linux-kernel - Re: [PATCH] mm: thp: move deferred split queue to memcg's nodeinfo

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Date:   Mon, 7 Oct 2019 16:19:59 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     kirill.shutemov@...ux.intel.com, ktkhai@...tuozzo.com,
        hannes@...xchg.org, hughd@...gle.com, shakeelb@...gle.com,
        rientjes@...gle.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: thp: move deferred split queue to memcg's nodeinfo

On 10/2/19 10:43 AM, Michal Hocko wrote:
> On Wed 02-10-19 06:16:43, Yang Shi wrote:
>> The commit 87eaceb3faa59b9b4d940ec9554ce251325d83fe ("mm: thp: make
>> deferred split shrinker memcg aware") makes deferred split queue per
>> memcg to resolve memcg pre-mature OOM problem.  But, all nodes end up
>> sharing the same queue instead of one queue per-node before the commit.
>> It is not a big deal for memcg limit reclaim, but it may cause global
>> kswapd shrink THPs from a different node.
>>
>> And, 0-day testing reported -19.6% regression of stress-ng's madvise
>> test [1].  I didn't see that much regression on my test box (24 threads,
>> 48GB memory, 2 nodes), with the same test (stress-ng --timeout 1
>> --metrics-brief --sequential 72  --class vm --exclude spawn,exec), I saw
>> average -3% (run the same test 10 times then calculate the average since
>> the test itself may have most 15% variation according to my test)
>> regression sometimes (not every time, sometimes I didn't see regression
>> at all).
>>
>> This might be caused by deferred split queue lock contention.  With some
>> configuration (i.e. just one root memcg) the lock contention my be worse
>> than before (given 2 nodes, two locks are reduced to one lock).
>>
>> So, moving deferred split queue to memcg's nodeinfo to make it NUMA
>> aware again.
>>
>> With this change stress-ng's madvise test shows average 4% improvement
>> sometimes and I didn't see degradation anymore.
> 
> My concern about this getting more and more complex
> (http://lkml.kernel.org/r/20191002084014.GH15624@dhcp22.suse.cz) holds
> here even more. Can we step back and reconsider the whole thing please?

What about freeing immediately after split via workqueue and also have a
synchronous version called before going oom? Maybe there would be also
other things that would benefit from this scheme instead of traditional
reclaim and shrinkers?