Re: Regression from 2.6.36

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 13 Apr 2011 04:37:36 +0200
Eric Dumazet <eric.dumazet@gmail.com> wrote:

> Le mardi 12 avril 2011 __ 18:31 -0700, Andrew Morton a __crit :
> > On Wed, 13 Apr 2011 09:23:11 +0800 Changli Gao <xiaosuo@gmail.com> wrote:
> > 
> > > On Wed, Apr 13, 2011 at 6:49 AM, Andrew Morton
> > > <akpm@linux-foundation.org> wrote:
> > > >
> > > > It's somewhat unclear (to me) what caused this regression.
> > > >
> > > > Is it because the kernel is now doing large kmalloc()s for the fdtable,
> > > > and this makes the page allocator go nuts trying to satisfy high-order
> > > > page allocation requests?
> > > >
> > > > Is it because the kernel now will usually free the fdtable
> > > > synchronously within the rcu callback, rather than deferring this to a
> > > > workqueue?
> > > >
> > > > The latter seems unlikely, so I'm thinking this was a case of
> > > > high-order-allocations-considered-harmful?
> > > >
> > > 
> > > Maybe, but I am not sure. Maybe my patch causes too many inner
> > > fragments. For example, when asking for 5 pages, get 8 pages, and 3
> > > pages are wasted, then memory thrash happens finally.
> > 
> > That theory sounds less likely, but could be tested by using
> > alloc_pages_exact().
> > 
> 
> Very unlikely, since fdtable sizes are powers of two, unless you hit
> sysctl_nr_open and it was changed (default value being 2^20)
> 

So am I correct in believing that this regression is due to the
high-order allocations putting excess stress onto page reclaim?

If so, then how large _are_ these allocations?  This perhaps can be
determined from /proc/slabinfo.  They must be pretty huge, because slub
likes to do excessively-large allocations and the system handles that
reasonably well.

I suppose that a suitable fix would be


From: Andrew Morton <akpm@linux-foundation.org>

Azurit reports large increases in system time after 2.6.36 when running
Apache.  It was bisected down to a892e2d7dcdfa6c76e6 ("vfs: use kmalloc()
to allocate fdmem if possible").

That patch caused the vfs to use kmalloc() for very large allocations and
this is causing excessive work (and presumably excessive reclaim) within
the page allocator.

Fix it by falling back to vmalloc() earlier - when the allocation attempt
would have been considered "costly" by reclaim.

Reported-by: azurIt <azurit@pobox.sk>
Cc: Changli Gao <xiaosuo@gmail.com>
Cc: Americo Wang <xiyou.wangcong@gmail.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 fs/file.c |   17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff -puN fs/file.c~a fs/file.c
--- a/fs/file.c~a
+++ a/fs/file.c
@@ -39,14 +39,17 @@ int sysctl_nr_open_max = 1024 * 1024; /*
  */
 static DEFINE_PER_CPU(struct fdtable_defer, fdtable_defer_list);
 
-static inline void *alloc_fdmem(unsigned int size)
+static void *alloc_fdmem(unsigned int size)
 {
-	void *data;
-
-	data = kmalloc(size, GFP_KERNEL|__GFP_NOWARN);
-	if (data != NULL)
-		return data;
-
+	/*
+	 * Very large allocations can stress page reclaim, so fall back to
+	 * vmalloc() if the allocation size will be considered "large" by the VM.
+	 */
+	if (size <= (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER) {
+		void *data = kmalloc(size, GFP_KERNEL|__GFP_NOWARN);
+		if (data != NULL)
+			return data;
+	}
 	return vmalloc(size);
 }
 
_