在 2020/11/12 下午8:19, Vlastimil Babka 写道:
> On 11/5/20 9:55 AM, Alex Shi wrote:
>> This patch moves per node lru_lock into lruvec, thus bring a lru_lock for
>> each of memcg per node. So on a large machine, each of memcg don't
>> have to suffer from per node pgdat->lru_lock competition. They could go
>> fast with their self lru_lock.
>>
>> After move memcg charge before lru inserting, page isolation could
>> serialize page's memcg, then per memcg lruvec lock is stable and could
>> replace per node lru lock.
>>
>> In func isolate_migratepages_block, compact_unlock_should_abort and
>> lock_page_lruvec_irqsave are open coded to work with compact_control.
>> Also add a debug func in locking which may give some clues if there are
>> sth out of hands.
>>
>> Daniel Jordan's testing show 62% improvement on modified readtwice case
>> on his 2P * 10 core * 2 HT broadwell box.
>> https://lore.kernel.org/lkml/20200915165807.kpp7uhiw7l3lo...@ca-dmjordan1.us.oracle.com/
>>
>> On a large machine with memcg enabled but not used, the page's lruvec
>> seeking pass a few pointers, that may lead to lru_lock holding time
>> increase and a bit regression.
>>
>> Hugh Dickins helped on the patch polish, thanks!
>>
>> Signed-off-by: Alex Shi <alex....@linux.alibaba.com>
>> Acked-by: Hugh Dickins <hu...@google.com>
>> Cc: Rong Chen <rong.a.c...@intel.com>
>> Cc: Hugh Dickins <hu...@google.com>
>> Cc: Andrew Morton <a...@linux-foundation.org>
>> Cc: Johannes Weiner <han...@cmpxchg.org>
>> Cc: Michal Hocko <mho...@kernel.org>
>> Cc: Vladimir Davydov <vdavydov....@gmail.com>
>> Cc: Yang Shi <yang....@linux.alibaba.com>
>> Cc: Matthew Wilcox <wi...@infradead.org>
>> Cc: Konstantin Khlebnikov <khlebni...@yandex-team.ru>
>> Cc: Tejun Heo <t...@kernel.org>
>> Cc: linux-kernel@vger.kernel.org
>> Cc: linux...@kvack.org
>> Cc: cgro...@vger.kernel.org
>
> I think I need some explanation about the rcu_read_lock() usage in
> lock_page_lruvec*() (and places effectively opencoding it).
> Preferably in form of some code comment, but that can be also added as a
> additional patch later, I don't want to block the series.
>
Hi Vlastimil,
Thanks for comments!
Oh, we did talk about the rcu_read_lock which is used to block memcg destroy
during locking.
and the spin_lock actually includes a rcu_read_lock(). Yes, we could add this
comments later.
> mem_cgroup_page_lruvec() comment says
>
> * This function relies on page->mem_cgroup being stable - see the
> * access rules in commit_charge().
>
> commit_charge() comment:
>
> * Any of the following ensures page->mem_cgroup stability:
> *
> * - the page lock
> * - LRU isolation
> * - lock_page_memcg()
> * - exclusive reference
>
> "LRU isolation" used to be quite clear, but now is it after
> TestClearPageLRU(page) or after deleting from the lru list as well?
> Also it doesn't mention rcu_read_lock(), should it?
The lru isolation still is same as old conception, a set actions that take a
page from a lru list, and commit_charge do
need a isoltion for the page.
but the condition of page_memcg could be change since we don't rely on lru
isolation for it. The comments
could be changed later.
>
> So what exactly are we protecting by rcu_read_lock() in e.g.
> lock_page_lruvec()?
>
> rcu_read_lock();
> lruvec = mem_cgroup_page_lruvec(page, pgdat);
> spin_lock(&lruvec->lru_lock);
> rcu_read_unlock();
>
> Looks like we are protecting the lruvec from going away and it can't go away
> anymore after we take the lru_lock?
>
> But then e.g. in __munlock_pagevec() we are doing this without an
> rcu_read_lock():
>
> new_lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
TestClearPageLRU could block the page from memcg migration/destory.
Thanks
Alex
>
> where new_lruvec is potentionally not the one that we have locked
>
> And the last thing mem_cgroup_page_lruvec() is doing is:
>
> if (unlikely(lruvec->pgdat != pgdat))
> lruvec->pgdat = pgdat;
> return lruvec;
>
> So without the rcu_read_lock() is this potentionally accessing the pgdat
> field of lruvec that might have just gone away?
>
> Thanks,
> Vlastimil
