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

[Shaohua Li]

On Wed, 2011-03-30 at 09:25 +0800, Andrew Morton wrote:
> On Wed, 30 Mar 2011 09:17:23 +0800 Shaohua Li <shaohua.li@xxxxxxxxx> 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@xxxxxxxxx> 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?
Hmm, it actually 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;
 } __attribute__((__aligned__(1 << (INTERNODE_CACHE_SHIFT))))
so lock and count are in one cache line.

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