 at 9:31 AM, Linus Torvalds
> > <torva...@linux-foundation.org> wrote:
> > >
> > > I'll go back and try to see why the page flag contention patch didn't
> > > get applied.
> >
> > Ahh, a combination of warring patches by Nick and PeterZ, and worry
> > about the page flag bits.
>
> I think Nick actually had a patch freeing up a pageflag, although Hugh
> had a comment on that.
>
> That said, I'm not a huge fan of his waiters patch, I'm still not sure
> why he wants to write another whole wait loop, but whatever. Whichever
> you prefer I suppose.
FWIW, here's mine.. compiles and boots on a NUMA x86_64 machine.
---
Subject: mm: Avoid slow path for unlock_page()
From: Peter Zijlstra <pet...@infradead.org>
Date: Thu, 27 Oct 2016 10:08:52 +0200
Currently we uncontidionally look up the page waitqueue and issue the
wakeup for unlock_page(), even though there might not be anybody
waiting.
Use another pageflag (PG_waiters) to keep track of the waitqueue state
such that we can avoid the work when there are in fact no waiters
queued.
Currently guarded by CONFIG_NUMA, which is not strictly correct as
I've been told there actually are 32bit architectures that have NUMA.
Ideally Nick manages to reclaim a pageflag and we can make this
unconditional.
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
include/linux/page-flags.h | 19 ++++++++
include/linux/pagemap.h | 25 ++++++++--
include/trace/events/mmflags.h | 7 +++
mm/filemap.c | 94 +++++++++++++++++++++++++++++++++++++----
4 files changed, 130 insertions(+), 15 deletions(-)
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -73,6 +73,14 @@
*/
enum pageflags {
PG_locked, /* Page is locked. Don't touch. */
+#ifdef CONFIG_NUMA
+ /*
+ * This bit must end up in the same word as PG_locked (or any other bit
+ * we're waiting on), as per all architectures their bitop
+ * implementations.
+ */
+ PG_waiters, /* The hashed waitqueue has waiters */
+#endif
PG_error,
PG_referenced,
PG_uptodate,
@@ -231,6 +239,9 @@ static __always_inline int TestClearPage
#define TESTPAGEFLAG_FALSE(uname) \
static inline int Page##uname(const struct page *page) { return 0; }
+#define TESTPAGEFLAG_TRUE(uname) \
+static inline int Page##uname(const struct page *page) { return 1; }
+
#define SETPAGEFLAG_NOOP(uname)
\
static inline void SetPage##uname(struct page *page) { }
@@ -249,10 +260,18 @@ static inline int TestClearPage##uname(s
#define PAGEFLAG_FALSE(uname) TESTPAGEFLAG_FALSE(uname)
\
SETPAGEFLAG_NOOP(uname) CLEARPAGEFLAG_NOOP(uname)
+#define PAGEFLAG_TRUE(uname) TESTPAGEFLAG_TRUE(uname) \
+ SETPAGEFLAG_NOOP(uname) CLEARPAGEFLAG_NOOP(uname)
+
#define TESTSCFLAG_FALSE(uname)
\
TESTSETFLAG_FALSE(uname) TESTCLEARFLAG_FALSE(uname)
__PAGEFLAG(Locked, locked, PF_NO_TAIL)
+#ifdef CONFIG_NUMA
+PAGEFLAG(Waiters, waiters, PF_NO_TAIL)
+#else
+PAGEFLAG_TRUE(Waiters);
+#endif
PAGEFLAG(Error, error, PF_NO_COMPOUND) TESTCLEARFLAG(Error, error,
PF_NO_COMPOUND)
PAGEFLAG(Referenced, referenced, PF_HEAD)
TESTCLEARFLAG(Referenced, referenced, PF_HEAD)
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -436,7 +436,7 @@ extern void __lock_page(struct page *pag
extern int __lock_page_killable(struct page *page);
extern int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
unsigned int flags);
-extern void unlock_page(struct page *page);
+extern void __unlock_page(struct page *page);
static inline int trylock_page(struct page *page)
{
@@ -467,6 +467,20 @@ static inline int lock_page_killable(str
return 0;
}
+static inline void unlock_page(struct page *page)
+{
+ page = compound_head(page);
+ VM_BUG_ON_PAGE(!PageLocked(page), page);
+ clear_bit_unlock(PG_locked, &page->flags);
+ /*
+ * Since PG_locked and PG_waiters are in the same word, Program-Order
+ * ensures the load of PG_waiters must not observe a value earlier
+ * than our clear_bit() store.
+ */
+ if (PageWaiters(page))
+ __unlock_page(page);
+}
+
/*
* lock_page_or_retry - Lock the page, unless this would block and the
* caller indicated that it can handle a retry.
