linux-kernel - Re: [PATCH 2/2] mm: memcg/slab: pre-allocate obj_cgroups for slab caches with SLAB

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Date:   Tue, 10 Nov 2020 13:51:52 -0800
From:   Roman Gushchin <guro@...com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     <linux-mm@...ck.org>, Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        <linux-kernel@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [PATCH 2/2] mm: memcg/slab: pre-allocate obj_cgroups for slab
 caches with SLAB_ACCOUNT

On Tue, Nov 10, 2020 at 12:50:08PM -0800, Andrew Morton wrote:
> On Tue, 10 Nov 2020 11:57:53 -0800 Roman Gushchin <guro@...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?

A very simplistic benchmark (allocating 10000000 64-bytes objects in a row)
shows ~15% win. In the real life it seems that most workloads are not very
sensitive to the speed of (accounted) slab allocations.