On Tue 05-03-13 17:10:55, Glauber Costa wrote:
> For the root memcg, there is no need to rely on the res_counters if hierarchy
> is enabled The sum of all mem cgroups plus the tasks in root itself, is
> necessarily the amount of memory used for the whole system. Since those figures
> are already kept somewhere anyway, we can just return them here, without too
> much hassle.
>
> Limit and soft limit can't be set for the root cgroup, so they are left at
> RESOURCE_MAX. Failcnt is left at 0, because its actual meaning is how many
> times we failed allocations due to the limit being hit. We will fail
> allocations in the root cgroup, but the limit will never the reason.
I do not like this very much to be honest. It just adds more hackery...
Why cannot we simply not account if nr_cgroups == 1 and move relevant
global counters to the root at the moment when a first group is
created?
The patch aims at reducing an overhead when there there are no other
groups, right?
> Signed-off-by: Glauber Costa <glommer@xxxxxxxxxxxxx>
> CC: Michal Hocko <mhocko@xxxxxxx>
> CC: Kamezawa Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> CC: Johannes Weiner <hannes@xxxxxxxxxxx>
> CC: Mel Gorman <mgorman@xxxxxxx>
> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
> mm/memcontrol.c | 64 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 64 insertions(+)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b8b363f..bfbf1c2 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4996,6 +4996,56 @@ static inline u64 mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
> return val << PAGE_SHIFT;
> }
>
> +static u64 memcg_read_root_rss(void)
> +{
> + struct task_struct *p;
> +
> + u64 rss = 0;
> + read_lock(&tasklist_lock);
> + for_each_process(p) {
> + if (!p->mm)
> + continue;
> + task_lock(p);
> + rss += get_mm_rss(p->mm);
> + task_unlock(p);
> + }
> + read_unlock(&tasklist_lock);
> + return rss;
> +}
> +
> +static u64 mem_cgroup_read_root(enum res_type type, int name)
> +{
> + if (name == RES_LIMIT)
> + return RESOURCE_MAX;
> + if (name == RES_SOFT_LIMIT)
> + return RESOURCE_MAX;
> + if (name == RES_FAILCNT)
> + return 0;
> + if (name == RES_MAX_USAGE)
> + return 0;
> +
> + if (WARN_ON_ONCE(name != RES_USAGE))
> + return 0;
> +
> + switch (type) {
> + case _MEM:
> + return (memcg_read_root_rss() +
> + atomic_long_read(&vm_stat[NR_FILE_PAGES])) << PAGE_SHIFT;
> + case _MEMSWAP: {
> + struct sysinfo i;
> + si_swapinfo(&i);
> +
> + return ((memcg_read_root_rss() +
> + atomic_long_read(&vm_stat[NR_FILE_PAGES])) << PAGE_SHIFT) +
> + i.totalswap - i.freeswap;
> + }
> + case _KMEM:
> + return 0;
> + default:
> + BUG();
> + };
> +}
> +
> static ssize_t mem_cgroup_read(struct cgroup *cont, struct cftype *cft,
> struct file *file, char __user *buf,
> size_t nbytes, loff_t *ppos)
> @@ -5012,6 +5062,19 @@ static ssize_t mem_cgroup_read(struct cgroup *cont, struct cftype *cft,
> if (!do_swap_account && type == _MEMSWAP)
> return -EOPNOTSUPP;
>
> + /*
> + * If we have root-level hierarchy, we can be certain that the charges
> + * in root are always global. We can then bypass the root cgroup
> + * entirely in this case, hopefuly leading to less contention in the
> + * root res_counters. The charges presented after reading it will
> + * always be the global charges.
> + */
> + if (mem_cgroup_disabled() ||
> + (mem_cgroup_is_root(memcg) && memcg->use_hierarchy)) {
> + val = mem_cgroup_read_root(type, name);
> + goto root_bypass;
> + }
> +
> switch (type) {
> case _MEM:
> if (name == RES_USAGE)
> @@ -5032,6 +5095,7 @@ static ssize_t mem_cgroup_read(struct cgroup *cont, struct cftype *cft,
> BUG();
> }
>
> +root_bypass:
> len = scnprintf(str, sizeof(str), "%llu\n", (unsigned long long)val);
> return simple_read_from_buffer(buf, nbytes, ppos, str, len);
> }
> --
> 1.8.1.2
>
> --
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--
Michal Hocko
SUSE Labs
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