linux-kernel - Re: [PATCH next] mm/swap.c: reduce lock contention in lru_cache

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Date:   Thu, 26 Nov 2020 12:05:28 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Alex Shi <alex.shi@...ux.alibaba.com>
Cc:     Konstantin Khlebnikov <koct9i@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>, Yu Zhao <yuzhao@...gle.com>,
        Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH next] mm/swap.c: reduce lock contention in lru_cache_add

On 11/26/20 4:12 AM, Alex Shi wrote:
> 
> 
> 在 2020/11/25 下午11:38, Vlastimil Babka 写道:
>> On 11/20/20 9:27 AM, Alex Shi wrote:
>>> The current relock logical will change lru_lock when found a new
>>> lruvec, so if 2 memcgs are reading file or alloc page at same time,
>>> they could hold the lru_lock alternately, and wait for each other for
>>> fairness attribute of ticket spin lock.
>>>
>>> This patch will sort that all lru_locks and only hold them once in
>>> above scenario. That could reduce fairness waiting for lock reget.
>>> Than, vm-scalability/case-lru-file-readtwice could get ~5% performance
>>> gain on my 2P*20core*HT machine.
>> 
>> Hm, once you sort the pages like this, it's a shame not to splice them instead of more list_del() + list_add() iterations. update_lru_size() could be also called once?
> 
> Yes, looks it's a good idea to use splice instead of list_del/add, but pages
> may on different lru list in a same lruvec, and also may come from different
> zones. That could involve 5 cycles for different lists, and more for zones...

Hmm, zones wouldn't affect splicing (there's a per-node lru these days), but 
would affect accounting. And yeah, there are 5 lru lists, and we probably need 
to be under lru lock to stabilize where a page belongs, so pre-sorting without 
lock wouldn't be safe? Bummer.

> I give up the try.
>