Subject: Re: [MM] Make mm counters per cpu instead of atomic
From: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
To: Christoph Lameter <cl@linux-foundation.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
       "hugh.dickins@tiscali.co.uk" <hugh.dickins@tiscali.co.uk>,
       linux-mm@kvack.org, linux-kernel@vger.kernel.org,
       akpm@linux-foundation.org, Tejun Heo <tj@kernel.org>,
       Andi Kleen <andi@firstfloor.org>
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Date: Tue, 17 Nov 2009 17:34:25 +0800
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On Tue, 2009-11-17 at 15:31 +0800, Zhang, Yanmin wrote:
> On Tue, 2009-11-17 at 14:48 +0800, Zhang, Yanmin wrote:
> > On Wed, 2009-11-04 at 14:14 -0500, Christoph Lameter wrote:
> > > From: Christoph Lameter <cl@linux-foundation.org>
> > > Subject: Make mm counters per cpu
> > > 
> > > Changing the mm counters to per cpu counters is possible after the introduction
> > > of the generic per cpu operations (currently in percpu and -next).
> > > 
> > > With that the contention on the counters in mm_struct can be avoided. The
> > > USE_SPLIT_PTLOCKS case distinction can go away. Larger SMP systems do not
> > > need to perform atomic updates to mm counters anymore. Various code paths
> > > can be simplified since per cpu counter updates are fast and batching
> > > of counter updates is no longer needed.
> > > 
> > > One price to pay for these improvements is the need to scan over all percpu
> > > counters when the actual count values are needed.
> > > 
> > > Signed-off-by: Christoph Lameter <cl@linux-foundation.org>
> > > 
> > > ---
> > >  fs/proc/task_mmu.c       |   14 +++++++++-
> > >  include/linux/mm_types.h |   16 ++++--------
> > >  include/linux/sched.h    |   61 ++++++++++++++++++++---------------------------
> > >  kernel/fork.c            |   25 ++++++++++++++-----
> > >  mm/filemap_xip.c         |    2 -
> > >  mm/fremap.c              |    2 -
> > >  mm/init-mm.c             |    3 ++
> > >  mm/memory.c              |   20 +++++++--------
> > >  mm/rmap.c                |   10 +++----
> > >  mm/swapfile.c            |    2 -
> > >  10 files changed, 84 insertions(+), 71 deletions(-)
> > > 
> > > Index: linux-2.6/include/linux/mm_types.h
> > > ===================================================================
> > > --- linux-2.6.orig/include/linux/mm_types.h	2009-11-04 13:08:33.000000000 -0600
> > > +++ linux-2.6/include/linux/mm_types.h	2009-11-04 13:13:42.000000000 -0600
> > > @@ -24,11 +24,10 @@ struct address_space;
> > 
> > > Index: linux-2.6/kernel/fork.c
> > > ===================================================================
> > > --- linux-2.6.orig/kernel/fork.c	2009-11-04 13:08:33.000000000 -0600
> > > +++ linux-2.6/kernel/fork.c	2009-11-04 13:14:19.000000000 -0600
> > > @@ -444,6 +444,8 @@ static void mm_init_aio(struct mm_struct
> > > 
> > >  static struct mm_struct * mm_init(struct mm_struct * mm, struct task_struct *p)
> > >  {
> > > +	int cpu;
> > > +
> > >  	atomic_set(&mm->mm_users, 1);
> > >  	atomic_set(&mm->mm_count, 1);
> > >  	init_rwsem(&mm->mmap_sem);
> > > @@ -452,8 +454,11 @@ static struct mm_struct * mm_init(struct
> > >  		(current->mm->flags & MMF_INIT_MASK) : default_dump_filter;
> > >  	mm->core_state = NULL;
> > >  	mm->nr_ptes = 0;
> > > -	set_mm_counter(mm, file_rss, 0);
> > > -	set_mm_counter(mm, anon_rss, 0);
> > > +	for_each_possible_cpu(cpu) {
> > > +		struct mm_counter *m;
> > > +
> > > +		memset(m, sizeof(struct mm_counter), 0);
> > Above memset is wrong.
> > 1) m isn't initiated;
> > 2) It seems the 2nd and the 3rd parameters should be interchanged.
> Changing it to below fixes the command hang issue.
> 
>         for_each_possible_cpu(cpu) {
>                 struct mm_counter *m = per_cpu(mm->rss->readers, cpu);
> 
>                 memset(m, 0, sizeof(struct mm_counter));
>         }

Sorry. I was too optimistic and used another kernel to boot.
The right change above should be:
 struct mm_counter *m = per_cpu_ptr(mm->rss, cpu);

Compiler doesn't report error/warning when I use any member.

With the change, command 'make oldconfig' and a boot command still
hangs.

Yanmin


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