linux-kernel - Re: [PATCH v7 02/10] mm/memcg: fold lru_lock in lock_page

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Date:   Mon, 13 Jan 2020 17:45:51 +0800
From:   Alex Shi <alex.shi@...ux.alibaba.com>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org,
        mgorman@...hsingularity.net, tj@...nel.org, hughd@...gle.com,
        daniel.m.jordan@...cle.com, yang.shi@...ux.alibaba.com,
        willy@...radead.org, shakeelb@...gle.com, hannes@...xchg.org
Cc:     Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>
Subject: Re: [PATCH v7 02/10] mm/memcg: fold lru_lock in lock_page_lru



在 2020/1/10 下午4:49, Konstantin Khlebnikov 写道:
> On 25/12/2019 12.04, Alex Shi wrote:
>>  From the commit_charge's explanations and mem_cgroup_commit_charge
>> comments, as well as call path when lrucare is ture, The lru_lock is
>> just to guard the task migration(which would be lead to move_account)
>> So it isn't needed when !PageLRU, and better be fold into PageLRU to
>> reduce lock contentions.
>>
>> Signed-off-by: Alex Shi <alex.shi@...ux.alibaba.com>
>> Cc: Johannes Weiner <hannes@...xchg.org>
>> Cc: Michal Hocko <mhocko@...nel.org>
>> Cc: Matthew Wilcox <willy@...radead.org>
>> Cc: Vladimir Davydov <vdavydov.dev@...il.com>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: cgroups@...r.kernel.org
>> Cc: linux-mm@...ck.org
>> Cc: linux-kernel@...r.kernel.org
>> ---
>>   mm/memcontrol.c | 9 ++++-----
>>   1 file changed, 4 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index c5b5f74cfd4d..0ad10caabc3d 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -2572,12 +2572,11 @@ static void cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages)
>>     static void lock_page_lru(struct page *page, int *isolated)
>>   {
>> -    pg_data_t *pgdat = page_pgdat(page);
>> -
>> -    spin_lock_irq(&pgdat->lru_lock);
>>       if (PageLRU(page)) {
>> +        pg_data_t *pgdat = page_pgdat(page);
>>           struct lruvec *lruvec;
>>   +        spin_lock_irq(&pgdat->lru_lock);
> 
> That's wrong. Here PageLRU must be checked again under lru_lock.
Hi, Konstantin,

For logical remain, we can get the lock and then release for !PageLRU. 
but I still can figure out the problem scenario. Would like to give more hints?


> 
> 
> Also I don't like these functions:
> - called lock/unlock but actually also isolates
> - used just once
> - pgdat evaluated twice

That's right. I will fold these functions into commit_charge.

Thanks
Alex