Re: [PATCH v17 21/21] mm/lru: revise the comments of lru_lock

[Alexander Duyck]

On Wed, Aug 5, 2020 at 6:39 PM Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> 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@xxxxxxxxxxxxxxxxx> 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. :)

Give me a day or so. I will submit a follow-on patch with some cleanup
for the comments.

Thanks.

- Alex