Re: [PATCH v5 2/6] memcg: stop vmscan when enough done.

From: Michal Hocko
Date: Thu Aug 18 2011 - 03:46:11 EST

On Thu 18-08-11 15:42:59, KAMEZAWA Hiroyuki wrote:
> On Thu, 18 Aug 2011 08:27:22 +0200
> Michal Hocko <mhocko@xxxxxxx> wrote:
>
> > On Thu 18-08-11 08:52:33, KAMEZAWA Hiroyuki wrote:
> > > On Wed, 17 Aug 2011 13:35:50 +0200
> > > Michal Hocko <mhocko@xxxxxxx> wrote:
> > >
> > > > On Wed 17-08-11 09:54:05, KAMEZAWA Hiroyuki wrote:
> > > > > On Thu, 11 Aug 2011 16:50:55 +0200
> > > > > > - mem_cgroup_force_empty asks for reclaiming all pages. I guess it should be
> > > > > > OK but will have to think about it some more.
> > > > >
> > > > > force_empty/rmdir() is allowed to be stopped by Ctrl-C. I think passing res->usage
> > > > > is overkilling.
> > > >
> > > > So, how many pages should be reclaimed then?
> > > >
> > >
> > > How about (1 << (MAX_ORDER-1))/loop ?
> >
> > Hmm, I am not sure I see any benefit. We want to reclaim all those
> > pages why shouldn't we do it in one batch? If we use a value based on
> > MAX_ORDER then we make a bigger chance that force_empty fails for big
> > cgroups (e.g. with a lot of page cache).
>
> Why bigger chance to fail ? retry counter is decreased only when we cannot
> make any reclaim. The number passed here is not problem against the faiulre.

Yes, you are right. I have overlooked that.


> I don't like very long vmscan which cannot be stopped by Ctrl-C.

Sure, now I see your point. Thanks for clarification.

> > Anyway, if we want to mimic the previous behavior then we should use
> > something like nr_nodes * SWAP_CLUSTER_MAX (the above value would be
> > sufficient for up to 32 nodes).
> >
>
> agreed.

Updated patch:
Changes since v1:
- reclaim nr_nodes * SWAP_CLUSTER_MAX in mem_cgroup_force_empty
---
From: Michal Hocko <mhocko@xxxxxxx>
Subject: memcg: add nr_pages argument for hierarchical reclaim

Now that we are doing memcg direct reclaim limited to nr_to_reclaim
pages (introduced by "memcg: stop vmscan when enough done.") we have to
be more careful. Currently we are using SWAP_CLUSTER_MAX which is OK for
most callers but it might cause failures for limit resize or force_empty
code paths on big NUMA machines.

Previously we might have reclaimed up to nr_nodes * SWAP_CLUSTER_MAX
while now we have it at SWAP_CLUSTER_MAX. Both resize and force_empty rely
on reclaiming a certain amount of pages and retrying if their condition is
still not met.

Let's add nr_pages argument to mem_cgroup_hierarchical_reclaim which will
push it further to try_to_free_mem_cgroup_pages. We still fall back to
SWAP_CLUSTER_MAX for small requests so the standard code (hot) paths are not
affected by this.

We have to be careful in mem_cgroup_do_charge and do not provide the
given nr_pages because we would reclaim too much for THP which can
safely fall back to single page allocations.

mem_cgroup_force_empty could try to reclaim all pages at once but it is much
better to limit the nr_pages to something reasonable so that we are able to
terminate it by a signal. Let's mimic previous behavior by asking for
MAX_NUMNODES * SWAP_CLUSTER_MAX.

Signed-off-by: Michal Hocko <mhocko@xxxxxxx>

Index: linus_tree/include/linux/memcontrol.h
===================================================================
--- linus_tree.orig/include/linux/memcontrol.h 2011-08-18 09:30:24.000000000 +0200
+++ linus_tree/include/linux/memcontrol.h 2011-08-18 09:30:36.000000000 +0200
@@ -130,7 +130,8 @@ extern void mem_cgroup_print_oom_info(st

extern unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem,
gfp_t gfp_mask, bool noswap,
- struct memcg_scanrecord *rec);
+ struct memcg_scanrecord *rec,
+ unsigned long nr_pages);
extern unsigned long mem_cgroup_shrink_node_zone(struct mem_cgroup *mem,
gfp_t gfp_mask, bool noswap,
struct zone *zone,
Index: linus_tree/mm/memcontrol.c
===================================================================
--- linus_tree.orig/mm/memcontrol.c 2011-08-18 09:30:34.000000000 +0200
+++ linus_tree/mm/memcontrol.c 2011-08-18 09:36:41.000000000 +0200
@@ -1729,12 +1729,15 @@ static void mem_cgroup_record_scanstat(s
* (other groups can be removed while we're walking....)
*
* If shrink==true, for avoiding to free too much, this returns immedieately.
+ * Given nr_pages tells how many pages are we over the soft limit or how many
+ * pages do we want to reclaim in the direct reclaim mode.
*/
static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
struct zone *zone,
gfp_t gfp_mask,
unsigned long reclaim_options,
- unsigned long *total_scanned)
+ unsigned long *total_scanned,
+ unsigned long nr_pages)
{
struct mem_cgroup *victim;
int ret, total = 0;
@@ -1743,11 +1746,8 @@ static int mem_cgroup_hierarchical_recla
bool shrink = reclaim_options & MEM_CGROUP_RECLAIM_SHRINK;
bool check_soft = reclaim_options & MEM_CGROUP_RECLAIM_SOFT;
struct memcg_scanrecord rec;
- unsigned long excess;
unsigned long scanned;

