Re: [PATCH 2/2] mm: add PageWaiters bit to indicate waitqueue should be checked

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Nov 02, 2016 at 06:03:46PM +1100, Nicholas Piggin wrote:
> Add a new page flag, PageWaiters. This bit is always set when the
> page has waiters on page_waitqueue(page), within the same synchronization
> scope as waitqueue_active(page) (i.e., it is manipulated under waitqueue
> lock). It may be set in some cases where that condition is not true
> (e.g., some scenarios of hash collisions or signals waking page waiters).
> 
> This bit can be used to avoid the costly waitqueue_active test for most
> cases where the page has no waiters (the hashed address effectively adds
> another line of cache footprint for most page operations). In cases where
> the bit is set when the page has no waiters, the slower wakeup path will
> end up clearing up the bit.
> 
> The generic bit-waitqueue infrastructure is no longer used for pages, and
> instead waitqueues are used directly with a custom key type. The generic
> code was not flexible enough to do PageWaiters manipulation under waitqueue
> lock, or always allow danging bits to be cleared when no waiters for this
> page on the waitqueue.
> 
> The upshot is that the page wait is much more flexible now, and could be
> easily extended to wait on other properties of the page (by carrying that
> data in the wait key).
> 
> This improves the performance of a streaming write into a preallocated
> tmpfs file by 2.2% on a POWER8 system with 64K pages (which is pretty
> significant if there is only a single unlock_page per 64K of copy_from_user).
> 
> Idea seems to have been around for a while, https://lwn.net/Articles/233391/
> 
> ---
>  include/linux/page-flags.h     |   2 +
>  include/linux/pagemap.h        |  23 +++---
>  include/trace/events/mmflags.h |   1 +
>  mm/filemap.c                   | 157 ++++++++++++++++++++++++++++++++---------
>  mm/swap.c                      |   2 +
>  5 files changed, 138 insertions(+), 47 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 58d30b8..da40a1d 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -73,6 +73,7 @@
>   */
>  enum pageflags {
>  	PG_locked,		/* Page is locked. Don't touch. */
> +	PG_waiters,		/* Page has waiters, check its waitqueue */
>  	PG_error,
>  	PG_referenced,
>  	PG_uptodate,
> @@ -255,6 +256,7 @@ static inline int TestClearPage##uname(struct page *page) { return 0; }
>  	TESTSETFLAG_FALSE(uname) TESTCLEARFLAG_FALSE(uname)
>  
>  __PAGEFLAG(Locked, locked, PF_NO_TAIL)
> +PAGEFLAG(Waiters, waiters, PF_NO_COMPOUND) __CLEARPAGEFLAG(Waiters, waiters, PF_NO_COMPOUND)

This should be at least PF_NO_TAIL to work with shmem/tmpfs huge pages.

-- 
 Kirill A. Shutemov

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