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

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Date:	Wed, 30 Mar 2011 10:35:51 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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, 2011-03-30 at 10:10 +0800, Andrew Morton wrote:
> On Wed, 30 Mar 2011 09:54:01 +0800 Shaohua Li <shaohua.li@...el.com> wrote:
> 
> > On Wed, 2011-03-30 at 09:41 +0800, Andrew Morton wrote:
> > > On Wed, 30 Mar 2011 09:36:40 +0800 Shaohua Li <shaohua.li@...el.com> wrote:
> > > 
> > > > > 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.
> > > 
> > > ____cacheline_aligned_in_smp would achieve that?
> > ____cacheline_aligned_in_smp can't guarantee the cache alignment for
> > multiple nodes, because the variable can be updated by multiple
> > nodes/cpus.
> 
> Confused.  If an object is aligned at a mulitple-of-128 address on one
> node, it is aligned at a multiple-of-128 address when viewed from other
> nodes, surely?
> 
> Even if the cache alignment to which you're referring is the internode
> cache, can a 34-byte, L1-cache-aligned structure ever span multiple
> internode cachelines?
ah, you are right, ____cacheline_aligned_in_smp is enough here, thanks
for correcting me here.

Make some variables have correct alignment/section to avoid cache issue.
In a workload which heavily does mmap/munmap, the variables will be used
frequently.

Signed-off-by: Shaohua Li <shaohua.li@...el.com>
---
 mm/mmap.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

Index: linux/mm/mmap.c
===================================================================
--- linux.orig/mm/mmap.c	2011-03-30 08:45:05.000000000 +0800
+++ linux/mm/mmap.c	2011-03-30 10:34:36.000000000 +0800
@@ -84,10 +84,14 @@ pgprot_t vm_get_page_prot(unsigned long
 }
 EXPORT_SYMBOL(vm_get_page_prot);
 
-int sysctl_overcommit_memory = OVERCOMMIT_GUESS;  /* heuristic overcommit */
-int sysctl_overcommit_ratio = 50;	/* default is 50% */
+int sysctl_overcommit_memory __read_mostly = OVERCOMMIT_GUESS;  /* heuristic overcommit */
+int sysctl_overcommit_ratio __read_mostly = 50;	/* default is 50% */
 int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT;
-struct percpu_counter vm_committed_as;
+/*
+ * 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_aligned_in_smp;
 
 /*
  * Check that a process has enough memory to allocate a new virtual


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