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Date: Thu, 2 Jun 2011 22:25:29 -0700
From: Ying Han <yinghan@...gle.com>
To: Johannes Weiner <hannes@...xchg.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
Balbir Singh <balbir@...ux.vnet.ibm.com>,
Michal Hocko <mhocko@...e.cz>,
Andrew Morton <akpm@...ux-foundation.org>,
Rik van Riel <riel@...hat.com>,
Minchan Kim <minchan.kim@...il.com>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Mel Gorman <mgorman@...e.de>, Greg Thelen <gthelen@...gle.com>,
Michel Lespinasse <walken@...gle.com>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [patch 4/8] memcg: rework soft limit reclaim
On Thu, Jun 2, 2011 at 2:55 PM, Ying Han <yinghan@...gle.com> wrote:
> On Tue, May 31, 2011 at 11:25 PM, Johannes Weiner <hannes@...xchg.org> wrote:
>> Currently, soft limit reclaim is entered from kswapd, where it selects
>> the memcg with the biggest soft limit excess in absolute bytes, and
>> reclaims pages from it with maximum aggressiveness (priority 0).
>>
>> This has the following disadvantages:
>>
>> 1. because of the aggressiveness, kswapd can be stalled on a memcg
>> that is hard to reclaim from for a long time, sending the rest of
>> the allocators into direct reclaim in the meantime.
>>
>> 2. it only considers the biggest offender (in absolute bytes, no
>> less, so very unhandy for setups with different-sized memcgs) and
>> does not apply any pressure at all on other memcgs in excess.
>>
>> 3. because it is only invoked from kswapd, the soft limit is
>> meaningful during global memory pressure, but it is not taken into
>> account during hierarchical target reclaim where it could allow
>> prioritizing memcgs as well. So while it does hierarchical
>> reclaim once triggered, it is not a truly hierarchical mechanism.
>>
>> Here is a different approach. Instead of having a soft limit reclaim
>> cycle separate from the rest of reclaim, this patch ensures that each
>> time a group of memcgs is reclaimed - be it because of global memory
>> pressure or because of a hard limit - memcgs that exceed their soft
>> limit, or contribute to the soft limit excess of one their parents,
>> are reclaimed from at a higher priority than their siblings.
>>
>> This results in the following:
>>
>> 1. all relevant memcgs are scanned with increasing priority during
>> memory pressure. The primary goal is to free pages, not to punish
>> soft limit offenders.
>>
>> 2. increased pressure is applied to all memcgs in excess of their
>> soft limit, not only the biggest offender.
>>
>> 3. the soft limit becomes meaningful for target reclaim as well,
>> where it allows prioritizing children of a hierarchy when the
>> parent hits its limit.
>>
>> 4. direct reclaim now also applies increased soft limit pressure,
>> not just kswapd anymore.
>>
>> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
>> ---
>> include/linux/memcontrol.h | 7 +++++++
>> mm/memcontrol.c | 26 ++++++++++++++++++++++++++
>> mm/vmscan.c | 8 ++++++--
>> 3 files changed, 39 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index 8f402b9..7d99e87 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -104,6 +104,7 @@ extern void mem_cgroup_end_migration(struct mem_cgroup *mem,
>> struct mem_cgroup *mem_cgroup_hierarchy_walk(struct mem_cgroup *,
>> struct mem_cgroup *);
>> void mem_cgroup_stop_hierarchy_walk(struct mem_cgroup *, struct mem_cgroup *);
>> +bool mem_cgroup_soft_limit_exceeded(struct mem_cgroup *, struct mem_cgroup *);
>>
>> /*
>> * For memory reclaim.
