linux-kernel - Re: [PATCH]mmap: add alignment for some variables

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Date:	Tue, 29 Mar 2011 18:25:44 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-mm <linux-mm@...ck.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH]mmap: add alignment for some variables

On Wed, 30 Mar 2011 09:17:23 +0800 Shaohua Li <shaohua.li@...el.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@...el.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?
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