linux-kernel - Re: [PATCH] mm: vmscan: check mem cgroup over reclaimed

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Date:	Tue, 24 Jan 2012 15:22:09 -0800
From:	Ying Han <yinghan@...gle.com>
To:	Hillf Danton <dhillf@...il.com>
Cc:	linux-mm@...ck.org, Michal Hocko <mhocko@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: vmscan: check mem cgroup over reclaimed

On Mon, Jan 23, 2012 at 7:45 PM, Hillf Danton <dhillf@...il.com> wrote:
> Hi all
>
> On Tue, Jan 24, 2012 at 3:04 AM, Ying Han <yinghan@...gle.com> wrote:
>> On Sun, Jan 22, 2012 at 5:55 PM, Hillf Danton <dhillf@...il.com> wrote:
>>> To avoid reduction in performance of reclaimee, checking overreclaim is added
>>> after shrinking lru list, when pages are reclaimed from mem cgroup.
>>>
>>> If over reclaim occurs, shrinking remaining lru lists is skipped, and no more
>>> reclaim for reclaim/compaction.
>>>
>>> Signed-off-by: Hillf Danton <dhillf@...il.com>
>>> ---
>>>
>>> --- a/mm/vmscan.c       Mon Jan 23 00:23:10 2012
>>> +++ b/mm/vmscan.c       Mon Jan 23 09:57:20 2012
>>> @@ -2086,6 +2086,7 @@ static void shrink_mem_cgroup_zone(int p
>>>        unsigned long nr_reclaimed, nr_scanned;
>>>        unsigned long nr_to_reclaim = sc->nr_to_reclaim;
>>>        struct blk_plug plug;
>>> +       bool memcg_over_reclaimed = false;
>>>
>>>  restart:
>>>        nr_reclaimed = 0;
>>> @@ -2103,6 +2104,11 @@ restart:
>>>
>>>                                nr_reclaimed += shrink_list(lru, nr_to_scan,
>>>                                                            mz, sc, priority);
>>> +
>>> +                               memcg_over_reclaimed = !scanning_global_lru(mz)
>>> +                                       && (nr_reclaimed >= nr_to_reclaim);
>>> +                               if (memcg_over_reclaimed)
>>> +                                       goto out;
>>
>> Why we need the change here? Do we have number to demonstrate?
>
> See below please 8-)
>
>>
>>
>>>                        }
>>>                }
>>>                /*
>>> @@ -2116,6 +2122,7 @@ restart:
>>>                if (nr_reclaimed >= nr_to_reclaim && priority < DEF_PRIORITY)
>>>                        break;
>>>        }
>>> +out:
>>>        blk_finish_plug(&plug);
>>>        sc->nr_reclaimed += nr_reclaimed;
>>>
>>> @@ -2127,7 +2134,8 @@ restart:
>>>                shrink_active_list(SWAP_CLUSTER_MAX, mz, sc, priority, 0);
>>>
>>>        /* reclaim/compaction might need reclaim to continue */
>>> -       if (should_continue_reclaim(mz, nr_reclaimed,
>>> +       if (!memcg_over_reclaimed &&
>>> +           should_continue_reclaim(mz, nr_reclaimed,
>>>                                        sc->nr_scanned - nr_scanned, sc))
>>
>> This changes the existing logic. What if the nr_reclaimed is greater
>> than nr_to_reclaim, but smaller than pages_for_compaction? The
>> existing logic is to continue reclaiming.
>>
> With soft limit available, what if nr_to_reclaim set to be the number of
> pages exceeding soft limit? With over reclaim abused, what are the targets
> of soft limit?

The nr_to_reclaim is set to SWAP_CLUSTER_MAX (32) for direct reclaim
and ULONG_MAX for background reclaim. Not sure we can set it, but it
is possible the res_counter_soft_limit_excess equal to that target
value. The current soft limit mechanism provides a clue of WHERE to
reclaim pages when there is memory pressure, it doesn't change the
reclaim target as it was before.

Overreclaim a cgroup under its softlimit is bad, but we should be
careful not introducing side effect before providing the guarantee.
Here, the should_continue_reclaim() has logic of freeing a bit more
order-0 pages for compaction. The logic got changed after this.

--Ying


> Thanks
> Hillf
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