linux-kernel - Re: [PATCH 5/5] mm/memcg: Optimize user context object stock access

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Message-ID: <49c03fb0-be46-5288-2c4c-6ad5ad194b4c@redhat.com>
Date:   Mon, 12 Apr 2021 15:58:46 -0400
From:   Waiman Long <llong@...hat.com>
To:     Roman Gushchin <guro@...com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Chris Down <chris@...isdown.name>,
        Yafang Shao <laoar.shao@...il.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Masayoshi Mizuma <msys.mizuma@...il.com>
Subject: Re: [PATCH 5/5] mm/memcg: Optimize user context object stock access

On 4/12/21 2:55 PM, Roman Gushchin wrote:
> On Fri, Apr 09, 2021 at 07:18:42PM -0400, Waiman Long wrote:
>> Most kmem_cache_alloc() calls are from user context. With instrumentation
>> enabled, the measured amount of kmem_cache_alloc() calls from non-task
>> context was about 0.01% of the total.
>>
>> The irq disable/enable sequence used in this case to access content
>> from object stock is slow.  To optimize for user context access, there
>> are now two object stocks for task context and interrupt context access
>> respectively.
>>
>> The task context object stock can be accessed after disabling preemption
>> which is cheap in non-preempt kernel. The interrupt context object stock
>> can only be accessed after disabling interrupt. User context code can
>> access interrupt object stock, but not vice versa.
>>
>> The mod_objcg_state() function is also modified to make sure that memcg
>> and lruvec stat updates are done with interrupted disabled.
>>
>> The downside of this change is that there are more data stored in local
>> object stocks and not reflected in the charge counter and the vmstat
>> arrays.  However, this is a small price to pay for better performance.
> I agree, the extra memory space is not a significant concern.
> I'd be more worried about the code complexity, but the result looks
> nice to me!
>
> Acked-by: Roman Gushchin <guro@...com>
>
> Btw, it seems that the mm tree ran a bit off, so I had to apply this series
> on top of Linus's tree to review. Please, rebase.

This patchset is based on the code in Linus' tree. I had applied the 
patchset to linux-next to see if there was any conflicts. Two of the 
patches had minor fuzzes around the edge but no actual merge conflict 
for now.

Cheers,
Longman