>> @@ -345,6 +346,12 @@ static inline void mem_cgroup_stop_hierarchy_walk(struct mem_cgroup *r,
>> {
>> }
>>
>> +static inline bool mem_cgroup_soft_limit_exceeded(struct mem_cgroup *root,
>> + struct mem_cgroup *mem)
>> +{
>> + return false;
>> +}
>> +
>> static inline void
>> mem_cgroup_print_oom_info(struct mem_cgroup *memcg, struct task_struct *p)
>> {
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 983efe4..94f77cc3 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -1460,6 +1460,32 @@ void mem_cgroup_stop_hierarchy_walk(struct mem_cgroup *root,
>> css_put(&mem->css);
>> }
>>
>> +/**
>> + * mem_cgroup_soft_limit_exceeded - check if a memcg (hierarchically)
>> + * exceeds a soft limit
>> + * @root: highest ancestor of @mem to consider
>> + * @mem: memcg to check for excess
>> + *
>> + * The function indicates whether @mem has exceeded its own soft
>> + * limit, or contributes to the soft limit excess of one of its
>> + * parents in the hierarchy below @root.
>> + */
>> +bool mem_cgroup_soft_limit_exceeded(struct mem_cgroup *root,
>> + struct mem_cgroup *mem)
>> +{
>> + for (;;) {
>> + if (mem == root_mem_cgroup)
>> + return false;
>> + if (res_counter_soft_limit_excess(&mem->res))
>> + return true;
>> + if (mem == root)
>> + return false;
>> + mem = parent_mem_cgroup(mem);
>> + if (!mem)
>> + return false;
>> + }
>> +}
>> +
>> static unsigned long mem_cgroup_reclaim(struct mem_cgroup *mem,
>> gfp_t gfp_mask,
>> unsigned long flags)
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index c7d4b44..0163840 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1988,9 +1988,13 @@ static void shrink_zone(int priority, struct zone *zone,
>> unsigned long reclaimed = sc->nr_reclaimed;
>> unsigned long scanned = sc->nr_scanned;
>> unsigned long nr_reclaimed;
>> + int epriority = priority;
>> +
>> + if (mem_cgroup_soft_limit_exceeded(root, mem))
>> + epriority -= 1;
>
> Here we grant the ability to shrink from all the memcgs, but only
> higher the priority for those exceed the soft_limit. That is a design
> change
> for the "soft_limit" which giving a hint to which memcgs to reclaim
> from first under global memory pressure.
Basically, we shouldn't reclaim from a memcg under its soft_limit
unless we have trouble reclaim pages from others. Something like the
following makes better sense:
diff --git a/mm/vmscan.c b/mm/vmscan.c
index bdc2fd3..b82ba8c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1989,6 +1989,8 @@ restart:
throttle_vm_writeout(sc->gfp_mask);
}
+#define MEMCG_SOFTLIMIT_RECLAIM_PRIORITY 2
+
static void shrink_zone(int priority, struct zone *zone,
struct scan_control *sc)
{
@@ -2001,13 +2003,13 @@ static void shrink_zone(int priority, struct zone *zone,
unsigned long reclaimed = sc->nr_reclaimed;
unsigned long scanned = sc->nr_scanned;
unsigned long nr_reclaimed;
- int epriority = priority;
- if (mem_cgroup_soft_limit_exceeded(root, mem))
- epriority -= 1;
+ if (!mem_cgroup_soft_limit_exceeded(root, mem) &&
+ priority > MEMCG_SOFTLIMIT_RECLAIM_PRIORITY)
+ continue;
sc->mem_cgroup = mem;
- do_shrink_zone(epriority, zone, sc);
+ do_shrink_zone(priority, zone, sc);
mem_cgroup_count_reclaim(mem, current_is_kswapd(),
mem != root, /* limit or hierarchy? */
sc->nr_scanned - scanned,
--Ying
>
> --Ying
>
>
>>
>> sc->mem_cgroup = mem;
>> - do_shrink_zone(priority, zone, sc);
>> + do_shrink_zone(epriority, zone, sc);
>> mem_cgroup_count_reclaim(mem, current_is_kswapd(),
>> mem != root, /* limit or hierarchy? */
>> sc->nr_scanned - scanned,
>> @@ -2480,7 +2484,7 @@ loop_again:
>> * Call soft limit reclaim before calling shrink_zone.
>> * For now we ignore the return value
>> */
>> - mem_cgroup_soft_limit_reclaim(zone, order, sc.gfp_mask);
>> + //mem_cgroup_soft_limit_reclaim(zone, order, sc.gfp_mask);
>>
>> /*
>> * We put equal pressure on every zone, unless
>> --
>> 1.7.5.2
>>
>>
>
--
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