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Date:   Tue, 10 Nov 2020 12:50:08 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     <linux-mm@kvack.org>, Shakeel Butt <shakeelb@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Christoph Lameter <cl@linux.com>,
        <linux-kernel@vger.kernel.org>, <kernel-team@fb.com>
Subject: Re: [PATCH 2/2] mm: memcg/slab: pre-allocate obj_cgroups for slab
 caches with SLAB_ACCOUNT
Message-Id: <20201110125008.003fb018e74c40109fd6723f@linux-foundation.org>
In-Reply-To: <20201110195753.530157-2-guro@fb.com>
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        <20201110195753.530157-2-guro@fb.com>
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On Tue, 10 Nov 2020 11:57:53 -0800 Roman Gushchin <guro@fb.com> wrote:

> In general it's unknown in advance if a slab page will contain
> accounted objects or not. In order to avoid memory waste, an
> obj_cgroup vector is allocated dynamically when a need to account
> of a new object arises. Such approach is memory efficient, but
> requires an expensive cmpxchg() to set up the memcg/objcgs pointer,
> because an allocation can race with a different allocation on another
> cpu.
> 
> But in some common cases it's known for sure that a slab page will
> contain accounted objects: if the page belongs to a slab cache with a
> SLAB_ACCOUNT flag set. It includes such popular objects like
> vm_area_struct, anon_vma, task_struct, etc.
> 
> In such cases we can pre-allocate the objcgs vector and simple assign
> it to the page without any atomic operations, because at this early
> stage the page is not visible to anyone else.

Was there any measurable performance change from this?