Re: [PATCH v17 18/21] mm/lru: introduce the relock_page_lruvec function

Alex Shi
2020-07-30T06:08:53Z

在 2020/7/30 上午1:52, Alexander Duyck 写道:
>> + rcu_read_lock();
>> + locked = mem_cgroup_page_lruvec(page, pgdat) == locked_lruvec;
>> + rcu_read_unlock();
>> +
>> + if (locked)
>> + return locked_lruvec;
>> +
>> + if (locked_lruvec)
>> + unlock_page_lruvec_irqrestore(locked_lruvec, *flags);
>> +
>> + return lock_page_lruvec_irqsave(page, flags);
>> +}
>> +
> So looking these over they seem to be pretty inefficient for what they
> do. Basically in worst case (locked_lruvec == NULL) you end up calling
> mem_cgoup_page_lruvec and all the rcu_read_lock/unlock a couple times
> for a single page. It might make more sense to structure this like:
> if (locked_lruvec) {

Uh, we still need to check this page's lruvec, that needs a rcu_read_lock.
to save a mem_cgroup_page_lruvec call, we have to open lock_page_lruvec
as your mentained before.

> if (lruvec_holds_page_lru_lock(page, locked_lruvec))
> return locked_lruvec;
>
> unlock_page_lruvec_irqrestore(locked_lruvec, *flags);
> }
> return lock_page_lruvec_irqsave(page, flags);
>
> The other piece that has me scratching my head is that I wonder if we
> couldn't do this without needing the rcu_read_lock. For example, what
> if we were to compare the page mem_cgroup pointer to the memcg back
> pointer stored in the mem_cgroup_per_node? It seems like ordering
> things this way would significantly reduce the overhead due to the
> pointer chasing to see if the page is in the locked lruvec or not.
>

If page->mem_cgroup always be charged. the following could be better.

+/* Don't lock again iff page's lruvec locked */
+static inline struct lruvec *relock_page_lruvec_irqsave(struct page *page,
+ struct lruvec *locked_lruvec, unsigned long *flags)
+{
+ struct lruvec *lruvec;
+
+ if (mem_cgroup_disabled())
+ return locked_lruvec;
+
+ /* user page always be charged */
+ VM_BUG_ON_PAGE(!page->mem_cgroup, page);
+
+ rcu_read_lock();
+ if (likely(lruvec_memcg(locked_lruvec) == page->mem_cgroup)) {
+ rcu_read_unlock();
+ return locked_lruvec;
+ }
+
+ if (locked_lruvec)
+ unlock_page_lruvec_irqrestore(locked_lruvec, *flags);
+
+ lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
+ spin_lock_irqsave(&lruvec->lru_lock, *flags);
+ rcu_read_unlock();
+ lruvec_memcg_debug(lruvec, page);
+
+ return lruvec;
+}
+

The user page is always be charged since readahead page is charged now.
and looks we also can apply this patch. I will test it to see if there is
other exception.

commit 826128346e50f6c60c513e166998466b593becad
Author: Alex Shi <alex.shi@linux.alibaba.com>
Date: Thu Jul 30 13:58:38 2020 +0800

mm/memcg: remove useless check on page->mem_cgroup

Since readahead page will be charged on memcg too. We don't need to
check this exception now.

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index af96217f2ec5..0c7f6bed199b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1336,12 +1336,6 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct pglist_data *pgd

VM_BUG_ON_PAGE(PageTail(page), page);
memcg = READ_ONCE(page->mem_cgroup);
- /*
- * Swapcache readahead pages are added to the LRU - and
- * possibly migrated - before they are charged.
- */
- if (!memcg)
- memcg = root_mem_cgroup;

mz = mem_cgroup_page_nodeinfo(memcg, page);
lruvec = &mz->lruvec;
@@ -6962,10 +6956,7 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage)
if (newpage->mem_cgroup)
return;

- /* Swapcache readahead pages can get replaced before being charged */
memcg = oldpage->mem_cgroup;
- if (!memcg)
- return;

/* Force-charge the new page. The old one will be freed soon */
nr_pages = thp_nr_pages(newpage);
@@ -7160,10 +7151,6 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)

memcg = page->mem_cgroup;

- /* Readahead page, never charged */
- if (!memcg)
- return;
-
/*
* In case the memcg owning these pages has been offlined and doesn't
* have an ID allocated to it anymore, charge the closest online