linux-kernel - Re: [PATCH v17 21/21] mm/lru: revise the comments of lru

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Date:   Thu, 6 Aug 2020 09:39:12 +0800
From:   Alex Shi <alex.shi@...ux.alibaba.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Tejun Heo <tj@...nel.org>, Hugh Dickins <hughd@...gle.com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        kbuild test robot <lkp@...el.com>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, cgroups@...r.kernel.org,
        Shakeel Butt <shakeelb@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Wei Yang <richard.weiyang@...il.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Rong Chen <rong.a.chen@...el.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v17 21/21] mm/lru: revise the comments of lru_lock



在 2020/8/4 下午10:29, Alexander Duyck 写道:
> On Tue, Aug 4, 2020 at 3:04 AM Alex Shi <alex.shi@...ux.alibaba.com> wrote:
>>
>>
>>
>> 在 2020/8/4 上午6:37, Alexander Duyck 写道:
>>>>
>>>>  shrink_inactive_list() also diverts any unevictable pages that it finds on the
>>>> -inactive lists to the appropriate zone's unevictable list.
>>>> +inactive lists to the appropriate node's unevictable list.
>>>>
>>>>  shrink_inactive_list() should only see SHM_LOCK'd pages that became SHM_LOCK'd
>>>>  after shrink_active_list() had moved them to the inactive list, or pages mapped
>>> Same here.
>>
>> lruvec is used per memcg per node actually, and it fallback to node if memcg disabled.
>> So the comments are still right.
>>
>> And most of changes just fix from zone->lru_lock to pgdat->lru_lock change.
> 
> Actually in my mind one thing that might work better would be to
> explain what the lruvec is and where it resides. Then replace zone
> with lruvec since that is really where the unevictable list resides.
> Then it would be correct for both the memcg and pgdat case.

Could you like to revise the doc as your thought?
> 
>>>
>>>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>>>> index 64ede5f150dc..44738cdb5a55 100644
>>>> --- a/include/linux/mm_types.h
>>>> +++ b/include/linux/mm_types.h
>>>> @@ -78,7 +78,7 @@ struct page {
>>>>                 struct {        /* Page cache and anonymous pages */
>>>>                         /**
>>>>                          * @lru: Pageout list, eg. active_list protected by
>>>> -                        * pgdat->lru_lock.  Sometimes used as a generic list
>>>> +                        * lruvec->lru_lock.  Sometimes used as a generic list
>>>>                          * by the page owner.
>>>>                          */
>>>>                         struct list_head lru;
>>>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>>>> index 8af956aa13cf..c92289a4e14d 100644
>>>> --- a/include/linux/mmzone.h
>>>> +++ b/include/linux/mmzone.h
>>>> @@ -115,7 +115,7 @@ static inline bool free_area_empty(struct free_area *area, int migratetype)
>>>>  struct pglist_data;
>>>>
>>>>  /*
>>>> - * zone->lock and the zone lru_lock are two of the hottest locks in the kernel.
>>>> + * zone->lock and the lru_lock are two of the hottest locks in the kernel.
>>>>   * So add a wild amount of padding here to ensure that they fall into separate
>>>>   * cachelines.  There are very few zone structures in the machine, so space
>>>>   * consumption is not a concern here.
>>> So I don't believe you are using ZONE_PADDING in any way to try and
>>> protect the LRU lock currently. At least you aren't using it in the
>>> lruvec. As such it might make sense to just drop the reference to the
>>> lru_lock here. That reminds me that we still need to review the
>>> placement of the lru_lock and determine if there might be a better
>>> placement and/or padding that might improve performance when under
>>> heavy stress.
>>>
>>
>> Right, is it the following looks better?
>>
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index ccc76590f823..0ed520954843 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -113,8 +113,7 @@ static inline bool free_area_empty(struct free_area *area, int migratetype)
>>  struct pglist_data;
>>
>>  /*
>> - * zone->lock and the lru_lock are two of the hottest locks in the kernel.
>> - * So add a wild amount of padding here to ensure that they fall into separate
>> + * Add a wild amount of padding here to ensure datas fall into separate
>>   * cachelines.  There are very few zone structures in the machine, so space
>>   * consumption is not a concern here.
>>   */
>>
>> Thanks!
>> Alex
> 
> I would maybe tweak it to make sure it is clear that we are using this
> to pad out items that are likely to cause cache thrash such as various
> hot spinocks and such.
> 

I appreciate if you like to change the doc better. :)

Thanks
Alex