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SubjectRe: [PATCH 4/7] memcg: update numa information based on event counter
On Wed, 22 Jun 2011 17:53:09 +0200
Michal Hocko <mhocko@suse.cz> wrote:

> On Thu 16-06-11 12:54:00, KAMEZAWA Hiroyuki wrote:
> > From 88090fe10e225ad8769ba0ea01692b7314e8b973 Mon Sep 17 00:00:00 2001
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > Date: Wed, 15 Jun 2011 16:19:46 +0900
> > Subject: [PATCH 4/7] memcg: update numa information based on event counter
> >
> > commit 889976 adds an numa node round-robin for memcg. But the information
> > is updated once per 10sec.
> >
> > This patch changes the update trigger from jiffies to memcg's event count.
> > After this patch, numa scan information will be updated when
> >
> > - the number of pagein/out events is larger than 3% of limit
> > or
> > - the number of pagein/out events is larger than 16k
> > (==64MB pagein/pageout if pagesize==4k.)
> >
> > The counter of mem->numascan_update the sum of percpu events counter.
> > When a task hits limit, it checks mem->numascan_update. If it's over
> > min(3% of limit, 16k), numa information will be updated.
>
> Yes, I like the event based approach more than the origin (time) based
> one.
>
> >
> > This patch also adds mutex for updating information. This will allow us
> > to avoid unnecessary scan.
> >
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > ---
> > mm/memcontrol.c | 51 +++++++++++++++++++++++++++++++++++++++++++++------
> > 1 file changed, 45 insertions(+), 6 deletions(-)
> >
> > Index: mmotm-0615/mm/memcontrol.c
> > ===================================================================
> > --- mmotm-0615.orig/mm/memcontrol.c
> > +++ mmotm-0615/mm/memcontrol.c
> > @@ -108,10 +108,12 @@ enum mem_cgroup_events_index {
> > enum mem_cgroup_events_target {
> > MEM_CGROUP_TARGET_THRESH,
> > MEM_CGROUP_TARGET_SOFTLIMIT,
> > + MEM_CGROUP_TARGET_NUMASCAN,
>
> Shouldn't it be defined only for MAX_NUMNODES > 1
>

Hmm, yes. But I want to reduce #ifdefs..


> > MEM_CGROUP_NTARGETS,
> > };
> > #define THRESHOLDS_EVENTS_TARGET (128)
> > #define SOFTLIMIT_EVENTS_TARGET (1024)
> > +#define NUMASCAN_EVENTS_TARGET (1024)
> >
> > struct mem_cgroup_stat_cpu {
> > long count[MEM_CGROUP_STAT_NSTATS];
> > @@ -288,8 +290,9 @@ struct mem_cgroup {
> > int last_scanned_node;
> > #if MAX_NUMNODES > 1
> > nodemask_t scan_nodes;
> > - unsigned long next_scan_node_update;
> > + struct mutex numascan_mutex;
> > #endif
> > + atomic_t numascan_update;
>
> Why it is out of ifdef?
>

This was for avoiding #ifdef in mem_cgroup_create()...but it's not used now.
I'll fix this.



> > /*
> > * Should the accounting and control be hierarchical, per subtree?
> > */
> > @@ -741,6 +744,9 @@ static void __mem_cgroup_target_update(s
> > case MEM_CGROUP_TARGET_SOFTLIMIT:
> > next = val + SOFTLIMIT_EVENTS_TARGET;
> > break;
> > + case MEM_CGROUP_TARGET_NUMASCAN:
> > + next = val + NUMASCAN_EVENTS_TARGET;
> > + break;
>
> MAX_NUMNODES > 1
>
> > default:
> > return;
> > }
> > @@ -764,6 +770,13 @@ static void memcg_check_events(struct me
> > __mem_cgroup_target_update(mem,
> > MEM_CGROUP_TARGET_SOFTLIMIT);
> > }
> > + if (unlikely(__memcg_event_check(mem,
> > + MEM_CGROUP_TARGET_NUMASCAN))) {
> > + atomic_add(MEM_CGROUP_TARGET_NUMASCAN,
> > + &mem->numascan_update);
> > + __mem_cgroup_target_update(mem,
> > + MEM_CGROUP_TARGET_NUMASCAN);
> > + }
> > }
>
> again MAX_NUMNODES > 1
>

Hmm, ok, I will add #ifdef only here.



> > }
> >
> > @@ -1616,17 +1629,32 @@ mem_cgroup_select_victim(struct mem_cgro
> > /*
> > * Always updating the nodemask is not very good - even if we have an empty
> > * list or the wrong list here, we can start from some node and traverse all
> > - * nodes based on the zonelist. So update the list loosely once per 10 secs.
> > + * nodes based on the zonelist.
> > *
> > + * The counter of mem->numascan_update is updated once per
> > + * NUMASCAN_EVENTS_TARGET. We update the numa information when we see
> > + * the number of event is larger than 3% of limit or 64MB pagein/pageout.
> > */
> > +#define NUMASCAN_UPDATE_RATIO (3)
> > +#define NUMASCAN_UPDATE_THRESH (16384UL) /* 16k events of pagein/pageout */
> > static void mem_cgroup_may_update_nodemask(struct mem_cgroup *mem)
> > {
> > int nid;
> > -
> > - if (time_after(mem->next_scan_node_update, jiffies))
> > + unsigned long long limit;
> > + /* if no limit, we never reach here */
> > + limit = res_counter_read_u64(&mem->res, RES_LIMIT);
> > + limit /= PAGE_SIZE;
> > + /* 3% of limit */
> > + limit = (limit * NUMASCAN_UPDATE_RATIO/100UL);
> > + limit = min_t(unsigned long long, limit, NUMASCAN_UPDATE_THRESH);
> > + /*
> > + * If the number of pagein/out event is larger than 3% of limit or
> > + * 64MB pagein/out, refresh numa information.
> > + */
> > + if (atomic_read(&mem->numascan_update) < limit ||
> > + !mutex_trylock(&mem->numascan_mutex))
> > return;
>
> I am not sure whether a mutex is not overkill here. What about using an
> atomic operation instead?
>

I think mutex is informative than atomic counter for code readers.
If influence of overhead is not big, I'd like to use mutex.

Thanks,
-Kame




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