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Date:   Thu, 15 Apr 2021 12:30:53 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Waiman Long <longman@redhat.com>
Cc:     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>, Roman Gushchin <guro@fb.com>,
        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>,
        Wei Yang <richard.weiyang@gmail.com>,
        Masayoshi Mizuma <msys.mizuma@gmail.com>,
        Xing Zhengjun <zhengjun.xing@linux.intel.com>
Subject: Re: [PATCH v3 2/5] mm/memcg: Introduce
 obj_cgroup_uncharge_mod_state()
Message-ID: <YHhqPYcajI9JgXk/@cmpxchg.org>
References: <20210414012027.5352-1-longman@redhat.com>
 <20210414012027.5352-3-longman@redhat.com>
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On Tue, Apr 13, 2021 at 09:20:24PM -0400, Waiman Long wrote:
> In memcg_slab_free_hook()/pcpu_memcg_free_hook(), obj_cgroup_uncharge()
> is followed by mod_objcg_state()/mod_memcg_state(). Each of these
> function call goes through a separate irq_save/irq_restore cycle. That
> is inefficient.  Introduce a new function obj_cgroup_uncharge_mod_state()
> that combines them with a single irq_save/irq_restore cycle.
>
> @@ -3292,6 +3296,25 @@ void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size)
>  	refill_obj_stock(objcg, size);
>  }
>  
> +void obj_cgroup_uncharge_mod_state(struct obj_cgroup *objcg, size_t size,
> +				   struct pglist_data *pgdat, int idx)

The optimization makes sense.

But please don't combine independent operations like this into a
single function. It makes for an unclear parameter list, it's a pain
in the behind to change the constituent operations later on, and it
has a habit of attracting more random bools over time. E.g. what if
the caller already has irqs disabled? What if it KNOWS that irqs are
enabled and it could use local_irq_disable() instead of save?

Just provide an __obj_cgroup_uncharge() that assumes irqs are
disabled, combine with the existing __mod_memcg_lruvec_state(), and
bubble the irq handling up to those callsites which know better.