@@ -491,11 +505,11 @@ extern int wait_on_page_bit_killable(str
extern int wait_on_page_bit_killable_timeout(struct page *page,
int bit_nr, unsigned long timeout);
+extern int wait_on_page_lock(struct page *page, int mode);
+
static inline int wait_on_page_locked_killable(struct page *page)
{
- if (!PageLocked(page))
- return 0;
- return wait_on_page_bit_killable(compound_head(page), PG_locked);
+ return wait_on_page_lock(page, TASK_KILLABLE);
}
extern wait_queue_head_t *page_waitqueue(struct page *page);
@@ -513,8 +527,7 @@ static inline void wake_up_page(struct p
*/
static inline void wait_on_page_locked(struct page *page)
{
- if (PageLocked(page))
- wait_on_page_bit(compound_head(page), PG_locked);
+ wait_on_page_lock(page, TASK_UNINTERRUPTIBLE);
}
/*
--- a/include/trace/events/mmflags.h
+++ b/include/trace/events/mmflags.h
@@ -55,6 +55,12 @@
__def_gfpflag_names \
) : "none"
+#ifdef CONFIG_NUMA
+#define IF_HAVE_PG_WAITERS(flag,string) {1UL << flag, string},
+#else
+#define IF_HAVE_PG_WAITERS(flag,string)
+#endif
+
#ifdef CONFIG_MMU
#define IF_HAVE_PG_MLOCK(flag,string) ,{1UL << flag, string}
#else
@@ -81,6 +87,7 @@
#define __def_pageflag_names \
{1UL << PG_locked, "locked" }, \
+IF_HAVE_PG_WAITERS(PG_waiters, "waiters" ) \
{1UL << PG_error, "error" }, \
{1UL << PG_referenced, "referenced" }, \
{1UL << PG_uptodate, "uptodate" }, \
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -809,15 +809,30 @@ EXPORT_SYMBOL_GPL(add_page_wait_queue);
* The mb is necessary to enforce ordering between the clear_bit and the read
* of the waitqueue (to avoid SMP races with a parallel wait_on_page_locked()).
*/
-void unlock_page(struct page *page)
+void __unlock_page(struct page *page)
{
- page = compound_head(page);
- VM_BUG_ON_PAGE(!PageLocked(page), page);
- clear_bit_unlock(PG_locked, &page->flags);
- smp_mb__after_atomic();
- wake_up_page(page, PG_locked);
+ wait_queue_head_t *wq = page_waitqueue(page);
+ unsigned long flags;
+
+ spin_lock_irqsave(&wq->lock, flags);
+ if (waitqueue_active(wq)) {
+ struct wait_bit_key key =
+ __WAIT_BIT_KEY_INITIALIZER(&page->flags, PG_locked);
+
+ __wake_up_locked_key(wq, TASK_NORMAL, &key);
+ } else {
+ /*
+ * We need to do ClearPageWaiters() under wq->lock such that
+ * we serialize against prepare_to_wait() adding waiters and
+ * setting task_struct::state.
+ *
+ * See lock_page_wait().
+ */
+ ClearPageWaiters(page);
+ }
+ spin_unlock_irqrestore(&wq->lock, flags);
}
-EXPORT_SYMBOL(unlock_page);
+EXPORT_SYMBOL(__unlock_page);
/**
* end_page_writeback - end writeback against a page
@@ -870,6 +885,55 @@ void page_endio(struct page *page, bool
}
EXPORT_SYMBOL_GPL(page_endio);
+static int lock_page_wait(struct wait_bit_key *word, int mode)
+{
+ struct page *page = container_of(word->flags, struct page, flags);
+
+ /*
+ * We cannot go sleep without having PG_waiters set. This would mean
+ * nobody would issue a wakeup and we'd be stuck.
+ */
+ if (!PageWaiters(page)) {
+
+ /*
+ * There are two orderings of importance:
+ *
+ * 1)
+ *
+ * [unlock] [wait]
+ *
+ * clear PG_locked set PG_waiters
+ * test PG_waiters test (and-set) PG_locked
+ *
+ * Since these are in the same word, and the clear/set
+ * operation are atomic, they are ordered against one another.
+ * Program-Order further constraints a CPU from speculating the
+ * later load to not be earlier than the RmW. So this doesn't
+ * need an explicit barrier. Also see unlock_page().
+ *
+ * 2)
+ *
+ * [unlock] [wait]
+ *
+ * LOCK wq->lock LOCK wq->lock
+ * __wake_up_locked || list-add
+ * clear PG_waiters set_current_state()
+ * UNLOCK wq->lock UNLOCK wq->lock
+ * set PG_waiters
+ *
+ * Since we're added to the waitqueue, we cannot get
+ * PG_waiters cleared without also getting TASK_RUNNING set,
+ * which will then void the schedule() call and we'll loop.
+ * Here wq->lock is sufficient ordering. See __unlock_page().
+ */
+ SetPageWaiters(page);
+
+ return 0;
+ }
+
+ return bit_wait_io(word, mode);
+}
+
/**
* __lock_page - get a lock on the page, assuming we need to sleep to get it
* @page: the page to lock
@@ -879,7 +943,7 @@ void __lock_page(struct page *page)
struct page *page_head = compound_head(page);
DEFINE_WAIT_BIT(wait, &page_head->flags, PG_locked);
- __wait_on_bit_lock(page_waitqueue(page_head), &wait, bit_wait_io,
+ __wait_on_bit_lock(page_waitqueue(page_head), &wait, lock_page_wait,
TASK_UNINTERRUPTIBLE);
}
EXPORT_SYMBOL(__lock_page);
@@ -890,10 +954,22 @@ int __lock_page_killable(struct page *pa
DEFINE_WAIT_BIT(wait, &page_head->flags, PG_locked);
return __wait_on_bit_lock(page_waitqueue(page_head), &wait,
- bit_wait_io, TASK_KILLABLE);
+ lock_page_wait, TASK_KILLABLE);
}
EXPORT_SYMBOL_GPL(__lock_page_killable);
+int wait_on_page_lock(struct page *page, int mode)
+{
+ struct page __always_unused *__page = (page = compound_head(page));
+ DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);
+
+ if (!PageLocked(page))
+ return 0;
+
+ return __wait_on_bit(page_waitqueue(page), &wait, lock_page_wait, mode);
+}
+EXPORT_SYMBOL(wait_on_page_lock);
+
/*
* Return values:
* 1 - page is locked; mmap_sem is still held.
