On Wed, Nov 02, 2016 at 06:50:35PM +1100, Nicholas Piggin wrote: > On Wed, 2 Nov 2016 10:31:57 +0300 > "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> wrote: > > > 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. > > I thought all paths using it already took the head page, but I'll double > check. Did you see a specific problem? PF_NO_COMPOUND doesn't allow both head and tail pages. CONFIG_DEBUG_VM_PGFLAGS=y will make it expode. -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>