[Devel] Re: [PATCH -mm 1/3] cgroup: use a hash table for css_set finding

Paul Menage menage at google.com
Wed Apr 2 03:23:59 PDT 2008


On Tue, Apr 1, 2008 at 7:16 PM, Li Zefan <lizf at cn.fujitsu.com> wrote:
> When we attach a process to a different cgroup, the css_set linked-list
>  will be run through to find a suitable existing css_set to use. This
>  patch implements a hash table for better performance.
>
>  The following benmarks has been tested:
>
>  For N in 1, 5, 10, 50, 100, 500, 1000, create N cgroups with one sleeping
>  task in each, and then move an additional task through each cgroup in
>  turn.
>
>  Here is a test result:
>
>  N       Loop    orig - Time(s)  hash - Time(s)
>  ----------------------------------------------
>  1       10000   1.201231728     1.196311177
>  5       2000    1.065743872     1.040566424
>  10      1000    0.991054735     0.986876440
>  50      200     0.976554203     0.969608733
>  100     100     0.998504680     0.969218270
>  500     20      1.157347764     0.962602963
>  1000    10      1.619521852     1.085140172
>
>
>  Signed-off-by: Li Zefan <lizf at cn.fujitsu.com>

Reviewed-by: Paul Menage <menage at google.com>

This looks good, thanks.

