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Subject: Re: [PATCH v8 4/5] locking/qspinlock: Introduce starvation avoidance
 into CNA
To:     Peter Zijlstra <peterz@infradead.org>,
        Alex Kogan <alex.kogan@oracle.com>
Cc:     linux@armlinux.org.uk, Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Arnd Bergmann <arnd@arndb.de>, linux-arch@vger.kernel.org,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>, hpa@zytor.com, x86@kernel.org,
        Hanjun Guo <guohanjun@huawei.com>,
        Jan Glauber <jglauber@marvell.com>,
        Steven Sistare <steven.sistare@oracle.com>,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        dave.dice@oracle.com
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From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <e26b3afa-80d6-71bf-39c8-0fa4b875cbb2@redhat.com>
Date:   Tue, 4 Feb 2020 12:39:30 -0500
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On 2/4/20 12:27 PM, Peter Zijlstra wrote:
> On Tue, Feb 04, 2020 at 11:54:02AM -0500, Alex Kogan wrote:
>>> On Feb 3, 2020, at 10:47 AM, Waiman Long <longman@redhat.com> wrote:
>>>
>>> On 2/3/20 10:28 AM, Peter Zijlstra wrote:
>>>> On Mon, Feb 03, 2020 at 09:59:12AM -0500, Waiman Long wrote:
>>>>> On 2/3/20 8:45 AM, Peter Zijlstra wrote:
>>>>>> Presumably you have a workload where CNA is actually a win? That i=
s,
>>>>>> what inspired you to go down this road? Which actual kernel lock i=
s so
>>>>>> contended on NUMA machines that we need to do this?
>> There are quite a few actually. files_struct.file_lock, file_lock_cont=
ext.flc_lock
>> and lockref.lock are some concrete examples that get very hot in will-=
it-scale
>> benchmarks.=20
> Right, that's all a variant of banging on the same resources across
> nodes. I'm not sure there's anything fundamental we can fix there.
>
>> And then there are spinlocks in __futex_data.queues,=20
>> which get hot when applications have contended (pthread) locks =E2=80=94=
=20
>> LevelDB is an example.
> A numa aware rework of futexes has been on the todo list for years :/
Now, we are going to get that for free with this patchset:-)
>
>> Our initial motivation was based on an observation that kernel qspinlo=
ck is not=20
>> NUMA-aware. So what, you may ask. Much like people realized in the pas=
t that
>> global spinning is bad for performance, and they switched from ticket =
lock to
>> locks with local spinning (e.g., MCS), I think everyone would agree th=
ese days that
>> bouncing a lock (and cache lines in general) across numa nodes is simi=
larly bad.
>> And as CNA demonstrates, we are easily leaving 2-3x speedups on the ta=
ble by
>> doing just that with the current qspinlock.
> Actual benchmarks with performance numbers are required. It helps
> motivate the patches as well as gives reviewers clues on how to
> reproduce / inspect the claims made.
>
I think the cover-letter does have some benchmark results listed.=C2=A0 A=
re
you saying that some benchmark results should be put into individual
patches themselves?

Cheers,
Longman