linux-kernel - Re: [PATCH-next v5 3/4] mm/memcg: Improve refill_obj

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Date:   Mon, 26 Apr 2021 12:24:02 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Waiman Long <longman@...hat.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>, Roman Gushchin <guro@...com>,
        LKML <linux-kernel@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        Muchun Song <songmuchun@...edance.com>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Chris Down <chris@...isdown.name>,
        Yafang Shao <laoar.shao@...il.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Masayoshi Mizuma <msys.mizuma@...il.com>,
        Xing Zhengjun <zhengjun.xing@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH-next v5 3/4] mm/memcg: Improve refill_obj_stock() performance

On Tue, Apr 20, 2021 at 12:30 PM Waiman Long <longman@...hat.com> wrote:
>
> There are two issues with the current refill_obj_stock() code. First of
> all, when nr_bytes reaches over PAGE_SIZE, it calls drain_obj_stock() to
> atomically flush out remaining bytes to obj_cgroup, clear cached_objcg
> and do a obj_cgroup_put(). It is likely that the same obj_cgroup will
> be used again which leads to another call to drain_obj_stock() and
> obj_cgroup_get() as well as atomically retrieve the available byte from
> obj_cgroup. That is costly. Instead, we should just uncharge the excess
> pages, reduce the stock bytes and be done with it. The drain_obj_stock()
> function should only be called when obj_cgroup changes.
>
> Secondly, when charging an object of size not less than a page in
> obj_cgroup_charge(), it is possible that the remaining bytes to be
> refilled to the stock will overflow a page and cause refill_obj_stock()
> to uncharge 1 page. To avoid the additional uncharge in this case,
> a new overfill flag is added to refill_obj_stock() which will be set
> when called from obj_cgroup_charge().
>
> A multithreaded kmalloc+kfree microbenchmark on a 2-socket 48-core
> 96-thread x86-64 system with 96 testing threads were run.  Before this
> patch, the total number of kilo kmalloc+kfree operations done for a 4k
> large object by all the testing threads per second were 4,304 kops/s
> (cgroup v1) and 8,478 kops/s (cgroup v2). After applying this patch, the
> number were 4,731 (cgroup v1) and 418,142 (cgroup v2) respectively. This
> represents a performance improvement of 1.10X (cgroup v1) and 49.3X
> (cgroup v2).
>
> Signed-off-by: Waiman Long <longman@...hat.com>

After incorporating Roman's suggestion, you can add:

Reviewed-by: Shakeel Butt <shakeelb@...gle.com>