- excess = res_counter_soft_limit_excess(&root_mem->res) >> PAGE_SHIFT;
-
/* If memsw_is_minimum==1, swap-out is of-no-use. */
if (!check_soft && !shrink && root_mem->memsw_is_minimum)
noswap = true;
@@ -1785,11 +1785,11 @@ static int mem_cgroup_hierarchical_recla
}
/*
* We want to do more targeted reclaim.
- * excess >> 2 is not to excessive so as to
+ * nr_pages >> 2 is not to excessive so as to
* reclaim too much, nor too less that we keep
* coming back to reclaim from this cgroup
*/
- if (total >= (excess >> 2) ||
+ if (total >= (nr_pages >> 2) ||
(loop > MEM_CGROUP_MAX_RECLAIM_LOOPS)) {
css_put(&victim->css);
break;
@@ -1816,7 +1816,7 @@ static int mem_cgroup_hierarchical_recla
*total_scanned += scanned;
} else
ret = try_to_free_mem_cgroup_pages(victim, gfp_mask,
- noswap, &rec);
+ noswap, &rec, nr_pages);
mem_cgroup_record_scanstat(&rec);
css_put(&victim->css);
/*
@@ -2331,8 +2331,14 @@ static int mem_cgroup_do_charge(struct m
if (!(gfp_mask & __GFP_WAIT))
return CHARGE_WOULDBLOCK;

+ /*
+ * We are lying about nr_pages because we do not want to
+ * reclaim too much for THP pages which should rather fallback
+ * to small pages.
+ */
ret = mem_cgroup_hierarchical_reclaim(mem_over_limit, NULL,
- gfp_mask, flags, NULL);
+ gfp_mask, flags, NULL,
+ 1);
if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
return CHARGE_RETRY;
/*
@@ -3567,7 +3573,8 @@ static int mem_cgroup_resize_limit(struc

mem_cgroup_hierarchical_reclaim(memcg, NULL, GFP_KERNEL,
MEM_CGROUP_RECLAIM_SHRINK,
- NULL);
+ NULL,
+ (val-memlimit) >> PAGE_SHIFT);
curusage = res_counter_read_u64(&memcg->res, RES_USAGE);
/* Usage is reduced ? */
if (curusage >= oldusage)
@@ -3628,7 +3635,8 @@ static int mem_cgroup_resize_memsw_limit
mem_cgroup_hierarchical_reclaim(memcg, NULL, GFP_KERNEL,
MEM_CGROUP_RECLAIM_NOSWAP |
MEM_CGROUP_RECLAIM_SHRINK,
- NULL);
+ NULL,
+ (val-memswlimit) >> PAGE_SHIFT);
curusage = res_counter_read_u64(&memcg->memsw, RES_USAGE);
/* Usage is reduced ? */
if (curusage >= oldusage)
@@ -3671,10 +3679,12 @@ unsigned long mem_cgroup_soft_limit_recl
break;

nr_scanned = 0;
+ excess = res_counter_soft_limit_excess(&mz->mem->res);
reclaimed = mem_cgroup_hierarchical_reclaim(mz->mem, zone,
gfp_mask,
MEM_CGROUP_RECLAIM_SOFT,
- &nr_scanned);
+ &nr_scanned,
+ excess >> PAGE_SHIFT);
nr_reclaimed += reclaimed;
*total_scanned += nr_scanned;
spin_lock(&mctz->lock);
@@ -3870,8 +3880,10 @@ try_to_free:
rec.context = SCAN_BY_SHRINK;
rec.mem = mem;
rec.root = mem;
+ /* reclaim from every node at least something */
progress = try_to_free_mem_cgroup_pages(mem, GFP_KERNEL,
- false, &rec);
+ false, &rec,
+ MAX_NUMNODES * SWAP_CLUSTER_MAX);
if (!progress) {
nr_retries--;
/* maybe some writeback is necessary */
Index: linus_tree/mm/vmscan.c
===================================================================
--- linus_tree.orig/mm/vmscan.c 2011-08-18 09:30:24.000000000 +0200
+++ linus_tree/mm/vmscan.c 2011-08-18 09:30:36.000000000 +0200
@@ -2340,7 +2340,8 @@ unsigned long mem_cgroup_shrink_node_zon
unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
gfp_t gfp_mask,
bool noswap,
- struct memcg_scanrecord *rec)
+ struct memcg_scanrecord *rec,
+ unsigned long nr_pages)
{
struct zonelist *zonelist;
unsigned long nr_reclaimed;
@@ -2350,7 +2351,7 @@ unsigned long try_to_free_mem_cgroup_pag
.may_writepage = !laptop_mode,
.may_unmap = 1,
.may_swap = !noswap,
- .nr_to_reclaim = SWAP_CLUSTER_MAX,
+ .nr_to_reclaim = max_t(unsigned long, nr_pages, SWAP_CLUSTER_MAX),
.order = 0,
.mem_cgroup = mem_cont,
.memcg_record = rec,

--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic
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