Re: [PATCH] memory-hotplug: fix zone stat mismatch

[Wen Congyang <wency@cn.fujitsu.com>]

At 09/20/2012 01:55 PM, Minchan Kim Wrote:
> On Thu, Sep 20, 2012 at 01:32:59PM +0800, Wen Congyang wrote:
>> At 09/20/2012 01:16 PM, Minchan Kim Wrote:
>>> On Thu, Sep 20, 2012 at 11:12:40AM +0800, Wen Congyang wrote:
>>>> At 09/20/2012 10:30 AM, Minchan Kim Wrote:
>>>>> On Thu, Sep 20, 2012 at 09:17:43AM +0800, Wen Congyang wrote:
>>>>>> At 09/20/2012 04:17 AM, Minchan Kim Wrote:
>>>>>>> Hi KOSAKI,
>>>>>>>
>>>>>>> On Wed, Sep 19, 2012 at 02:05:20PM -0400, KOSAKI Motohiro wrote:
>>>>>>>> On Wed, Sep 19, 2012 at 3:29 AM, Minchan Kim <minchan@kernel.org> wrote:
>>>>>>>>> During memory-hotplug stress test, I found NR_ISOLATED_[ANON|FILE]
>>>>>>>>> are increasing so that kernel are hang out.
>>>>>>>>>
>>>>>>>>> The cause is that when we do memory-hotadd after memory-remove,
>>>>>>>>> __zone_pcp_update clear out zone's ZONE_STAT_ITEMS in setup_pageset
>>>>>>>>> without draining vm_stat_diff of all CPU.
>>>>>>>>>
>>>>>>>>> This patch fixes it.
>>>>>>>>
>>>>>>>> zone_pcp_update() is called from online pages path. but IMHO,
>>>>>>>> the statistics should be drained offline path. isn't it?
>>>>>>>
>>>>>>> It isn't necessary because statistics is right until we reset it to zero
>>>>>>> in online path.
>>>>>>> Do you have something on your mind that we have to drain it in offline path?
>>>>>>
>>>>>> When a node is offlined and onlined again. We create node_data[i] in the
>>>>>
>>>>> I would like to clarify your word.
>>>>> Create or recreate?
>>>>> Why I have a question is as I look over the source code, hotadd_new_pgdat
>>>>> seem to be called if we do hotadd *new* memory. It's not the case for
>>>>> offline and online again you mentioned. If you're right, I should find 
>>>>> arch_free_nodedata to free pgdat when node is disappear but I can't find it.
>>>>> Do I miss something?
>>>>
>>>> Hmm, when a memory is removed, we don't do cleanup now. We(Fujitsu) posted
>>>> a patchset to do this:
>>>> https://lkml.org/lkml/2012/9/5/201
>>>>
>>>> We don't free pgdat in this patchset now. We have two choice:
>>>> 1. free pgdat
>>>> 2. don't free it, and reuse it when it is onlined again
>>>>
>>>> I'm not sure which choice is better.
>>>
>>> I have no idea because I don't know how you guys uses.
>>> If there is use case that sometime you ues many node burstly but
>>> ues a few node in most time, 1) would be good POV memory efficiency
>>> although it makes code rather complicated.
>>>
>>> Anyway, it's another story with this patch because it's not merged yet.
>>>
>>>>
>>>>>
>>>>>
>>>>>> function hotadd_new_pgdat(), and we will lost the statistics stored in
>>>>>> zone->pageset. So we should drain it in offline path.
>>>>>
>>>>> Even we drain in offline patch, it still has a problem.
>>>>>
>>>>> 1. offline
>>>>> 2. drain -> OKAY 
>>>>> 3. schedule
>>>>> 4. Process A increase zone stat
>>>>> 5. Process B increase zone stat
>>>>> 6. online
>>>>> 7. reset it -> we ends up lost zone stat counter which is modified between 2-6
>>>>>
>>>>
>>>> I understand why you drain it in online path now. But it still should drain it
>>>> in offline path because if all pages in this zone are offlined, we will call
>>>> zone_pcp_reset() to reset zone's pcp. We should also drop it in the function
>>>> zone_pcp_reset().
>>>
>>> Good point.
>>> How about this?
>>>
>>> >From e92bf3e96720c89cb18ec32c5db095a27ad4133c Mon Sep 17 00:00:00 2001