>  ---
>   include/linux/cgroup.h |    7 +++++-
>   kernel/cgroup.c        |   58 ++++++++++++++++++++++++++++++++++++++----------
>   2 files changed, 52 insertions(+), 13 deletions(-)
>
>  diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
>  index 785a01c..c15c5e0 100644
>  --- a/include/linux/cgroup.h
>  +++ b/include/linux/cgroup.h
>  @@ -156,6 +156,12 @@ struct css_set {
>         struct list_head list;
>
>         /*
>  +        * List running through all cgroup groups in the same hash
>  +        * slot. Protected by css_set_lock
>  +        */
>  +       struct hlist_node hlist;
>  +
>  +       /*
>          * List running through all tasks using this cgroup
>          * group. Protected by css_set_lock
>          */
>  @@ -174,7 +180,6 @@ struct css_set {
>          * during subsystem registration (at boot time).
>          */
>         struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];
>  -
>   };
>
>   /*
>  diff --git a/kernel/cgroup.c b/kernel/cgroup.c
>  index e8e8ec4..78e5bde 100644
>  --- a/kernel/cgroup.c
>  +++ b/kernel/cgroup.c
>  @@ -44,6 +44,7 @@
>   #include <linux/kmod.h>
>   #include <linux/delayacct.h>
>   #include <linux/cgroupstats.h>
>  +#include <linux/hash.h>
>
>   #include <asm/atomic.h>
>
>  @@ -193,6 +194,26 @@ static struct cg_cgroup_link init_css_set_link;
>   static DEFINE_RWLOCK(css_set_lock);
>   static int css_set_count;
>
>  +/* hash table for cgroup groups. This improves the performance to
>  + * find an existing css_set */
>  +#define CSS_SET_HASH_BITS      7
>  +#define CSS_SET_TABLE_SIZE     (1 << CSS_SET_HASH_BITS)
>  +static struct hlist_head css_set_table[CSS_SET_TABLE_SIZE];
>  +
>  +static struct hlist_head *css_set_hash(struct cgroup_subsys_state *css[])
>  +{
>  +       int i;
>  +       int index;
>  +       unsigned long tmp = 0UL;
>  +
>  +       for (i = 0; i < CGROUP_SUBSYS_COUNT; i++)
>  +               tmp += (unsigned long)css[i];
>  +
>  +       index = hash_long(tmp, CSS_SET_HASH_BITS);
>  +
>  +       return &css_set_table[index];
>  +}
>  +
>   /* We don't maintain the lists running through each css_set to its
>   * task until after the first call to cgroup_iter_start(). This
>   * reduces the fork()/exit() overhead for people who have cgroups
>  @@ -219,6 +240,7 @@ static int use_task_css_set_links;
>   static void unlink_css_set(struct css_set *cg)
>   {
>         write_lock(&css_set_lock);
>  +       hlist_del(&cg->hlist);
>         list_del(&cg->list);
>         css_set_count--;
>         while (!list_empty(&cg->cg_links)) {
>  @@ -284,9 +306,7 @@ static inline void put_css_set_taskexit(struct css_set *cg)
>   /*
>   * find_existing_css_set() is a helper for
>   * find_css_set(), and checks to see whether an existing
>  - * css_set is suitable. This currently walks a linked-list for
>  - * simplicity; a later patch will use a hash table for better
>  - * performance
>  + * css_set is suitable.
>   *
>   * oldcg: the cgroup group that we're using before the cgroup
>   * transition
>  @@ -303,7 +323,9 @@ static struct css_set *find_existing_css_set(
>   {
>         int i;
>         struct cgroupfs_root *root = cgrp->root;
>  -       struct list_head *l = &init_css_set.list;
>  +       struct hlist_head *hhead;
>  +       struct hlist_node *node;
>  +       struct css_set *cg;
>
>         /* Built the set of subsystem state objects that we want to
>          * see in the new css_set */
>  @@ -320,18 +342,13 @@ static struct css_set *find_existing_css_set(
>                 }
>         }
>
>  -       /* Look through existing cgroup groups to find one to reuse */
>  -       do {
>  -               struct css_set *cg =
>  -                       list_entry(l, struct css_set, list);
>  -
>  +       hhead = css_set_hash(template);
>  +       hlist_for_each_entry(cg, node, hhead, hlist) {
>                 if (!memcmp(template, cg->subsys, sizeof(cg->subsys))) {
>                         /* All subsystems matched */
>                         return cg;
>                 }
>  -               /* Try the next cgroup group */
>  -               l = l->next;
>  -       } while (l != &init_css_set.list);
>  +       }
>
>         /* No existing cgroup group matched */
>         return NULL;
>  @@ -393,6 +410,8 @@ static struct css_set *find_css_set(
>         struct list_head tmp_cg_links;
>         struct cg_cgroup_link *link;
>
>  +       struct hlist_head *hhead;
>  +
>         /* First see if we already have a cgroup group that matches
>          * the desired set */
>         write_lock(&css_set_lock);
>  @@ -417,6 +436,7 @@ static struct css_set *find_css_set(
>         kref_init(&res->ref);
>         INIT_LIST_HEAD(&res->cg_links);
>         INIT_LIST_HEAD(&res->tasks);
>  +       INIT_HLIST_NODE(&res->hlist);
>
>         /* Copy the set of subsystem state objects generated in
>          * find_existing_css_set() */
>  @@ -459,6 +479,11 @@ static struct css_set *find_css_set(
>         /* Link this cgroup group into the list */
>         list_add(&res->list, &init_css_set.list);
>         css_set_count++;
>  +
>  +       /* Add this cgroup group to the hash table */
>  +       hhead = css_set_hash(res->subsys);
>  +       hlist_add_head(&res->hlist, hhead);
>  +
>         write_unlock(&css_set_lock);
>
>         return res;
>  @@ -2499,6 +2524,7 @@ int __init cgroup_init_early(void)
>         INIT_LIST_HEAD(&init_css_set.list);
>         INIT_LIST_HEAD(&init_css_set.cg_links);
>         INIT_LIST_HEAD(&init_css_set.tasks);
>  +       INIT_HLIST_NODE(&init_css_set.hlist);
>         css_set_count = 1;
>         init_cgroup_root(&rootnode);
>         list_add(&rootnode.root_list, &roots);
>  @@ -2511,6 +2537,9 @@ int __init cgroup_init_early(void)
>         list_add(&init_css_set_link.cg_link_list,
>                  &init_css_set.cg_links);
>
>  +       for (i = 0; i < CSS_SET_TABLE_SIZE; i++)
>  +               INIT_HLIST_HEAD(&css_set_table[i]);
>  +
>         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
>                 struct cgroup_subsys *ss = subsys[i];
>
>  @@ -2540,6 +2569,7 @@ int __init cgroup_init(void)
>   {
>         int err;
>         int i;
>  +       struct hlist_head *hhead;
>
>         err = bdi_init(&cgroup_backing_dev_info);
>         if (err)
>  @@ -2551,6 +2581,10 @@ int __init cgroup_init(void)
>                         cgroup_init_subsys(ss);
>         }
>
>  +       /* Add init_css_set to the hash table */
>  +       hhead = css_set_hash(init_css_set.subsys);
>  +       hlist_add_head(&init_css_set.hlist, hhead);
>  +
>         err = register_filesystem(&cgroup_fs_type);
>         if (err < 0)
>                 goto out;
>  --
>  1.5.4.rc3
>
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