Re: [PATCH 5/7] Add /proc trigger for memory compaction

[David Rientjes <rientjes@google.com>]

On Wed, 6 Jan 2010, Mel Gorman wrote:

> This patch adds a proc file /proc/sys/vm/compact_node. When a NID is written
> to the file, each zone in that node is compacted. This should be done with
> debugfs but this was what was available to rebase quickly and I suspect
> debugfs either did not exist or was in development during the first
> implementation.
> 
> If this interface is to exist in the long term, it needs to be thought
> about carefully. For the moment, it's handy to have to test compaction
> under a controlled setting.
> 

With Lee's work on mempolicy-constrained hugepage allocations, there is a 
use-case for this explicit trigger to be exported via sysfs in the 
longterm: we should be able to determine how successful the allocation of 
hugepages will be on a particular node without actually allocating them 
depending on the degree of external fragmentation to form our mempolicy.  
Since node-targeted hugepage allocation and freeing now exists in the 
kernel and compaction is used primary for the former, I don't see why it 
shouldn't be exposed.

I like the (slightly racy) interface that avoids having a trigger for each 
node under /sys/devices/system/node as well.

> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> ---
>  include/linux/compaction.h |    5 ++++
>  kernel/sysctl.c            |   11 +++++++++
>  mm/compaction.c            |   52 ++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 68 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> index 6201371..5965ef2 100644
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -5,4 +5,9 @@
>  #define COMPACT_INCOMPLETE	0
>  #define COMPACT_COMPLETE	1
>  
> +#ifdef CONFIG_MIGRATION
> +extern int sysctl_compaction_handler(struct ctl_table *table, int write,
> +			void __user *buffer, size_t *length, loff_t *ppos);
> +#endif /* CONFIG_MIGRATION */
> +
>  #endif /* _LINUX_COMPACTION_H */
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 8a68b24..6202e95 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -50,6 +50,7 @@
>  #include <linux/ftrace.h>
>  #include <linux/slow-work.h>
>  #include <linux/perf_event.h>
> +#include <linux/compaction.h>
>  
>  #include <asm/uaccess.h>
>  #include <asm/processor.h>
> @@ -80,6 +81,7 @@ extern int pid_max;
>  extern int min_free_kbytes;
>  extern int pid_max_min, pid_max_max;
>  extern int sysctl_drop_caches;
> +extern int sysctl_compact_node;
>  extern int percpu_pagelist_fraction;
>  extern int compat_log;
>  extern int latencytop_enabled;
> @@ -1109,6 +1111,15 @@ static struct ctl_table vm_table[] = {
>  		.mode		= 0644,
>  		.proc_handler	= drop_caches_sysctl_handler,
>  	},
> +#ifdef CONFIG_MIGRATION
> +	{
> +		.procname	= "compact_node",
> +		.data		= &sysctl_compact_node,
> +		.maxlen		= sizeof(int),
> +		.mode		= 0644,

This should only need 0200?

> +		.proc_handler	= sysctl_compaction_handler,
> +	},
> +#endif /* CONFIG_MIGRATION */
>  	{
>  		.procname	= "min_free_kbytes",
>  		.data		= &min_free_kbytes,
> diff --git a/mm/compaction.c b/mm/compaction.c
> index d36760a..a8bcae2 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -11,6 +11,7 @@
>  #include <linux/migrate.h>
>  #include <linux/compaction.h>
>  #include <linux/mm_inline.h>
> +#include <linux/sysctl.h>
>  #include "internal.h"
>  
>  /*
> @@ -338,3 +339,54 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
>  	return ret;
>  }
>  
> +/* Compact all zones within a node */
> +int compact_node(int nid)
> +{
> +	int zoneid;
> +	pg_data_t *pgdat;
> +	struct zone *zone;
> +
> +	if (nid < 0 || nid > nr_node_ids || !node_online(nid))
> +		return -EINVAL;
> +	pgdat = NODE_DATA(nid);
> +
> +	/* Flush pending updates to the LRU lists */
> +	lru_add_drain_all();
> +
> +	printk(KERN_INFO "Compacting memory in node %d\n", nid);
> +	for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
> +		struct compact_control cc;
> +
> +		zone = &pgdat->node_zones[zoneid];
> +		if (!populated_zone(zone))
> +			continue;
> +
> +		cc.nr_freepages = 0;
> +		cc.nr_migratepages = 0;
> +		cc.zone = zone;
> +		INIT_LIST_HEAD(&cc.freepages);
> +		INIT_LIST_HEAD(&cc.migratepages);
> +
> +		compact_zone(zone, &cc);
> +
> +		VM_BUG_ON(!list_empty(&cc.freepages));
> +		VM_BUG_ON(!list_empty(&cc.migratepages));
> +	}
> +	printk(KERN_INFO "Compaction of node %d complete\n", nid);

If it's exposed through sysfs, the printk's should probably be demoted to 
pr_debug().

> +
> +	return 0;
> +}
> +
> +/* This is global and fierce ugly but it's straight-forward */
> +int sysctl_compact_node;
> +
> +/* This is the entry point for compacting nodes via /proc/sys/vm */
> +int sysctl_compaction_handler(struct ctl_table *table, int write,
> +			void __user *buffer, size_t *length, loff_t *ppos)
> +{
> +	proc_dointvec(table, write, buffer, length, ppos);
> +	if (write)
> +		return compact_node(sysctl_compact_node);
> +
> +	return 0;
> +}

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