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Date:   Mon, 12 Apr 2021 11:55:57 -0700
From:   Roman Gushchin <guro@fb.com>
To:     Waiman Long <longman@redhat.com>
CC:     Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>, Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        <linux-kernel@vger.kernel.org>, <cgroups@vger.kernel.org>,
        <linux-mm@kvack.org>, Shakeel Butt <shakeelb@google.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Alex Shi <alex.shi@linux.alibaba.com>,
        Chris Down <chris@chrisdown.name>,
        Yafang Shao <laoar.shao@gmail.com>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Wei Yang <richard.weiyang@gmail.com>,
        Masayoshi Mizuma <msys.mizuma@gmail.com>
Subject: Re: [PATCH 5/5] mm/memcg: Optimize user context object stock access
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References: <20210409231842.8840-1-longman@redhat.com>
 <20210409231842.8840-6-longman@redhat.com>
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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@fb.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.

Thanks!