>>> From: Minchan Kim <minchan@kernel.org>
>>> Date: Thu, 20 Sep 2012 14:11:49 +0900
>>> Subject: [PATCH v2] memory-hotplug: fix zone stat mismatch
>>>
>>> During memory-hotplug, I found NR_ISOLATED_[ANON|FILE]
>>> are increasing so that kernel are hang out.
>>>
>>> The cause is that when we do memory-hotadd after memory-remove,
>>> __zone_pcp_update clear out zone's ZONE_STAT_ITEMS in setup_pageset
>>> although vm_stat_diff of all CPU still have value.
>>>
>>> In addtion, when we offline all pages of the zone, we reset them
>>> in zone_pcp_reset without drain so that we lost zone stat item.
>>>
>>> This patch fixes it.
>>>
>>> * from v1
>>>   * drain offline patch - KOSAKI, Wen
>>>
>>> Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>>> Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
>>> Cc: Wen Congyang <wency@cn.fujitsu.com>
>>> Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
>>> Signed-off-by: Minchan Kim <minchan@kernel.org>
>>> ---
>>>  include/linux/vmstat.h |    4 ++++
>>>  mm/page_alloc.c        |    7 +++++++
>>>  mm/vmstat.c            |   12 ++++++++++++
>>>  3 files changed, 23 insertions(+)
>>>
>>> diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
>>> index ad2cfd5..5d31876 100644
>>> --- a/include/linux/vmstat.h
>>> +++ b/include/linux/vmstat.h
>>> @@ -198,6 +198,8 @@ extern void __dec_zone_state(struct zone *, enum zone_stat_item);
>>>  void refresh_cpu_vm_stats(int);
>>>  void refresh_zone_stat_thresholds(void);
>>>  
>>> +void drain_zonestat(struct zone *zone, struct per_cpu_pageset *);
>>> +
>>>  int calculate_pressure_threshold(struct zone *zone);
>>>  int calculate_normal_threshold(struct zone *zone);
>>>  void set_pgdat_percpu_threshold(pg_data_t *pgdat,
>>> @@ -251,6 +253,8 @@ static inline void __dec_zone_page_state(struct page *page,
>>>  static inline void refresh_cpu_vm_stats(int cpu) { }
>>>  static inline void refresh_zone_stat_thresholds(void) { }
>>>  
>>> +static inline void drain_zonestat(struct zone *zone,
>>> +			struct per_cpu_pageset *pset) { }
>>>  #endif		/* CONFIG_SMP */
>>>  
>>>  extern const char * const vmstat_text[];
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index ab58346..980f2e7 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -5904,6 +5904,7 @@ static int __meminit __zone_pcp_update(void *data)
>>>  		local_irq_save(flags);
>>>  		if (pcp->count > 0)
>>>  			free_pcppages_bulk(zone, pcp->count, pcp);
>>> +		drain_zonestat(zone, pset);
>>>  		setup_pageset(pset, batch);
>>>  		local_irq_restore(flags);
>>>  	}
>>> @@ -5920,10 +5921,16 @@ void __meminit zone_pcp_update(struct zone *zone)
>>>  void zone_pcp_reset(struct zone *zone)
>>>  {
>>>  	unsigned long flags;
>>> +	int cpu;
>>> +	struct per_cpu_pageset *pset;
>>>  
>>>  	/* avoid races with drain_pages()  */
>>>  	local_irq_save(flags);
>>>  	if (zone->pageset != &boot_pageset) {
>>> +		for_each_online_cpu(cpu) {
>>
>> A cpu can be offlined before the pages in the zone are offlined. So
>> I think you should drain it on all possible cpu, not online cpu.
>>
> 
> I'm not CPU hotplug expert but IMHO, it should be handled
> by vmstat_cpuup_callback when the CPU is down. Otherwise,
> it should never drain until zone is empty by memory-hotplug.
> 
> And if it is, it should be another patch.

It has been done in page_alloc_cpu_notify(). Your patch is OK to me now.

Thanks
Wen Congyang

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