Re: [PATCH]mmap: add alignment for some variables

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

On Wed, 30 Mar 2011 09:17:23 +0800 Shaohua Li <shaohua.li@intel.com> wrote:

> On Wed, 2011-03-30 at 09:06 +0800, Andrew Morton wrote:
> > On Wed, 30 Mar 2011 09:01:22 +0800 Shaohua Li <shaohua.li@intel.com> wrote:
> > 
> > > +/*
> > > + * Make sure vm_committed_as in one cacheline and not cacheline shared with
> > > + * other variables. It can be updated by several CPUs frequently.
> > > + */
> > > +struct percpu_counter vm_committed_as ____cacheline_internodealigned_in_smp;
> > 
> > The mystery deepens.  The only cross-cpu writeable fields in there are
> > percpu_counter.lock and its companion percpu_counter.count.  If CPUs
> > are contending for the lock then that itself is a problem - how does
> > adding some padding to the struct help anything?
> I had another patch trying to address the lock contention (for case
> OVERCOMMIT_GUESS), will send out soon. But thought better to have the
> correct alignment for OVERCOMMIT_NEVER case.

I still don't understand why adding
____cacheline_internodealigned_in_smp to vm_committed_as improves
anything.

Here it is:

struct percpu_counter {
	spinlock_t lock;
	s64 count;
#ifdef CONFIG_HOTPLUG_CPU
	struct list_head list;	/* All percpu_counters are on a list */
#endif
	s32 __percpu *counters;
};

and your patch effectively converts this to

  struct percpu_counter {
	spinlock_t lock;
  	s64 count;
  #ifdef CONFIG_HOTPLUG_CPU
	struct list_head list;	/* All percpu_counters are on a list */
  #endif
	s32 __percpu *counters;
+	char large_waste_of_space[lots];
  };

how is it that this improves things?