Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <1597061872-58724-1-git-send-email-xlpang@linux.alibaba.com>
In-Reply-To: <1597061872-58724-1-git-send-email-xlpang@linux.alibaba.com>
From:   Pekka Enberg <penberg@gmail.com>
Date:   Thu, 20 Aug 2020 17:02:10 +0300
Message-ID: <CAOJsxLE9SkLY5V=D=Ot1oyLkbAORagfozYqxk1iNDW6b7QZrwg@mail.gmail.com>
Subject: Re: [PATCH v2 0/3] mm/slub: Fix count_partial() problem
To:     Xunlei Pang <xlpang@linux.alibaba.com>
Cc:     Vlastimil Babka <vbabka@suse.cz>, Christoph Lameter <cl@linux.com>,
        Wen Yang <wenyang@linux.alibaba.com>,
        Roman Gushchin <guro@fb.com>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        David Rientjes <rientjes@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Aug 10, 2020 at 3:18 PM Xunlei Pang <xlpang@linux.alibaba.com> wrote:
>
> v1->v2:
> - Improved changelog and variable naming for PATCH 1~2.
> - PATCH3 adds per-cpu counter to avoid performance regression
>   in concurrent __slab_free().
>
> [Testing]
> On my 32-cpu 2-socket physical machine:
> Intel(R) Xeon(R) CPU E5-2650 v2 @ 2.60GHz
> perf stat --null --repeat 10 -- hackbench 20 thread 20000
>
> == original, no patched
>       19.211637055 seconds time elapsed                                          ( +-  0.57% )
>
> == patched with patch1~2
>  Performance counter stats for 'hackbench 20 thread 20000' (10 runs):
>
>       21.731833146 seconds time elapsed                                          ( +-  0.17% )
>
> == patched with patch1~3
>  Performance counter stats for 'hackbench 20 thread 20000' (10 runs):
>
>       19.112106847 seconds time elapsed                                          ( +-  0.64% )
>
>
> Xunlei Pang (3):
>   mm/slub: Introduce two counters for partial objects
>   mm/slub: Get rid of count_partial()
>   mm/slub: Use percpu partial free counter
>
>  mm/slab.h |   2 +
>  mm/slub.c | 124 +++++++++++++++++++++++++++++++++++++++++++-------------------
>  2 files changed, 89 insertions(+), 37 deletions(-)

We probably need to wrap the counters under CONFIG_SLUB_DEBUG because
AFAICT all the code that uses them is also wrapped under it.

An alternative approach for this patch would be to somehow make the
lock in count_partial() more granular, but I don't know how feasible
that actually is.

Anyway, I am OK with this approach:

Reviewed-by: Pekka Enberg <penberg@kernel.org>

You still need to convince Christoph, though, because he had
objections over this approach.

- Pekka