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